Bacterial Growth — Microbiology

MICROBIOLOGY

ULTIMATE STUDY GUIDE

DATE: Tuesday 31 March / Wednesday 1 April 2026

DURATION: 60 minutes | WEIGHT: 15% of total marks | ATTEMPT: Single attempt only

FORMAT: 10 Multiple Choice + 1 Matching + 3 Extended Short Answer

CHEAT SHEET: 1 x A4 page, both sides, handwritten, with name and student ID

BROWSER: Lockdown browser — no Canvas access during exam

DEVICES: Laptops/tablets only — phones NOT permitted

STRATEGY FOR 4.0 GPA:

• Understand theory behind every practical — questions will be applied, not just recall

• Know the interpretation of ALL results — colour changes, what they mean, why

• Link tests to organisms — which test differentiates which bacteria

• Don't rely solely on cheat sheet — 60 min is tight; you need material internalised

• Master the flowcharts — identification is a decision tree, not isolated facts

CONTENTS

Section Topic Covers

Part 1 Bacterial Growth Requirements Temperature, O2, Nutrition, UV Effects

Part 2 Microscopy, Morphology & Gram Stain Procedure, Colony/Cell Morphology, Applied Scenarios

Part 3 Taxonomy & Classification Linnaean System, Molecular Clock, Three Domains

Part 4 Gram-Negative Bacteria Proteobacteria, Enterobacteriaceae, MacConkey Agar

Part 5 Biochemical Identification Tests IMViC, Catalase, Oxidase, Rapid Methods, Flowcharts

Part 6 Gram-Positive Bacteria & Identification Firmicutes, Actinobacteria, Endospores, HBA, MSA

Appendix A Master Organism Reference Table All organisms, all features, one table

Appendix B Practice Questions & Answers 30+ exam-style questions with model answers

Appendix C Cheat Sheet Planning Guide What to prioritise on your A4 handwritten sheet

Microbiology — Ultimate Study Guide | Page 1 of 27

PART 1

BACTERIAL GROWTH

REQUIREMENTS

Understanding what bacteria need to grow is foundational. This topic covers the environmental and nutritional

factors that control microbial growth — and directly connects to your practicals on UV exposure and culture

conditions.

1.1 Temperature Classifications

Every bacterial species has three cardinal temperatures: a minimum, an optimum (fastest growth), and a

maximum. Bacteria are grouped by their optimal range.

Growth Rate

Temperature (°C)

-10 0 10 20 30 40 50 60 70 80 90 100 110

Psychrophiles

Psychrotrophs

Mesophiles

Thermophiles

Hyperthermo-

philes

37°C

(Body temp)

Classification Optimal Range Etymology Clinical/Practical Significance Examples

Psychrophiles 10–15°C (range:

-10 to 20°C)

psychro = cold

phile = love

Found in polar/deep ocean

environments. Rarely cause human

disease.

Polaromonas

Psychrotrophs 20–30°C (can

grow at 0°C)

troph = energy/

nourishment

IMPORTANT: Spoil refrigerated

food. Grow slowly at fridge temp

(4°C). Why food still goes off in the

fridge.

Listeria

monocytogenes

Mesophiles 25–40°C (range:

20-45°C)

meso = middle MOST IMPORTANT CLINICALLY.

Optimal at ~37°C = human body

temp. Most pathogens are

mesophiles.

E. coli, S. aureus,

Salmonella

Thermophiles 50–60°C (range:

40-80°C)

thermo = heat Form heat-resistant endospores.

Some are pathogens. Used in

industrial processes.

Bacillus

stearothermo-

philus

Hyperthermophil

80–110°C hyper = ultra/

excessive

Found near volcanic vents, hot

springs. Mostly Archaea. Not

clinically relevant.

Pyrolobus fumarii

Microbiology — Ultimate Study Guide | Page 2 of 27

■ Why does your lecturer emphasise mesophiles? Because the exam is about CLINICALLY relevant bacteria.

Nearly all pathogens discussed in this course (E. coli, S. aureus, Klebsiella, etc.) are mesophiles that thrive at

37°C — which is why lab incubation is at 37°C.

★ MNEMONIC: P-P-M-T-H = Please Pass Me The Hamburger (Psychrophile, Psychrotroph,

Mesophile, Thermophile, Hyperthermophile) — from cold to hot, left to right.

Practice Questions — Temperature

Q: A patient develops food poisoning from chicken that was stored in the fridge for 2 weeks. Which type

of organism likely caused this?

A: A psychrotroph — these organisms grow slowly at refrigeration temperatures (4°C) and spoil stored food.

Example: Listeria monocytogenes.

Q: Why is 37°C used as the standard incubation temperature in clinical microbiology labs?

A: Because most human pathogens are mesophiles with an optimal growth temperature around 37°C, which

matches human body temperature. Incubating at this temperature maximises growth of clinically relevant organisms.

Q: Clostridium botulinum endospores survive boiling. Does this make C. botulinum a thermophile?

A: No. C. botulinum is a mesophile (optimal growth ~37°C). Its endospores are heat-resistant, but the vegetative

cells grow at mesophilic temperatures. Don't confuse spore resistance with growth temperature classification.

1.2 Oxygen Requirements

Bacteria have dramatically different relationships with oxygen. This classification is critical for understanding which

culture conditions to use and why certain organisms cause infections in specific body sites (e.g., wound infections

deep in tissue where O2 is low).

O■

Obligate

Aerobe

O■

Obligate

Anaerobe

O■

Facultative

Anaerobe

O■

Microaero-

phile

O■

Aerotolerant

Anaerobe

Classification O2 Relationship Energy Strategy Where to Find

Them

Key Example

Obligate aerobes REQUIRE O2.

Cannot grow

without it.

Aerobic respiration

only (electron

transport chain with

O2 as final acceptor)

Surfaces, lungs,

well-oxygenated

tissues

Mycobacterium

tuberculosis,

Pseudomonas

aeruginosa

Aerotolerant

anaerobes

TOLERATE O2

but cannot use it

for energy.

Cannot reproduce

in O2.

Fermentation only

(anaerobic). Must

have anaerobic

conditions for

reproduction.

Various, can

survive brief O2

exposure

Lactobacillus

Microbiology — Ultimate Study Guide | Page 3 of 27

Classification O2 Relationship Energy Strategy Where to Find

Them

Key Example

Facultative

anaerobes

Grow WITH or

WITHOUT O2.

Most versatile.

Switch between

aerobic respiration

(more ATP) and

fermentation

Very adaptable —

GI tract, blood,

urinary tract

E. coli, S. aureus,

Salmonella

Obligate

anaerobes

O2 is TOXIC.

Cannot survive

exposure.

Anaerobic

metabolism only

(fermentation or

anaerobic

respiration)

Deep wounds, GI

tract, abscesses,

soil (deep)

Clostridium

botulinum, C.

tetani

Microaerophiles Need O2 but at

LOW levels.

Excess O2 is

toxic.

Aerobic respiration

but with limited O2

(2-10% O2 optimal)

Intestinal mucosa,

stomach lining

Campylobacter

jejuni,

Helicobacter

pylori

■ EXAM TIP: The word "facultative" means "optional" — the organism has the OPTION to use O2 or not.

"Obligate" means "required/mandatory" — they are OBLIGATED to have (or avoid) O2.

★ MNEMONIC: F.A.N. = Facultative ANaerobe — they're FANs of both environments. O.A. = Only

Aerobic or Only Anaerobic — they're locked into one.

Q: Campylobacter jejuni is a microaerophile. Explain why standard aerobic incubation (21% O2) would

NOT be appropriate for culturing this organism.

