Historical Perspective: From Animalcules to Germ Theory and the Molecular Era

Practice questions (17)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

1. Which worker first described bacteria and protozoa, seen through hand-ground single-lens microscopes, in letters to the Royal Society of London?

   1. Robert Hooke, whose Micrographia illustrated moulds and cork cells seen with a compound microscope
   2. Louis Pasteur, who studied the microbes of fermentation and of spoiled wine and broth
   3. Antonie van Leeuwenhoek, a Delft cloth merchant who ground his own tiny bead lenses
   4. Lazzaro Spallanzani, who boiled infusions in sealed flasks to test spontaneous generation
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   Answer: C — Antonie van Leeuwenhoek, a Delft cloth merchant who ground his own tiny bead lenses

   Leeuwenhoek's letters to the Royal Society from the 1670s describe animalcules in rainwater, infusions and dental scrapings, including rods, cocci and spirals. Hooke is the tempting distractor: he used a compound microscope and illustrated moulds and cork cells, but did not report bacteria. Pasteur and Spallanzani worked a century and more later.

2. A bead lens of the kind used in early single-lens microscopes has a focal length of 1.25 mm. Taking the magnification of a simple lens as M = D/f with D = 250 mm (the near point of the eye), the lens magnifies ____ times.

   Numerical answer — type the value.

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   Answer: 200

   M = D/f = 250 mm ÷ 1.25 mm = 200. The shorter the focal length (the smaller and more strongly curved the bead), the higher the magnification, which is why Leeuwenhoek's tiny beads outperformed the compound microscopes of his day.

3. Robert Hooke's Micrographia (1665) is remembered in microbiology because it

   1. illustrated the fruiting structures of a mould and introduced the word cell
   2. gave the first report of bacteria in scrapings taken from human teeth
   3. disproved the spontaneous generation of maggots by covering jars of meat
   4. described the first pure culture of a bacterium on a solid medium
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   Answer: A — illustrated the fruiting structures of a mould and introduced the word cell

   Micrographia used a compound microscope to draw, among other things, a mould's sporangia and the empty boxes of cork, which Hooke called cells. Bacteria in dental scrapings are Leeuwenhoek's report, the covered jars are Redi's (1668), and pure culture on solid media came from Koch's laboratory two centuries later.

4. Why did Pasteur's swan-neck flask experiment answer the objection that had been raised against Spallanzani's sealed-flask results?

   1. The neck was melted shut after boiling, so no microbe could reach the cooled broth at any time
   2. The broth was boiled far longer than before, which killed even the most heat-resistant spores
   3. The curved neck was packed with cotton, so the air reaching the broth had been filtered sterile
   4. Air reached the broth freely through the open neck while dust and germs settled in its bend
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   Answer: D — Air reached the broth freely through the open neck while dust and germs settled in its bend

   Spallanzani's critics said sealing kept out the air (and a vital force) that life needs. Pasteur's neck stayed open, so air entered, yet the broth stayed sterile until dust trapped in the bend reached it. A sealed or cotton-plugged neck would have reopened the very objection he was answering, and long boiling was not the point of the design.

5. A hay infusion boiled for one hour in a flask kept in a dust-free chamber becomes turbid after two days, while a meat broth boiled for five minutes in the same chamber stays clear. Which explanation fits both results and also points to a remedy?

   1. Hay holds a vital force that withstands boiling and generates new microbes once the infusion cools in still air
   2. Hay carries heat-resistant endospores; boiling on successive days, after they germinate, would sterilise it
   3. Dust-free air still carries vegetative bacteria, and these settle preferentially in the richer hay infusion
   4. Long boiling breaks hay sugars into products that later polymerise into turbid material with no microbes at all
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   Answer: B — Hay carries heat-resistant endospores; boiling on successive days, after they germinate, would sterilise it

   Hay carries Bacillus endospores that survive an hour of boiling; meat broth lacks them, so a short boil sterilises it. Intermittent heating (tyndallisation) lets spores germinate between boilings and kills the heat-sensitive cells they become. Airborne bacteria cannot be the cause, because the meat broth in the same dust-free air stays clear, and the vital-force idea is the very claim the experiment refutes.

6. In a repeat of Pasteur's swan-neck experiment with a nutrient broth, which of the following outcomes are expected?

   1. Boiled broth in a flask whose curved neck is left intact stays clear for months
   2. Boiled broth turns turbid within days once the neck is snapped off near the body of the flask
   3. Unboiled broth in a flask with an intact curved neck stays clear, because the neck traps all dust
   4. Boiled broth turns turbid after the flask is tilted so that broth runs into the bend of the neck and back
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   Answer: A — Boiled broth in a flask whose curved neck is left intact stays clear for months; B — Boiled broth turns turbid within days once the neck is snapped off near the body of the flask; D — Boiled broth turns turbid after the flask is tilted so that broth runs into the bend of the neck and back

   The intact neck admits air but traps dust, so boiled broth stays sterile; removing the neck, or washing the trapped dust into the broth by tilting, brings germs in and the broth spoils. Unboiled broth already contains living microbes from its ingredients, so the neck cannot keep it clear; the experiment depends on boiling first.

