Introduction to Microbiology

Microorganisms drive health (normal flora, pathogens, antimicrobial resistance), industry (fermentation, enzymes), and ecosystems (nutrient cycling). Emerging respiratory viruses (SARS-CoV-2, influenza, earlier SARS/MERS) remind us that infection control basics — barriers (hand hygiene, masks), vaccines, and surveillance — remain central.

Your course likely asks you to explain growth requirements, identify organisms by tests and colony morphology, and connect microbes to disease. Week 1 sets the vocabulary and historical frame so those practical weeks feel connected, not arbitrary.

Bacteria are well characterised, easy to grow on defined media, large enough for simple light microscopy, and relatively safe in teaching strains. Skills in streaking, staining, and aseptic technique transfer to fungi and many clinical protocols.

Caveat: only about 0.1% of bacterial species in nature have been cultured — environmental microbiology and DNA sequencing revealed enormous “dark matter.”

Germ theory: a particular microbe can cause a particular disease (contrast with vague “miasma” ideas). Bassi (fungus & silkworms), Pasteur (protozoan silkworm disease), Koch (anthrax bacterium) built the evidence.

Pasteurization applies heat high enough and long enough to reduce pathogens and spoilage organisms in liquids (historically: wine/milk) without relying on full sterilization — exact times/temps are standards you may memorise in food microbiology.

Koch’s postulates (idealised): isolate from diseased → pure culture → cause disease in healthy experimental host → re-isolate same agent. Limitations today: unculturable organisms, ethical human infection, asymptomatic carriage.

Vaccination — from vacca (cow): Jenner used cowpox exposure to protect against smallpox. Core idea: immune memory without full disease.

Chemotherapy (infectious disease sense) = treat with chemicals — natural (e.g. quinine historically for malaria) or synthetic. Antibiotics are microbial natural products (or derivatives) that inhibit or kill other microbes — Fleming’s penicillin is the iconic example.

Salvarsan (Ehrlich) showed a targeted chemical could hit a pathogen — the “magic bullet” narrative.

UK-style life characteristics: MRS GREN — Movement, Respiration, Sensation, Growth, Reproduction, Excretion, Nutrition. Viruses (and prions) fail several criteria — not considered living cells; they are obligate intracellular parasites with a virion particle stage.

What all “microorganisms” share in your intro course: they are microscopic — you need microscopy (or indirect detection) to study them properly.

Prokaryotes (Bacteria, Archaea): no nucleus, no membrane-bound organelles. Eukaryotes (fungi, protozoa, algae): nucleus + organelles. Endosymbiotic theory: mitochondria (and chloroplasts in plants/algae) derived from ancient bacteria — explains double membranes and ribosome similarity.

In a closed batch culture, population changes follow four phases: lag (adaptation), log (exponential growth), stationary (growth ≈ death; nutrient/waste limits), death (viability drops). Use the interactive to fix the vocabulary.

Bacteriostatic drugs stop growth — organisms may survive until immune clearance or drug removal. Bactericidal drugs kill — under lab definitions, they do not regrow when subcultured without drug. That is why courses stress completing antibiotic courses: eradicate persisters and reduce resistance selection.

Lectures usually group media by purpose, not only by physical form (liquid broth vs solid agar). Know the definitions and one classic example each — exams often mix the terms.

Many plates are both selective and differential (e.g. MacConkey). “Selective” answers who grows; “differential” answers how they look when they do.

Incubation temperature and atmosphere are chosen to match the pathogen’s ecological niche. Wrong conditions → false negatives or slow growth.

Morphology on a Gram stain or wet prep is described by shape (coccus, bacillus, spiral/curved) and arrangement (how daughter cells stayed together after division).

Streptococcus vs Staphylococcus is a classic naming clue: chains vs clusters — both are Gram-positive cocci but the arrangement hints at genus before biochemical ID.

Fluency with metric steps matters: 1 m = 10³ mm = 10⁶ μm = 10⁹ nm. A typical coccus might be ~0.5–1.5 μm diameter; many rods ~2–5 μm long. Viruses are often tens–hundreds of nm — below the resolving power of light microscopy, hence EM or molecular detection.

Magnification makes the image bigger; resolution is the minimum distance at which two points are distinguished as separate. High magnification without resolution = empty zoom. Light microscopy practical limit ~200 nm lateral resolution; two adjacent cocci (~1 μm) are resolvable; a 30 nm virion is not.