A: Microaerophiles require O2 for growth but only at low concentrations (2–10%). At atmospheric O2 levels (21%),

reactive oxygen species (superoxide, hydrogen peroxide) accumulate faster than the organism's detoxification

enzymes can handle, causing toxic damage. Campylobacter requires a special microaerobic incubation environment

(e.g., 5% O2, 10% CO2).

1.3 Nutritional Growth Factors

All bacteria need certain elements and compounds to build cellular structures and generate energy. Understanding

these explains why different media are formulated with specific ingredients.

Growth Factor Role in the Cell Why It Matters for the Exam

Carbon Backbone of ALL organic molecules.

Structural compounds (cell wall,

membranes). Primary energy source.

Autotrophs fix CO2; heterotrophs need organic carbon.

This is why media contain glucose, lactose, citrate etc.

Nitrogen Component of amino acids → proteins.

Component of nucleotides → DNA/RNA.

Nitrogen fixation by some bacteria.

Peptone in media = nitrogen source. Nitrogen cycle:

some bacteria convert N2 → NH3 → NO2 → NO3.

Alphaproteobacteria (Nitrobacter) do nitrification.

Sulphur (S) Component of amino acids (cysteine,

methionine). Essential for protein structure

(disulfide bonds). Some chemotrophs use

S for energy.

Deltaproteobacteria (Desulfovibrio) are sulphur reducers.

H2S production tested in some biochemical tests.

Phosphorus (P) In DNA and RNA (phosphodiester bonds).

In ATP (energy currency). In

phospholipids (cell membrane).

Without P, no DNA replication or energy production.

Phosphorus is why ATP = adenosine TRIphosphate.

Trace elements Minerals (Fe, Mn, Zn, Cu, Co). Act as

enzyme cofactors. Essential for enzyme

catalytic function.

Without cofactors, enzymes don't work. Fe is especially

important for pathogenesis — many pathogens compete

with host for iron.

Microbiology — Ultimate Study Guide | Page 4 of 27

Growth Factor Role in the Cell Why It Matters for the Exam

Organic growth

factors

Vitamins: assist metabolic reactions.

Amino acids: protein building blocks.

Purines & pyrimidines: DNA/RNA bases.

Some bacteria are 'fastidious' — they cannot synthesize

these and need them supplied in media. Mycoplasma

needs sterols added to media.

1.4 Effect of UV on Microbial Growth (Prac 2 Connection)

This practical tested the bactericidal effect of UV radiation on Serratia marcescens. UV light causes thymine

dimers in DNA, preventing replication. The experiment tested how lid status (closed vs. open) and exposure

duration affected bacterial survival.

★ Key experimental design points:

• All plates were inoculated with S. marcescens for lawn growth

• Single circles = CLOSED lid (UV blocked by plastic)

• Double circles = OPEN lid (UV reaches bacteria directly)

• After UV exposure, all plates incubated at 37°C (mesophile!) with O2 for 24 hours

Plate Lid UV (min) Expected

Growth

Reasoning

1 Closed 0 Full lawn Positive control — no UV, normal growth

2 Open 10 Reduced lawn UV kills some bacteria; short exposure

3 Closed 10 Full lawn Lid blocks UV; bacteria protected

4 Open 20 Sparse growth Longer UV exposure kills more bacteria

5 Open 30 Very sparse Significant UV damage to DNA

6 Open 60 Minimal/none Prolonged UV — most bacteria killed

7 Closed 60 Full lawn Even 60 min UV blocked by closed lid

■ APPLIED QUESTION ALERT: Nahar explicitly said questions will be applied. You may be given a plate result

(e.g., 'Plate shows full lawn after 60 min UV') and asked to explain why. The answer: the lid was CLOSED,

blocking UV. You may also be asked why S. marcescens was chosen — it produces a red pigment making

colonies easy to see.

Q: A student accidentally leaves the lid OPEN on plate 3 instead of closed. Predict the result and explain.

A: With the lid open, UV would directly reach the bacteria. After 10 minutes of UV exposure, you'd expect reduced

growth (similar to plate 2) rather than a full lawn. The UV causes thymine dimers in bacterial DNA, preventing

normal replication. Only bacteria that can repair this damage (via photolyase or excision repair) would survive.

Q: Why were all plates incubated at 37°C specifically?

A: Serratia marcescens is a mesophile with optimal growth near 37°C. Incubating at this temperature ensures

maximum growth of surviving bacteria, making the UV killing effect clearly visible by comparison with the controls.

Note: S. marcescens actually produces more red pigment at lower temperatures (~25°C), but 37°C is standard for

clinical lab conditions.

Microbiology — Ultimate Study Guide | Page 5 of 27

PART 2

MICROSCOPY, MORPHOLOGY &

GRAM STAIN

The Gram stain is the single most important technique in clinical microbiology. It divides bacteria into two

fundamental groups and, combined with morphological observation, gives immediate preliminary identification.

Every step has a specific purpose, and understanding WHY (not just WHAT) is critical for applied questions.

2.1 Gram Stain — Procedure & Deep Analysis

1. Smear &

Heat Fix

2. Crystal

Violet

3. Iodine

(Mordant)

4. Acetone

(Decolourise)

5. Safranin

(Counterstain)

6. Observe

Oil Immersion

GRAM POSITIVE

PURPLE / VIOLET

Thick peptidoglycan

retains crystal violet

GRAM NEGATIVE

PINK / RED

Thin peptidoglycan + outer membrane

loses CV, takes up safranin

Step Action Purpose & Mechanism What Goes Wrong If Done Incorrectly

1 Suspend colony

in sterile water

on slide

Creates a thin smear (~1.5cm diameter).

Too much = overly thick, can't see individual

cells.

Too thick: cells pile up, stain unevenly,

morphology obscured. Too little: not enough

cells to observe.

2 Heat fix on heat

block

Kills bacteria. Adheres cells to slide so they

aren't washed off by subsequent liquid

steps. Denatures proteins for stain uptake.

Under-heated: cells wash off during

staining. Over-heated: cells distort,

morphology ruined.

3 Crystal Violet

(primary stain)

Stains ALL bacteria purple/violet. Both

Gram+ and Gram- absorb the dye at this

stage.

If skipped: no initial colour to retain/lose. If

too brief: weak staining overall.

4 Iodine mordant

(after rinsing

CV)

Forms large crystal violet-iodine (CV-I)

complexes INSIDE the cells. These

complexes are too big to be easily washed

out.

If skipped: CV washes out of ALL cells

easily. CV-I complex is key to differential

staining.

5 Acetone/alcohol

(decolouriser)

THE CRITICAL STEP. Dissolves outer

membrane of Gram-neg cells → CV-I

washes out → cells become colourless.

Gram-pos thick peptidoglycan traps CV-I.

TOO LONG: Over-decolourises Gram+ →

they lose CV → look Gram-neg (FALSE

NEGATIVE). TOO SHORT: Gram-neg

retain some CV → look Gram-pos (FALSE

POSITIVE).

Microbiology — Ultimate Study Guide | Page 6 of 27

MY NOTES & CONCLUSIONS

Part 1: Bacterial Growth Requirements

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Part 1: Bacterial Growth Requirements

Step Action Purpose & Mechanism What Goes Wrong If Done Incorrectly

6 Safranin/Fuchsi

n (counterstain)

Adds pink/red colour to the now-colourless

Gram-neg cells so they're visible. Does

NOT change Gram-pos (purple stays).

If skipped: Gram-neg cells are

clear/invisible under microscope. Only

Gram-pos visible.

7 Rinse, blot dry,

oil immersion

Removes excess stain. Oil immersion

(100x) allows visualisation of individual cells

(shape, size, arrangement).

If oil immersion not used: bacteria are too

small (~1-5µm) to see clearly at lower

magnification.