7. Eduard Buchner obtained alcoholic fermentation of sugar with juice pressed from ground yeast and filtered free of intact cells. What did this result show?

   1. Fermentation is catalysed by enzymes that stay active outside the living cell
   2. Fermentation needs intact living yeast cells, exactly as Pasteur had argued
   3. Fermentation is an uncatalysed decay of sugar needing no yeast material at all
   4. Yeast juice carries live bacteria that take over fermentation once yeast is broken
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   Answer: A — Fermentation is catalysed by enzymes that stay active outside the living cell

   A cell-free extract still fermented, so the activity lies in molecules (Buchner's zymase, a mixture of enzymes) rather than in the living cell. That contradicted the strict vitalist reading of Pasteur, which is the tempting distractor, while also showing that yeast material is essential, against the idea of uncatalysed decay. The juice was filtered free of cells, so contaminating live bacteria are excluded.

8. Winogradsky's studies of Beggiatoa and of nitrifying bacteria established which concept?

   1. Fermentation, using an organic compound as both electron donor and acceptor
   2. Symbiotic nitrogen fixation, reduction of N₂ to ammonia in root nodules
   3. Chemolithotrophy, gaining energy by oxidising inorganic H₂S, NH₃ or NO₂⁻
   4. Anoxygenic photosynthesis by purple and green sulphur bacteria in ponds
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   Answer: C — Chemolithotrophy, gaining energy by oxidising inorganic H₂S, NH₃ or NO₂⁻

   Beggiatoa oxidises H₂S and nitrifiers oxidise ammonia and nitrite, using the energy to fix CO₂: chemolithotrophy, which Winogradsky discovered. Purple and green sulphur bacteria also use H₂S, which makes anoxygenic photosynthesis the tempting distractor, but they use it as an electron donor for light-driven growth, not as an energy source in the dark. Nodule nitrogen fixation was shown by others.

9. Soil is incubated in air in a liquid medium of mannitol and mineral salts with no combined nitrogen. After several transfers into fresh medium, large aerobic cells able to fix N₂ dominate. The approach and the organism most likely enriched are

   1. Koch's pure-culture streaking, enriching the anaerobic nitrogen fixer Clostridium pasteurianum
   2. The Winogradsky column, enriching the light-dependent heterocystous cyanobacterium Anabaena
   3. Pasteur's fermentation method, enriching the acid-tolerant lactic acid bacterium Lactobacillus
   4. Beijerinck's enrichment culture, enriching the free-living aerobic nitrogen fixer Azotobacter
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   Answer: D — Beijerinck's enrichment culture, enriching the free-living aerobic nitrogen fixer Azotobacter

   A medium lacking combined nitrogen favours N₂ fixers; aerobic incubation with an organic carbon source favours aerobic heterotrophs; serial transfer is the hallmark of Beijerinck's enrichment culture, by which he isolated Azotobacter in 1901. Clostridium pasteurianum fixes N₂ but is a strict anaerobe and would not thrive in aerated medium; Anabaena needs light, and Lactobacillus cannot fix N₂.

10. Soil microbes completely oxidise 45 g of glucose (C₆H₁₂O₆) to CO₂ and water under aerobic conditions. Taking C = 12, H = 1 and O = 16, the mass of CO₂ released is ____ g.

    Numerical answer — type the value.

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    Answer: 66

    C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O. M(glucose) = 6(12) + 12(1) + 6(16) = 180 g mol⁻¹, so 45 g = 0.25 mol. CO₂ formed = 6 × 0.25 = 1.5 mol, and M(CO₂) = 44 g mol⁻¹, so 1.5 × 44 = 66 g. Forgetting the factor 6 gives 11 g.

11. Which observation conflicts specifically with Koch's first postulate, rather than with the second, third or fourth?

    1. Healthy people carry Vibrio cholerae in the gut without symptoms during an outbreak
    2. Mycobacterium leprae cannot be grown on any cell-free laboratory culture medium
    3. No animal species develops the human disease when inoculated with the cultured organism
    4. The organism recovered from an inoculated animal differs from the one that was inoculated
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    Answer: A — Healthy people carry Vibrio cholerae in the gut without symptoms during an outbreak

    The first postulate demands the microbe in every case of the disease and not in healthy hosts; asymptomatic carriers break the second half of that requirement. Failure to grow M. leprae in culture violates the second postulate, the lack of an animal model the third, and a different organism on re-isolation the fourth.

