🦫 Microbiology

What you’ll learn

1. The Water Drop Full of AnimalsExplain why microbial life went undiscovered until the 1670s, and identify the two facts — smallness and doubling — that organise the whole subject.Antonie van Leeuwenhoek, a Delft draper with self-ground lenses, reported swimming 'animalcules' to the Royal Society in the 1670s, including more creatures in his own dental plaque than there were people in the Netherlands. The delay was a matter of scale: microbes are roughly 100 times below the eye's resolving limit. Two facts — that microbes are very small and that they double — organise everything that follows.
2. Why Small Is a StrategyUse the surface-area-to-volume ratio to explain why bacteria need no organs, why they are metabolically intense, and why cells cannot simply grow large.Surface-to-volume ratio scales as 6/diameter, so a 1 µm bacterium has about 20 times more surface per unit volume than a 20 µm human cell. That ratio lets diffusion do the work organs do in large organisms — no mouth, no blood, no heart — and makes bacteria the most metabolically intense life on Earth per gram. It also caps how large a cell can get before its interior starves.
3. Two Kinds of Cell, One Deep DivideDistinguish prokaryotic from eukaryotic cells and the three domains of life, and explain why the Gram stain still guides antibiotic choice.The deepest division in biology is whether a cell encloses its DNA in a nucleus. Carl Woese's ribosomal RNA comparisons in the 1970s further split the prokaryotes into Bacteria and Archaea — three domains, not two. Hans Christian Gram's 1884 stain reads envelope architecture: thick peptidoglycan retains the dye (Gram-positive), while a thin layer under an outer membrane does not (Gram-negative) — and that outer membrane excludes many antibiotics.
4. The Arithmetic of DoublingApply exponential growth and the four-phase growth curve to explain why doubling time governs spoilage, infection and food safety.E. coli doubles about every 20 minutes under optimal laboratory conditions, so one cell becomes roughly 69 billion (2^36) in twelve hours. Real populations follow a four-phase curve — lag, log, stationary, death — because exponential growth always meets a finite world; in nature almost all microbes live permanently in stationary phase. Refrigeration does not sterilise: it stretches the doubling time, which is why food safety is governed by a clock rather than by smell.
5. Microbes Eat EverythingDescribe the metabolic range of microbes and explain why planetary element cycles, including the nitrogen cycle, depend on them.Animals run one energy chemistry; bacteria and archaea run essentially every energy-yielding reaction thermodynamics permits, from hydrogen and iron to sulfur and methane. Cyanobacterial oxygen production drove the Great Oxidation Event and laid down banded iron formations. Nitrogen fixation by nitrogenase-bearing microbes is the only biological route from atmospheric N₂ into living matter — the Haber–Bosch process is the sole industrial alternative.
6. Viruses Are Not Alive, and That MattersExplain what a virus is and is not, walk the replication cycle, and derive why antibiotics cannot treat viral infections.A virus is genetic material in a protein coat, with no ribosomes, no metabolism and no independent movement — it fails standard tests for life yet is the most numerous biological entity on Earth, at roughly ten million particles per millilitre of surface seawater. It replicates only by redirecting a host cell's machinery through attachment, entry, replication, assembly and release. Antibiotics work by selective toxicity against bacterial structures a virus does not possess, so they cannot act on viruses at all.
7. How a Microbe Makes You SickExplain how pathogens cause disease — adhesion, invasion, toxins, and the host's own response — and state both the power and the limits of Koch's postulates.Virulence is a side effect of a way of making a living, not an intention; cholera's symptoms are its transmission mechanism. Koch's four postulates established causation by intervention rather than association, but fail for unculturable organisms, human-only pathogens, and asymptomatic carriers, so molecular versions now do the work. Much of what illness feels like — fever, aches, fatigue, and in sepsis the lethal event itself — is the host's own regulated immune response rather than direct microbial damage.
8. Resistance Is Evolution You Can WatchExplain antibiotic resistance as selection acting on bacterial populations plus horizontal gene transfer — not as a change in the patient — and state the sourced global burden.Fleming warned in his 1945 Nobel lecture that underdosing would cultivate resistant organisms, and penicillin-resistant Staphylococcus aureus appeared within a few years. Resistance is selection: variants that blunt the drug already exist in a large population, and the antibiotic removes their competition, after which 20-minute doubling refills the field. Horizontal gene transfer moves resistance genes sideways between species; the Lancet's 2022 GBD analysis estimated 1.27 million deaths attributable to and 4.95 million associated with bacterial AMR in 2019.
9. The Microbes That Keep You AliveDescribe the functions of the human microbiome and use the 2016 correction of the 10:1 cell ratio as a case study in scientific self-correction.The widely repeated '10 bacteria per human cell' figure traces to a 1972 estimate; Sender, Fuchs and Milo's 2016 PLOS Biology analysis put a 70 kg reference man at about 3.8×10^13 bacteria to 3.0×10^13 human cells — roughly 1:1. Gut microbes ferment fibre into short-chain fatty acids that feed the colon lining, synthesise vitamin K and B vitamins, provide colonisation resistance against pathogens such as C. difficile, and help train the immune system. Humans have deliberately cultivated microbes for millennia in bread, beer, cheese and every fermented food.
10. The Ninety-Nine Percent We Cannot GrowExplain the great plate count anomaly and how culture-independent sequencing revealed the microbial majority — then restate the course's through-line.Koch's agar plate founded microbiology and simultaneously limited its field of view: Staley and Konopka's 1985 'great plate count anomaly' found that plating recovers a maximum of about 1% of the direct microscopic count. The unculturable majority is not dead but is poorly served by a Petri dish's flood of nutrients and absence of neighbours. Culture-independent ribosomal RNA sequencing revealed whole phyla with no cultured representative, including SAR11 — plausibly the most abundant organism on Earth, unseen until the 1990s.

Questions this course answers

Grounded in trusted sources

• OpenStax — Microbiology (2016)
• Madigan, Bender, Buckley, Sattley & Stahl — Brock Biology of Microorganisms, 15th ed. (2018)
• Ron Milo & Rob Phillips — Cell Biology by the Numbers (2015)
• Antimicrobial Resistance Collaborators — 'Global burden of bacterial antimicrobial resistance in 2019', The Lancet 399 (2022)
• Sender, Fuchs & Milo — 'Revised Estimates for the Number of Human and Bacteria Cells in the Body', PLOS Biology 14(8) (2016)
• Staley & Konopka — Annual Review of Microbiology 39 (1985)
• Carl R. Woese & George E. Fox — PNAS 74(11) (1977)
• Alexander Fleming — Nobel Lecture (1945)

Every Wunder lesson is built from real, reputable sources — never invented.

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