■ The acetone step determines the entire result. The underlying science: Gram-negative bacteria have a thin

peptidoglycan layer + outer lipid membrane. Acetone dissolves the outer membrane, creating gaps for CV-I

to escape. Gram-positive bacteria have a thick peptidoglycan layer (up to 90% of cell wall) with no outer

membrane — the dense mesh traps CV-I inside, unless you leave acetone on too long.

Gram Stain Results Summary:

Feature Gram-Positive Gram-Negative

Final colour PURPLE / VIOLET PINK / RED

Primary stain retained? YES — CV-I trapped NO — CV-I washed out

Counterstain visible? No (masked by purple) YES — safranin gives pink

Cell wall structure Thick peptidoglycan (20-80nm, up to 90% of

wall)

Thin peptidoglycan (2-3nm) + outer membrane

with LPS

Outer membrane? ABSENT PRESENT (contains LPS/endotoxin)

Periplasmic space? Narrow or absent Present (between inner/outer membrane)

Teichoic acids? Present (in cell wall) Absent

LPS (endotoxin)? Absent Present (in outer membrane)

2.2 Colony Morphology (Macroscopic Observation)

When you look at a culture plate, you describe colonies using three features. This is the FIRST step in identification

— before any staining or biochemical tests.

Feature What You're Looking At Types Memory Aid

Form Shape when viewed from

ABOVE (bird's eye view)

Circular, Irregular, Filamentous,

Rhizoid

Think of looking DOWN at the

colony — most common is circular

Elevation Height/profile when viewed

from the SIDE

(cross-section)

Raised, Convex, Flat, Umbonate,

Crateriform

Think of slicing the colony in half —

what does the side profile look like?

Margin Edge pattern around the

colony border

Entire (smooth), Undulate (wavy),

Filiform (hair-like), Curled, Lobate

Think of zooming into the EDGE —

is it smooth or jagged?

■ Additional colony features to note: colour/pigmentation (e.g., S. marcescens = red), opacity (opaque, translucent,

transparent), texture (smooth, rough, mucoid), and odour.

2.3 Cell Morphology (Microscopic Observation)

Under oil immersion microscopy, record: (1) Gram stain result, (2) cell shape, (3) cell arrangement, and (4)

approximate size.

Microbiology — Ultimate Study Guide | Page 7 of 27

Cocci (spherical cells) — diameter 0.5–1.5 µm

Arrangement Description Clinical Significance Key Example

Single cells Individual cocci Less common as a defining arrangement Various

Chains

(Strepto-)

Cocci in a line,

end-to-end

Streptococcus = chains of cocci. This is

WHERE THE NAME COMES FROM.

Streptococcus

pyogenes

Clusters

(Staphylo-)

Grape-like

irregular

clusters

Staphylococcus = clusters of cocci.

NAME LITERALLY MEANS 'grape

cluster'.

Staphylococcus

aureus

Tetrads Groups of 4 in

a square

Less common clinically Micrococcus

Diplococci Pairs of 2 Several important pathogens are

diplococci

Neisseria

(Gram-neg!)

Lance-shaped Elongated

pairs (pointed

ends)

Distinctive of pneumococci S. pneumoniae

★ MNEMONIC: STREPto = STRIP (think of a strip/chain). STAPHylo = STAFF meeting (everyone

clusters around the table).

Bacilli (rod-shaped cells) — length typically 2–5 µm

Shape Variant Size Description Key Example

Standard rod 2–5 µm long Typical straight rod with rounded or square

ends

E. coli, Bacillus

Coccobacilli 1–2 µm Short, plump rods. EASILY MISTAKEN for

cocci. Record as 'coccobacillus' not just

'coccus'

Klebsiella

(shorter, thicker)

Endosporing 2–5 µm Rod with visible bulge where endospore

sits. Spore can be central, subterminal, or

terminal.

Clostridium,

Bacillus

Curved / vibrio 2–5 µm Comma-shaped single curve Campylobacter

(comma-shaped)

Spiral forms Up to 15 µm Helical/corkscrew with multiple turns Helicobacter

(curved + flagella)

Fusiform Variable Spindle-shaped: tapered at both ends Fusobacterium

■ TRICKY: Klebsiella pneumoniae is described as 'shorter and thicker' than other Enterobacteriaceae. On a

Gram stain, it could look like a coccobacillus. If you see short, plump Gram-negative rods — think Klebsiella.

Microbiology — Ultimate Study Guide | Page 8 of 27

PART 3

TAXONOMY & CLASSIFICATION

3.1 Linnaean System & Binomial Nomenclature

Taxonomy is the science of classifying organisms. Carolus Linnaeus created a hierarchical system that gives every

organism a unique two-part (binomial) name. While imperfect, it remains the foundation of naming conventions.

Taxonomic Hierarchy (broadest → most specific):

★ MNEMONIC: D-P-C-O-F-G-S = Dear Professor, Can Our Family Get Spaghetti? = Domain, Phylum,

Class, Order, Family, Genus, Species

Binomial Nomenclature Rules:

Rule Correct Example Incorrect Example Why It Matters

Genus capitalised,

species lowercase

Staphylococcus aureus staphylococcus Aureus Standardised naming across

all biology

Italicised (typed) or

underlined

(handwritten)

Escherichia coli Escherichia coli (no format) Shows it's a scientific

binomial name

Genus can be

abbreviated after

first mention

S. aureus Staph. aureus Only the standard

abbreviation is correct

Species needs

genus (never

standalone)

E. coli coli "coli" alone is meaningless

— many genera could have

■ EXAM TIP: In the exam, if you write bacterial names, underline them since you're handwriting. 'S. aureus'

with underlining is correct. Without underlining, you may lose marks for nomenclature errors.

3.2 Five Limitations of the Linnaean System

# Limitation Explanation Microbiology Impact

1 Look alike ≠

Coevolution: organisms in the same

environment develop similar

structures independently

(convergent evolution).

Two bacteria may look identical under

microscope but be genetically very

different.

2 Look different ≠

unrelated

Related organisms can diverge in

appearance over time (divergent

evolution).

Bacillus (rod) and Staphylococcus

(cocci) are BOTH in Order Bacillale!

3 Relied on visibility Microorganisms were invisible to

Linnaeus. Entire domains of life

were unknown.

Bacteria and Archaea were entirely

missed in the original system.

4 Limited capacity Only accommodated ~10,000

species with 5 hierarchical levels.

We now know millions of microbial

species. Need many more levels

(domain, phylum, etc.)

5 Totally linear Assumed organisms evolved in

neat, branching lines (vertical

descent).

Bacteria do horizontal gene transfer —

genes jump between unrelated species.

This breaks a purely linear tree.

Microbiology — Ultimate Study Guide | Page 9 of 27

MY NOTES & CONCLUSIONS

Part 2: Microscopy, Morphology & Gram Stain

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Part 2: Microscopy, Morphology & Gram Stain

3.3 Ribosomal rRNA as a Molecular Clock

Carl Woese revolutionised classification by comparing ribosomal RNA sequences. Here's why this works:

• Ribosomes are essential for ALL life — every cell needs them to make proteins

• The genes encoding rRNA are therefore highly conserved (mutations are usually lethal)

• Over billions of years, slow mutations accumulate in rRNA genes

• By measuring the number of differences in rRNA between two organisms and dividing by estimated time,

you get a mutation rate = molecular clock

• More differences = more distantly related (more time since common ancestor)

• Prokaryotes use 16S rRNA; eukaryotes use 18S rRNA for this analysis

Central Dogma connection: DNA (chromosome) → transcription → mRNA → ribosome → translation →

protein. The ribosome is where mRNA is read and tRNA brings amino acids. Without ribosomes, no proteins =

no life.