12. In an illustrative ward record, 600 of 5000 mothers died of puerperal fever in the year before doctors began washing their hands in chlorinated lime, and 90 of 4500 died in the year after. The relative reduction in the death rate is ____ % (to one decimal place).

    Numerical answer — type the value.

    Show answer

    Answer: 83.3

    Rate before = 600/5000 = 0.12 (12%). Rate after = 90/4500 = 0.02 (2%). Relative reduction = (12 − 2)/12 × 100 = 83.33%, which is 83.3 to one decimal place. The absolute reduction is 10 percentage points, a common slip; comparing raw deaths (600 to 90, 85%) ignores the different number of births.

13. Treating wounds, dressings and surgical instruments with carbolic acid to prevent infection after operations was introduced by

    1. Ignaz Semmelweis
    2. Joseph Lister
    3. John Snow
    4. Robert Koch
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    Answer: B — Joseph Lister

    Lister published his antiseptic method with carbolic acid (phenol) in 1867. Semmelweis is the tempting distractor, but his measure (1847) was handwashing in chlorinated lime before examining women in a maternity ward, not the treatment of surgical wounds. Snow traced cholera to a water pump, and Koch proved specific causes of anthrax and tuberculosis.

14. Which of the following are genuine limitations of Koch's postulates as originally stated?

    1. Mycobacterium leprae and Treponema pallidum cannot be grown in cell-free culture media
    2. Some pathogens are carried by healthy people who never develop the disease
    3. Bacillus anthracis forms spores and can be cultured on solid media and injected into mice
    4. Deliberately infecting people to test the third postulate for a human-only pathogen is unethical
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    Answer: A — Mycobacterium leprae and Treponema pallidum cannot be grown in cell-free culture media; B — Some pathogens are carried by healthy people who never develop the disease; D — Deliberately infecting people to test the third postulate for a human-only pathogen is unethical

    Unculturable pathogens block the second postulate, healthy carriers the first, and the ethics of human inoculation the third. The anthrax bacillus is not a limitation at all: it satisfied every postulate and was Koch's own model case in 1876.

15. Ivanovsky's 1892 finding that sap from mosaic-diseased tobacco stayed infectious after passing through a filter that retains bacteria was important because it

    1. proved the disease was caused by a toxin secreted by bacteria multiplying in the leaves
    2. showed that the agent was a nucleic acid enclosed within a coat of protein subunits
    3. revealed an infectious agent smaller than any bacterium, later recognised as a virus
    4. showed that the agent could be grown in pure culture on nutrient agar like a bacterium
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    Answer: C — revealed an infectious agent smaller than any bacterium, later recognised as a virus

    Passing a bacteria-proof filter showed an agent below bacterial size; Beijerinck (1898) then argued it was a new kind of agent that multiplies only in living tissue, and the word virus stuck. The toxin idea is the tempting distractor because Ivanovsky himself considered it, but his filtrate was never shown to contain a toxin. Its nucleic acid and protein make-up came decades later, and the agent cannot grow on agar.

16. Which of the following discoverer–discovery pairs are correctly matched?

    1. Paul Ehrlich: Salvarsan, a synthetic arsenical drug against syphilis
    2. Selman Waksman's laboratory: penicillin from a Penicillium mould
    3. Gerhard Domagk: Prontosil, the first sulfonamide drug used clinically
    4. Emil von Behring and Kitasato Shibasaburo: antitoxin against diphtheria and tetanus
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    Answer: A — Paul Ehrlich: Salvarsan, a synthetic arsenical drug against syphilis; C — Gerhard Domagk: Prontosil, the first sulfonamide drug used clinically; D — Emil von Behring and Kitasato Shibasaburo: antitoxin against diphtheria and tetanus

    Ehrlich's Salvarsan (1910), Domagk's Prontosil (1930s) and Behring and Kitasato's antitoxin (1890) are correctly paired. Penicillin was Fleming's discovery (1928); Waksman's laboratory found streptomycin, from Streptomyces, in the 1940s.

17. An extract of heat-killed smooth (S) pneumococci converts live rough (R) cells into S cells. Portions of the extract are treated with single enzymes before being added to R cells. Which result identifies DNA as the transforming principle?

    1. Activity is lost after protease treatment but survives deoxyribonuclease treatment
    2. Activity is lost after deoxyribonuclease but survives protease and ribonuclease
    3. Activity is lost after ribonuclease treatment but survives deoxyribonuclease treatment
    4. Activity survives all three enzymes but is lost when the extract is heated to boiling
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    Answer: B — Activity is lost after deoxyribonuclease but survives protease and ribonuclease

    If DNA carries the activity, only the enzyme that destroys DNA abolishes it, and enzymes that destroy protein or RNA leave it intact; this was the logic of Avery, MacLeod and McCarty (1944). Loss after protease would point to protein and loss after ribonuclease to RNA. Loss on boiling cannot discriminate, and the extract was made from heat-killed cells in the first place.