3.4 The Three-Domain System

Three Domain System

ARCHAEA

Prokaryotic

No peptidoglycan

Ether-linked lipids

Branched chains

Met (1st AA)

No antibiotic sens.

BACTERIA

Prokaryotic

Has peptidoglycan

Ester-linked lipids

Straight chains

fMet (1st AA)

Antibiotic sensitive

EUKARYA

Eukaryotic

No peptidoglycan

Ester-linked lipids

Straight chains

Met (1st AA)

No antibiotic sens.

Feature Archaea Bacteria Eukarya

Cell type Prokaryotic Prokaryotic Eukaryotic

Cell wall Varies

(pseudopeptidoglycan,

polysaccharides, protein)

NO peptidoglycan

Contains PEPTIDOGLYCAN

(unique to Bacteria — target of

antibiotics)

Varies (cellulose, chitin,

none) Contains

carbohydrates

Membrane lipids BRANCHED chains

attached by ETHER linkage

STRAIGHT chains attached by

ESTER linkage

STRAIGHT chains attached

by ESTER linkage

1st amino acid in

protein

synthesis

Methionine (Met) Formylmethionine (fMet) (UNIQUE

to bacteria)

Methionine (Met)

Antibiotic

sensitivity

NO YES (peptidoglycan + fMet are

antibiotic targets)

rRNA loop* Lacking Present Lacking

Common arm of

tRNA†

Lacking Present Present

*rRNA loop binds to ribosomal protein; found in all bacteria. †Common tRNA arm sequence (guanine-thymine-pseudouridine-cytosine-guanine) found in

bacteria and eukaryotes.

Microbiology — Ultimate Study Guide | Page 10 of 27

★ HIGH-YIELD for matching questions:

• ONLY Bacteria have peptidoglycan → why antibiotics like penicillin target bacteria but not archaea or human

cells

• ONLY Bacteria use formylmethionine → another antibiotic target

• Archaea have ETHER-linked lipids (unique) while Bacteria and Eukarya both have ESTER-linked

• Archaea LACK both rRNA loop AND common tRNA arm — most unlike bacteria

Microbiology — Ultimate Study Guide | Page 11 of 27

PART 4

GRAM-NEGATIVE BACTERIA —

DIVERSITY & ID

Gram-negative bacteria are classified primarily within the phylum Proteobacteria, which is divided into five classes

(Alpha through Epsilon). The most clinically important class is Gammaproteobacteria, which includes the family

Enterobacteriaceae.

4.1 Proteobacteria — Five Classes

Class Environment Key Features Important

Genera

Exam-Relevant Facts

Alpha Nutrient-POO

(oligotrophic)

Some fix nitrogen

Some convert N

→ nitrites Tend to

be small

Agrobacterium

Nitrobacter

Nitrosomonas

Alpha = 'A' = 'A little' nutrients (poor

environment) Nitrifying bacteria

Beta Nutrient-RIC

H (eutrophic)

Opposite of

Alpha! Rich in

organics, Fe, S,

Neisseria

Bordetella

Beta = 'B' = 'Big' nutrients Neisseria

= diplococci (Gram-neg) Bordetella

= whooping cough

Gamma Diverse

habitats

MOST DIVERSE

Greatest medical

& economic

significance

Serratia, E. coli

Klebsiella

Enterobacter

Pseudomonas

This is where most of your practical

organisms live! Includes

Enterobacteriaceae

Delta Anaerobic

habitats

Sulphur reducers

Predators of

bacteria Biofilm

formers

Desulfovibrio

Myxococcus

Bdellovibrio

Bdellovibrio is a PREDATOR of

other bacteria — unique!

Myxococcus forms biofilms

Epsilon GI tracts (low

O2)

Microaerophilic

Curved/spiral GI

pathogens

Campylobacter

Helicobacter

Both are microaerophiles!

Campylobacter = food poisoning H.

pylori = stomach ulcers/cancer

★ MNEMONIC: A-B-G-D-E = Always Buy Gamma Delta Epsilon — Gamma is the 'most valuable'

(most diverse/important clinically)

4.2 Key Gammaproteobacteria — Organism Identity Cards

These are the organisms you've worked with in practicals. KNOW them inside out.

Organism Gra

Shape Motile

Oxidas

Lactose

Ferment

er?

Distinguishing Features

Serratia

marcescens

− Rod Yes − Yes RED PIGMENT Catalase + Prac 2: UV

experiment organism

Escherichia coli − Rod Yes − Yes Formic acid usage No citrate use Indole

+ | MR + | VP − | Citrate −

Klebsiella

pneumoniae

− Short/

thick rod

NO − Yes NON-MOTILE (unusual for Enterobact.)

Shorter and thicker than others Large

mucoid colonies

Microbiology — Ultimate Study Guide | Page 12 of 27

MY NOTES & CONCLUSIONS

Part 3: Taxonomy & Classification

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Part 3: Taxonomy & Classification

Organism Gra

Shape Motile

Oxidas

Lactose

Ferment

er?

Distinguishing Features

Enterobacter

aerogenes

− Rod Yes − Yes VP + | MR − | Citrate + OFTEN confused

with E. coli (IMViC pattern distinguishes

them)

Pseudomonas

aeruginosa

− Rod Yes + No OXIDASE POSITIVE = NOT Enterobact.

Blue-green pigment (pyocyanin)

Grape-like odour Opportunistic pathogen

■ Pseudomonas aeruginosa is the ODD ONE OUT. It is Oxidase + while ALL Enterobacteriaceae are Oxidase

−. This is the FIRST test to separate them. If oxidase is positive, it's NOT Enterobacteriaceae.

4.3 Enterobacteriaceae Identification — Decision Flowchart

Unknown Gram-Negative Rod

Catalase +, Oxidase -, Fac. Anaerobe

Oxidase Test

(Are they Enterobacteriaceae?)

Oxidase +

P. aeruginosa

Oxidase - (Enterobact.)

MacConkey Agar

Lactose +

Pink colonies

Lactose -

Colourless

Escherichia

Klebsiella

Enterobacter

Serratia

Salmonella

Shigella

Yersinia

Proteus

Further Differentiation: IMViC + Other Tests

ONPG: beta-galactosidase (yellow = +)

Citrate: Simmons agar (blue = +)

Methyl Red: mixed acid (red = +)

VP: butanediol/acetoin (red = +)

Indole: tryptophanase (red ring = +)

Urease: urea breakdown (pink = +)

Nitrate: NO3 reduction (red = +)

Motility: E. coli = motile

Klebsiella = non-motile

Shared traits of ALL Enterobacteriaceae (cannot differentiate within family):

• All Catalase POSITIVE

• All Oxidase NEGATIVE

• All Facultative anaerobes

• All express ECA (Enterobacteriaceae Common Antigen)

■ EXAM TIP: If a question asks 'Which of the following tests would NOT help differentiate between E. coli and

Klebsiella?' — the answer is catalase or oxidase (both are the same for all Enterobacteriaceae).

4.4 MacConkey Agar (MAC) — Deep Dive

MacConkey agar is BOTH selective AND differential — understand what each term means:

Microbiology — Ultimate Study Guide | Page 13 of 27

Property How It Works What It Shows

SELECTIVE Contains bile salts and crystal violet which

inhibit Gram-positive growth. ONLY

Gram-negative bacteria can grow.

If you see growth on MAC, the organism is

Gram-negative. If no growth, it may be

Gram-positive.

DIFFERENTIA

Contains lactose + neutral red indicator.

Lactose fermenters produce acid → neutral

red turns PINK/RED at low pH.

Non-fermenters don't produce acid.

Pink/red colonies = lactose fermenter

(Escherichia, Klebsiella, Enterobacter, Serratia)

Colourless colonies = non-fermenter

(Salmonella, Shigella, Yersinia, Proteus)

★ MNEMONIC: Lactose fermenters on MAC = EEKS! = Escherichia, Enterobacter, Klebsiella,

Serratia

4.5 Epsilonproteobacteria — Campylobacter & Helicobacter

Feature Campylobacter (C. jejuni) Helicobacter (H. pylori)

Shape Comma-shaped (single curve) Curved rods with MULTIPLE flagella

O2 requirement Microaerophilic Microaerophilic

Habitat Intestinal tracts; contaminates water, soil, meat,

milk

Stomach lining (survives acid by producing

urease → ammonia)

Disease Main cause of bacterial food poisoning

Cramping + diarrhoea

Most common cause of stomach ulcers Can

cause stomach cancer

Transmission Contaminated food/water (especially poultry) Person-to-person (oral-oral, faecal-oral)

Microbiology — Ultimate Study Guide | Page 14 of 27

PART 5

BIOCHEMICAL IDENTIFICATION

TESTS

These tests are the core of your practicals. For each test, understand: (1) what enzyme/pathway it detects, (2)

what reagent/medium is used, (3) what a positive result looks like and why, (4) what a negative result looks like

and why, and (5) which organisms give which result.

Biochemical Test Results - Colour Guide

Catalase Bubbles No bubbles +: O2 bubbles

Oxidase Blue/Purple No change +: P. aeruginosa

ONPG Yellow Colourless +: E. coli

Citrate Blue Green +: Enterobacter

Methyl Red Red Yellow +: E. coli

VP Red/Pink No colour +: Enterobacter

Indole Red ring No ring +: E. coli

Urease Pink Yellow +: Proteus

Nitrate Red No colour* *then add zinc

POSITIVE NEGATIVE KEY ORGANISM

5.1 ONPG Test (Beta-Galactosidase Detection)

Aspect Detail

What it tests Presence of beta-galactosidase enzyme (involved in lactose fermentation)

How it works ONPG disk contains ortho-nitrophenyl-b-D-galactopyranose (synthetic galactoside).

Beta-galactosidase cleaves it just like lactose. The released ortho-nitrophenol is YELLOW in

alkaline solution.

Positive result YELLOW solution = organism has beta-galactosidase = lactose fermenter

Negative result COLOURLESS = no beta-galactosidase = non-fermenter

Key organisms + : E. coli | − : Proteus mirabilis

Why not just use

MAC?

MAC shows fermentation by colony colour but ONPG specifically confirms the enzyme. Some 'late

lactose fermenters' may look negative on MAC but positive on ONPG.

5.2 Citrate Utilisation Test

Aspect Detail

What it tests Can the bacterium use CITRATE as its sole carbon source?

Medium Simmons citrate agar slant — contains sodium citrate + bromothymol blue indicator

Enzyme Citrase cleaves citrate into smaller carbon molecules for metabolism

Microbiology — Ultimate Study Guide | Page 15 of 27

MY NOTES & CONCLUSIONS

Part 4: Gram-Negative Bacteria

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Part 4: Gram-Negative Bacteria

Aspect Detail

How colour

changes

When citrate is metabolised, the by-products make the medium ALKALINE. Bromothymol blue:

GREEN at acidic/neutral pH → BLUE at alkaline pH

Positive result BLUE slant = citrate used as carbon source

Negative result GREEN slant (no change) = citrate NOT used

Key organisms + : Enterobacter, Klebsiella | − : E. coli

■ EXAM TIP: E. coli is CITRATE NEGATIVE. This is a key differentiator from Enterobacter and Klebsiella (both

citrate positive). If the exam gives you 'citrate +' in a scenario, rule out E. coli immediately.

5.3 Methyl Red (MR) & Voges-Proskauer (VP) Tests

These two tests use the SAME broth (MRVP broth with peptone, glucose, buffer) but test for different fermentation

pathways. They are ALWAYS interpreted together.

Glucose Fermentation: MR vs VP Pathways

GLUCOSE

MIXED ACID PRODUCTS

Lactate, Acetate, Formate

BUTANEDIOL (neutral)

Acetoin (AMC)

METHYL RED = RED (+)

VP = Negative

VP = RED (+)

Methyl Red = Negative

E. coli Enterobacter

Methyl Red Test Voges-Proskauer Test

Pathway

detected

Mixed acid fermentation → lactate, acetate,

formate, succinate

Butanediol pathway → acetoin (acetyl methyl

carbinol)

Reagent added Methyl red pH indicator Alpha-naphthol + KOH (Barritt's reagents)

Positive = ? RED = stable acid end-products overwhelm the

buffer in medium

RED / dark pink = acetoin reacts with Barritt's

reagents

Negative = ? Yellow/orange = acids consumed or neutral

products formed

No colour change = no acetoin

E. coli MR POSITIVE VP NEGATIVE

Enterobacter MR NEGATIVE VP POSITIVE

★ Critical concept: MR and VP are generally OPPOSITE results. If an organism makes mixed acids (MR+), it's

NOT making butanediol (VP−), and vice versa. There are rare exceptions, but for this exam: MR+ = VP− and

MR− = VP+

5.4 Catalase & Oxidase Tests

Catalase Test Oxidase Test

Enzyme detected Catalase Cytochrome c oxidase

Microbiology — Ultimate Study Guide | Page 16 of 27

Catalase Test Oxidase Test

Reaction H2O2 → H2O + O2 (hydrogen peroxide broken

down)

Electron transfer from donor to acceptor (part of

electron transport chain)

Reagent 3% hydrogen peroxide (H2O2) Phenylenediamine dye (artificial electron donor)

on filter paper

Positive = ? BUBBLES (O2 gas produced) BLUE / PURPLE (dye is reduced)

Negative = ? No bubbles No colour change

Clinical use Differentiates: Staphylococcus (cat +) vs

Streptococcus (cat −)

THE KEY GATEKEEPER TEST: Oxidase + =

NOT Enterobact. (e.g., Pseudomonas) Oxidase −

= Enterobacteriaceae

5.5 Indole Test

• Enzyme: Tryptophanase

• Reaction: Tryptophan → ammonia + pyruvate + indole

• Reagent: Kovac's reagent (reacts with indole)

• Positive: RED RING at surface of broth (indole + Kovac's)

• Negative: No red ring / yellow layer

• Key result: E. coli = Indole + | Enterobacter = Indole −

5.6 Urease Test

• Enzyme: Urease

• Reaction: Urea → ammonia (alkaline) + CO2

• Indicator: Phenol red (yellow at acidic pH, PINK at alkaline pH)

• Positive: PINK / RED = urease breaks down urea, ammonia raises pH

• Negative: YELLOW = no urease, urea remains intact (acidic)

• Key result: Proteus mirabilis = Urease + | E. coli = Urease −

■ H. pylori also produces urease — this is how it survives in stomach acid! The ammonia neutralises acid in its

immediate environment.

5.7 Nitrate Reduction Test

Microbiology — Ultimate Study Guide | Page 17 of 27

Nitrate Reduction Test - Decision Flowchart

Add Reagents A + B

to nitrate broth

RED colour

POSITIVE-1: NO3 → NO2

NO colour

Add ZINC DUST

No colour + GAS

POSITIVE-2: NO3 → N2

Turns RED

TRUE NEGATIVE

Why is this test confusing? Because 'no colour' after reagents A+B has TWO possible meanings:

• The bacteria reduced nitrate all the way past nitrite to N2 gas (Positive-2) — there's no nitrite left for the

reagents to detect, but gas is in the Durham tube

• The bacteria did nothing and nitrate is still sitting there unchanged (True Negative)

• The zinc dust step resolves this: zinc chemically reduces any remaining nitrate to nitrite. If zinc produces a

red colour, the nitrate was still there = the bacteria didn't reduce it = TRUE NEGATIVE

5.8 IMViC Pattern Summary

The IMViC tests (Indole, Methyl Red, Voges-Proskauer, Citrate) are the GOLD STANDARD for differentiating

common Enterobacteriaceae.

Organism I M V C IMViC Pattern Memory Aid

E. coli + + − − + + − − E. coli is the 'mixed acid champion'

Enterobacter

aerogenes

− − + + − − + + Exact OPPOSITE of E. coli!

Klebsiella

pneumoniae

− − + + − − + + Same as Enterobacter. Differentiate by

motility (Kleb = non-motile)

★ MEMORISE THIS: E. coli = + + − − and Enterobacter/Klebsiella = − − + +. They are exact mirrors. To tell

Enterobacter from Klebsiella: Klebsiella is NON-MOTILE (and shorter/thicker rods).

5.9 Rapid Identification Methods

Method Principle Speed What You Identified in Lab

MicroBact™ 12A

strip

24 miniaturised biochemical tests in

wells. Colour changes read against

chart. Results generate numerical profile

matched to database.

18–24 hours

(incubation

needed)

Klebsiella aerogenes Escherichia

coli

RapID™ ONE 18 cavities with dried substrates. Add

bacterial suspension → incubate 4h.

Colour changes compared to colour

guide.

4 hours

(much faster)

Oxidase-negative Gram-negative

rods

Microbiology — Ultimate Study Guide | Page 18 of 27

Method Principle Speed What You Identified in Lab

VITEK 2 FULLY AUTOMATED

Fluorescence-based monitoring of test

cards. Broth Microdilution MIC

technique. Continuously monitors

growth.

4–18 hours

(automated)

Identification + Antibiotic

Susceptibility Testing (AST) MIC

= Minimum Inhibitory

Concentration

MALDI-TOF MS Sample mixed with matrix on target

plate. Laser ionises molecules → ions fly

through tube. Separated by

mass-to-charge ratio (m/z). Peptide

Mass Fingerprint matched to database.

Minutes

(fastest!)

Identifies species by unique

protein 'fingerprint'. Gold

standard in modern clinical labs.

■ EXAM TIP: For MALDI-TOF: remember the acronym — Matrix-Assisted Laser Desorption Ionisation - Time Of

Flight. The key concept: smaller ions fly faster through the tube, so ions separate by mass. The pattern of

masses = fingerprint = species ID.

Microbiology — Ultimate Study Guide | Page 19 of 27

PART 6

GRAM-POSITIVE BACTERIA &

IDENTIFICATION

Gram-positive bacteria are divided into two major phyla based on their DNA base composition: Firmicutes (low

G+C content) and Actinobacteria (high G+C, >60%). This is a MOLECULAR classification that overrides

morphology.

6.1 G+C Content Classification

Gram-Positive Classification by G+C Content

Low G+C High G+C (>60%)

FIRMICUTES

Clostridium (anaerobe, spores, botulism)

Bacillus (aerobe, spores, food poisoning)

Staphylococcus (cocci! fac. anaerobe, skin)

Streptococcus (cocci! fac. anaerobe, throat)

Lactobacillus (fac. anaerobe, fermentation)

Mycoplasma (NO cell wall, pleomorphic)

ACTINOBACTERIA

Corynebacterium (rod, aerobe, cattle)

Mycobacterium (rod, obl. aerobe, TB/leprosy)

Streptomyces (rod, obl. aerobe, antibiotics!)

>700 species = largest bacterial genus

Produces: streptomycin, rapamycin,

tetracyclines

6.2 Phylum Firmicutes (Low G+C) — Complete Analysis

Order Genus Shape O2 Spores

Key

Disease/Use

Critical Exam Points

Clostridiale Clostridium

botulinum

Rod Obligate

anaerob

YES Botulism

(Bo-tox)

Neurotoxin

prevents muscle

contraction

Anaerobic wound

infections. Endospores

survive cooking. Toxin =

most potent known.

Bacillale Bacillus

subtilis

Rod Obligate

aerobe

YES Food poisoning

Genetic

engineering

Model organism.

Endospore = heat

resistant.

Bacillale Staphylococ

cus

COCCI Facultati

ve anaer

obe

No Food poisoning,

skin infections,

opportunistic

ORDER BACILLALE BUT

COCCI SHAPED! Covers

skin (normal flora).

Clusters under microscope.

Lactobacill

ale

Lactobacillu

Rod Facultati

ve anaer

obe

No Food

fermentation

(yogurt, cheese)

Probiotic. Non-spore

forming.

Lactobacill

ale

Streptococc

COCCI Facultati

ve anaer

obe

No Tooth decay,

abscesses, sore

throat

Chains under microscope.

Haemolysis on blood agar.

■ The BIGGEST TRAP in this topic: Staphylococcus is in Order Bacillale but is COCCI-shaped, not

rod-shaped! The order name does not dictate cell morphology. Similarly, Streptococcus is in Lactobacillale but

is cocci, not bacilli. ALWAYS check the actual shape, don't assume from the order name.

Microbiology — Ultimate Study Guide | Page 20 of 27

MY NOTES & CONCLUSIONS

Part 5: Biochemical Identification Tests

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Part 5: Biochemical Identification Tests

6.3 Mycoplasmatales — The Exception to Every Rule

Mycoplasma is the most unusual bacterium you'll encounter. It breaks many 'rules' of bacteriology.

Feature Mycoplasma Why It Matters

Cell wall NONE — completely absent Antibiotics targeting cell wall (penicillin,

cephalosporins) are USELESS against

Mycoplasma

Gram stain USELESS — cannot classify

as + or − without a cell wall

The Gram stain relies on cell wall structure to

differentiate. No wall = no result.

Shape Pleomorphic (highly variable) Without a rigid cell wall, the cell takes on whatever

shape the environment imposes.

Size 0.1–0.25 µm (smallest living

cell)

Once confused with viruses due to tiny size. But

has both DNA AND RNA (viruses have one or

other).

Colony

appearance

FRIED EGG appearance Very distinctive! Central dense core with thin

spreading periphery.

Growth

medium

MUST contain sterols Mycoplasma cannot synthesize sterols. Without

sterols, the membrane is unstable.

Lifestyle Obligate intracellular parasites Cannot survive independently for long. Small

genome from evolutionary gene loss.

★ Mycoplasma is a VERY likely exam question because it's so unique. Remember 6 key facts: No cell wall,

Gram stain useless, pleomorphic, smallest living cell, fried egg colonies, needs sterols in media.

6.4 Phylum Actinobacteria (High G+C)

Genus Shape O2 Key Disease/Use Exam Points

Corynebacteriu

Rod Aerobe C. renale: kidney infection in

cattle

Rod-shaped. 'Coryne' =

club-shaped.

Mycobacterium Rod Obligate

aerobe

Tuberculosis (M. tuberculosis)

Leprosy (M. leprae)

Waxy cell wall (mycolic acids).

Acid-fast staining (not Gram

stain). Very slow growing.

Streptomyces Rod (fila

mentous

)

Obligate

aerobe

Produces MANY antibiotics:

Streptomycin, Rapamycin,

Tetracyclines

LARGEST bacterial genus (>700

species). Soil organisms. Earthy

smell of soil = Streptomyces!

6.5 Endospores — Survival Structures

Endospores are NOT reproductive structures — one cell makes one spore to survive harsh conditions. They are

the most resistant biological structures known.

Aspect Detail

What they are Dormant, dehydrated structures with thick protective coats (dipicolinic acid + calcium)

Who makes them Mainly Clostridium and Bacillus (both Firmicutes)

What they resist High temperatures, UV radiation, desiccation, chemical disinfectants, extreme pH

Why they matter

clinically

Spores can survive standard cooking/cleaning → food poisoning (B. cereus, C. perfringens)

Autoclaving (121°C, 15 min) is needed to kill them — boiling (100°C) is NOT enough

Microbiology — Ultimate Study Guide | Page 21 of 27

Aspect Detail

Endospore stain Green endospores inside red/pink vegetative cells (Schaeffer-Fulton stain)

Spore positions Central, subterminal, or terminal (position within the rod can help identify species)

6.6 Gram-Positive Identification Tests

Horse Blood Agar (HBA) — Haemolysis Patterns

Haemolysis Patterns on Horse Blood Agar (HBA)

Alpha (α)

PARTIAL breakdown

Green zone

e.g. S. pneumoniae

Beta (β)

COMPLETE breakdown

Clear zone

e.g. S. pyogenes

Gamma (γ)

NO haemolysis

No change

e.g. Enterococcus

HBA is BOTH differential (shows haemolysis patterns) AND enriched (blood provides nutrients that encourage

growth of most bacteria including fastidious ones).

Coagulase Test

Differentiates Staphylococcus species into two clinically important groups:

• Coagulase POSITIVE: S. aureus — clot forms (slide test: clumping; tube test: clot in tube)

• Coagulase NEGATIVE: S. epidermidis, S. saprophyticus — no clot

■ Coagulase converts fibrinogen to fibrin, forming a clot. S. aureus uses this as a virulence factor to 'hide' from the

immune system inside a fibrin shield.

Mannitol Salt Agar (MSA)

MSA is BOTH selective (high salt inhibits most bacteria except Staphylococci) AND differential (mannitol

fermentation changes colour).

Organism Growth? Mannitol

Fermentation?

Colony Colour Media Colour

S. aureus YES YES (produces acid) YELLOW YELLOW (pH drops: 8.4 →

6.8)

Coag-neg Staph YES NO RED / small Stays RED/pink

Most other bacteria NO (killed

by salt)

N/A No growth Stays RED/pink

■ The indicator in MSA is phenol red: red/pink at pH 8.4 (alkaline) and yellow at pH 6.8 (acidic). S. aureus ferments

mannitol → acid → drops pH → yellow.

Oxidation/Fermentation (O/F) Test

Tests whether bacteria break down glucose by oxidation (aerobic), fermentation (anaerobic), both, or neither.

• Two tubes inoculated: one OPEN (aerobic) and one sealed with oil (anaerobic)

Microbiology — Ultimate Study Guide | Page 22 of 27

• O+/F+ (both yellow) = fermentative (can do both)

• O+/F− (open yellow, sealed green) = oxidative only

• O−/F− (both green) = non-saccharolytic (doesn't use glucose)

DNase Test

• Tests for DNase enzyme that hydrolyses (breaks down) DNA

• Clear zone around bacterial growth = POSITIVE (DNA hydrolysed)

• No clear zone = NEGATIVE (DNA intact)

• S. aureus is typically DNase positive

Microbiology — Ultimate Study Guide | Page 23 of 27

PART A

MASTER ORGANISM REFERENCE

TABLE

This is your one-stop reference. Every organism from the course, with all testable features in one table.

Organism Gram Shape O2 Motilit

Cat/Ox Lactos

IMViC Key ID Features

E. coli Gram − Rod Fac. ana

erobe

Motile Cat + / Ox

−

Lactos

e +

I+ M+ V−

C−

Formic acid, no citrate

Klebsiella

pneumoniae

Gram − Short/thi

ck rod

Fac. ana

erobe

Non-m

otile

Cat + / Ox

−

Lactos

e +

I− M− V+

C+

Mucoid colonies

Enterobacter

aerogenes

Gram − Rod Fac. ana

erobe

Motile Cat + / Ox

−

Lactos

e +

I− M− V+

C+

Same IMViC as Kleb

but MOTILE

Serratia

marcescens

Gram − Rod Fac. ana

erobe

Motile Cat + / Ox

−

Lactos

e +

Variable RED PIGMENT

Pseudomona

s aeruginosa

Gram − Rod Obligate

aerobe

Motile Cat + / Ox

Lactos

e −

N/A OXIDASE +

Blue-green pigment

Salmonella Gram − Rod Fac. ana

erobe

Motile Cat + / Ox

−

Lactos

e −

Variable Non-fermenter on

MAC

Campylobact

er jejuni

Gram − Comma Microaer

ophile

Motile Cat + / Ox

N/A N/A #1 bacterial food

poisoning

H. pylori Gram − Curved

rod

flagella

Microaer

ophile

Motile Urease + N/A N/A Stomach ulcers/cancer

S. aureus Gram + Cocci

clusters

Fac. ana

erobe

Non-m

otile

Cat + / Ox

−

N/A N/A Coag +, MSA yellow

DNase +, Beta

haemolysis

Streptococcu

Gram + Cocci

chains

Fac. ana

erobe

Non-m

otile

Cat − / Ox

−

N/A N/A Alpha/beta/gamma

haemolysis

Bacillus

subtilis

Gram + Rod Obligate

aerobe

Motile Cat + N/A N/A Endospore forming

C. botulinum Gram + Rod Obligate

anaerob

Motile Cat − N/A N/A Endospore forming

Neurotoxin (botulism)

Mycoplasma N/A Pleomor

phic

Variable Non-m

otile

N/A N/A N/A No cell wall! Fried egg

colonies Needs sterols

Microbiology — Ultimate Study Guide | Page 24 of 27

MY NOTES & CONCLUSIONS

Part 6: Gram-Positive Bacteria & Identification

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Part 6: Gram-Positive Bacteria & Identification

PART B

PRACTICE QUESTIONS & MODEL

ANSWERS

These questions are designed to mimic the applied style Nahar has indicated. The extended short answer

questions especially require you to EXPLAIN mechanisms, not just state facts.

MULTIPLE CHOICE PRACTICE

Q: 1. Which of the following organisms is Oxidase POSITIVE?

(a) E. coli (b) Klebsiella (c) Pseudomonas aeruginosa (d) Enterobacter

A: (c) Pseudomonas aeruginosa. All Enterobacteriaceae are Oxidase negative. P. aeruginosa is the key Oxidase +

organism you need to know.

Q: 2. MacConkey agar is described as selective. What does this mean?

(a) It differentiates lactose fermenters (b) It only allows Gram-negative bacteria to grow (c) It contains antibiotics (d)

It kills all bacteria except E. coli

A: (b) The bile salts and crystal violet in MAC inhibit Gram-positive bacteria, allowing only Gram-negative organisms

to grow. This is the selective function. (Note: differentiating lactose fermenters is the DIFFERENTIAL function.)

Q: 3. A Gram stain shows purple cocci in grape-like clusters. What is the most likely genus?

(a) Streptococcus (b) Neisseria (c) Staphylococcus (d) Enterococcus

A: (c) Staphylococcus. Purple = Gram-positive. Cocci in clusters = 'staphylo' arrangement. The name literally means

'grape cluster of spheres.'

Q: 4. Which of the following is NOT a shared trait of all Enterobacteriaceae?

(a) Catalase positive (b) Oxidase negative (c) Lactose fermenter (d) Facultative anaerobe

A: (c) Not all Enterobacteriaceae ferment lactose. Salmonella, Shigella, Yersinia, and Proteus are non-fermenters.

However, all are catalase +, oxidase −, and facultative anaerobes.

Q: 5. Which organism lacks a cell wall and cannot be Gram stained?

(a) Mycobacterium (b) Streptomyces (c) Mycoplasma (d) Bacillus

A: (c) Mycoplasma. It lacks a cell wall entirely, making it pleomorphic and immune to cell wall-targeting antibiotics

(penicillin). Mycobacterium HAS a cell wall (waxy mycolic acids) but uses acid-fast staining instead of Gram.

Q: 6. A citrate utilisation test on Simmons agar turns BLUE. This indicates:

(a) The organism cannot use citrate (b) The organism used citrate, raising pH (c) Acid production (d) Lactose

fermentation

A: (b) Blue = alkaline pH = citrate was metabolised. Bromothymol blue is the indicator (green at neutral/acidic, blue

at alkaline). The by-products of citrate metabolism raise the pH.

Q: 7. In the nitrate reduction test, reagents A+B produce no colour. Zinc dust is then added and the

solution turns RED. This means:

(a) Positive-1: nitrate reduced to nitrite (b) Positive-2: nitrate reduced to N2 (c) True negative: no nitrate reduction

occurred (d) Invalid test

A: (c) True negative. When A+B gave no colour, it could mean nitrate was either fully reduced past nitrite

(Positive-2) or not reduced at all. Adding zinc chemically reduces any remaining nitrate to nitrite. The red colour

confirms nitrate was still present = bacteria did NOT reduce it.

Microbiology — Ultimate Study Guide | Page 25 of 27

MY NOTES & CONCLUSIONS

Appendix A: Master Organism Reference Table

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Appendix A: Master Organism Reference Table

Q: 8. S. aureus on Mannitol Salt Agar appears as yellow colonies on yellow agar. Why?

(a) S. aureus produces yellow pigment (b) S. aureus ferments mannitol, producing acid that changes phenol red to

yellow (c) The salt concentration causes colour change (d) S. aureus produces urease

A: (b) S. aureus ferments mannitol → acid production → pH drops from 8.4 to 6.8 → phenol red indicator changes

from red to yellow. This is the DIFFERENTIAL function of MSA.

EXTENDED SHORT ANSWER PRACTICE

These mirror the 3 extended short answer questions in your exam. Practice writing concise but complete answers.

Q: Extended Q1: You are given an unknown Gram-negative rod isolated from a patient's urine sample.

Describe a systematic approach to identify this organism, including at least 4 specific tests and the

expected results for E. coli.

A: Step 1: Confirm Gram stain result — Gram-negative rod. Step 2: Oxidase test — E. coli is oxidase NEGATIVE

(confirms Enterobacteriaceae). If positive, consider Pseudomonas. Step 3: MacConkey agar — E. coli is a lactose

fermenter → PINK colonies on MAC. Step 4: IMViC tests — E. coli pattern is I+ M+ V− C− (Indole positive, Methyl

Red positive, VP negative, Citrate negative). Step 5: Motility test — E. coli is MOTILE (distinguishes from non-motile

Klebsiella which shares some IMViC results). Additional: ONPG test would be positive (has beta-galactosidase),

confirming lactose fermentation ability.

Q: Extended Q2: Explain why Mycoplasma cannot be identified using Gram staining. Describe 3 unique

characteristics of Mycoplasma and explain why antibiotics like penicillin would be ineffective.

A: Mycoplasma LACKS A CELL WALL entirely. The Gram stain works by differentiating bacteria based on cell wall

structure — Gram-positive bacteria retain crystal violet due to thick peptidoglycan, while Gram-negative cells lose it

through their thin peptidoglycan layer. Without any cell wall, Mycoplasma cannot retain or lose the crystal violet in a

meaningful way, making Gram staining useless. 3 Unique characteristics: (1) PLEOMORPHIC shape — without a

rigid cell wall, the cell membrane alone determines shape, which varies constantly. (2) FRIED EGG colony

appearance — distinctive morphology with dense centre and thin periphery, colonies are tiny (0.1–0.25 µm). (3)

Requires STEROLS in growth media — Mycoplasma cannot synthesize sterols, which other bacteria don't need

because their cell wall provides structural support. Penicillin is ineffective because it works by inhibiting

peptidoglycan synthesis (specifically, it blocks transpeptidase enzymes that cross-link peptidoglycan chains). Since

Mycoplasma has NO peptidoglycan (no cell wall at all), penicillin has no target.

Q: Extended Q3: Compare and contrast the Methyl Red and Voges-Proskauer tests. Include: what they

test for, what medium is used, what reagents are added, and how to interpret the results. Give an

example organism for each positive result.

A: Both tests use the same medium: MRVP broth (contains peptone, glucose, and buffer). Both test how bacteria

ferment glucose, but they detect DIFFERENT end products from two alternative pathways. METHYL RED TEST:

Tests for mixed acid fermentation products (lactate, acetate, formate). After incubation, the pH indicator methyl red

is added. If stable acid end products are present, they overcome the buffer and maintain an acidic pH → methyl red

stays RED = POSITIVE. Example: E. coli (MR+). If the organism produces neutral products instead (butanediol), the

acids are consumed and the medium becomes less acidic → methyl red turns YELLOW = NEGATIVE.

VOGES-PROSKAUER TEST: Tests for the butanediol pathway, specifically for the intermediate acetoin (acetyl

methyl carbinol). After incubation, alpha-naphthol and KOH (Barritt's reagents) are added. If acetoin is present, it

reacts with these reagents to produce a RED/dark pink colour = POSITIVE. Example: Enterobacter aerogenes

(VP+). If no acetoin is present, no colour change = NEGATIVE. The key relationship: these pathways are generally

mutually exclusive. An organism that is MR+ is typically VP− (E. coli: MR+ VP−), and vice versa (Enterobacter: MR−

VP+). This makes the MR/VP pair a powerful differential tool.

Microbiology — Ultimate Study Guide | Page 26 of 27

PART C

CHEAT SHEET PLANNING GUIDE

You get 1 x A4 page (both sides), handwritten, with your name and student ID. Here's how to maximise its value in

60 minutes.

STRATEGY: Don't write things you'll easily remember. Write things you'll forget under pressure.

Priority What to Include

MUST include

(easy to forget

under pressure)

• IMViC patterns: E. coli = ++−−, Enterobacter/Kleb = −−++ • Nitrate test flowchart (3 outcomes) •

Biochem test colour results (all 9 tests) • Three-domain comparison table (membrane lipids, 1st AA,

etc.) • Proteobacteria 5 classes with key genera • Organism-specific features table (motility, oxidase,

unique traits) • Mycoplasma 6 key facts • Haemolysis types (α=green, β=clear, γ=none)

SHOULD include

(moderately

difficult)

• Growth factor table (C, N, S, P, trace, organic) • Oxygen requirement definitions • Colony

morphology terms (form, elevation, margin) • Gram stain steps with purposes • MAC agar: lactose

fermenters vs non-fermenters • G+C split: Firmicutes vs Actinobacteria genera

DON'T waste

space on (you'll

remember)

• Basic definitions you know cold (mesophile = middle temp, etc.) • The exam format itself • Things

with obvious logic (UV + open lid = killing) • Binomial nomenclature rules (you know these)

FORMAT TIPS • Use tiny handwriting and fine-tip pen (0.3mm) • Draw mini tables (much more space-efficient than

sentences) • Use abbreviations: + / − / Ox / Cat / Fac / Ob • Colour-code: red = Gram-neg tests,

purple = Gram-pos tests • Leave margins for quick reference — don't pack edge-to-edge

You've got this, Hamidreza. Study smart, trust the process, and go get that

4.0.

Bismillah.

Microbiology — Ultimate Study Guide | Page 27 of 27

MY NOTES & CONCLUSIONS

Appendix C: Cheat Sheet Planning Guide

Write your own summary, key takeaways, weak areas, and connections between concepts.

Microbiology — Appendix C: Cheat Sheet Planning Guide