NDA Biology · Microbiology and Disease

Antibiotics and Useful Microbes

An antibiotic is a chemical made by one microbe that kills or stops another; this subtopic covers Fleming's discovery of penicillin, why antibiotics miss viruses, how bacteria resist them, and the friendly microbes we put to work.

All Microbiology and Disease notes NDA Biology strategy

Why this matters

Seven PYQs, all EASY or MODERATE recall. The cornerstone fact is Fleming → penicillin (1928), tested directly in 2025. Three follow-ups recur: antibiotics act on bacterial metabolic pathways so they do NOTHING to viruses; resistant bacteria destroy penicillin with the enzyme β-lactamase; and antibiotics themselves come FROM microbes. The subtopic also folds in the useful side of microbiology — Lactobacillus curdling milk, probiotics as live microbial supplements — and one worm-disease recall (elephantiasis = Wuchereria).

Concept 1 of 4

Antibiotics — discovery and how they work

Intuition

Penicillin was the first antibiotic, found by Alexander Fleming in 1928 when a stray Penicillium mould killed the bacteria on his culture plate. An antibiotic works by jamming a step in bacterial metabolism (cell-wall building, protein synthesis) — which is exactly why it does nothing to a virus, since a virus has no metabolism to jam.

Definition

The discovery and mechanism facts:

Penicillin — the first antibiotic, discovered by Alexander Fleming (1928) from the mould *Penicillium*. (Not Crick, Wilkins or Darwin.)
Antibiotics are obtained from microbes themselves — fungi and bacteria (e.g. penicillin from a fungus, streptomycin from a bacterium). The statement 'no antibiotic has been obtained from any microbe' is false.
Antibiotics act on bacterial metabolic pathways (cell-wall synthesis, protein synthesis). Viruses have no such pathways, so antibiotics do not work on viral infections.

Fact	Answer
Penicillin was discovered by	Alexander Fleming (1928)Q From the Penicillium mould killing Staphylococcus. NDA 2025.
Are antibiotics obtained from microbes?	Yes — fungi and bacteriaQ 'No antibiotic from any microbe' is the FALSE statement. NDA 2017.
Do antibiotics affect viruses?	No — viruses lack metabolic pathwaysQ Antibiotics target bacterial pathways; viruses have none, so taking antibiotics does NOT cure a viral infection. NDA 2020.

Antibiotics fight bacteria by attacking their metabolism — a virus has none, so antibiotics are useless against viral disease.

Practice this conceptself-check · 4 quick reps

Try it yourself

A patient with a viral cold is given antibiotics and does not improve. Explain, in terms of how antibiotics work, why they had no effect.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Who discovered penicillin?
2.
Are antibiotics obtained from microbes?
3.
Do antibiotics work against viruses?
4.
Penicillin comes from which kind of organism?

From the bank · past-year question

Example 1Microbiology and DiseaseEASY

Initial discovery of antibiotic penicillin was done by

[Q112 · Apr · 2025]

Antibiotics do nothing to viruses

Taking antibiotics does not cure a viral infection. Antibiotics attack bacterial metabolic pathways; viruses have none. The correct statement is 'viruses do not possess metabolic pathways on which antibiotics can function, whereas bacteria do'.

Antibiotics DO come from microbes

A 'which statement is NOT correct?' item plants 'no antibiotic has been obtained from any microbe' — this is false. Penicillin comes from a fungus, streptomycin from a bacterium; microbes are the source of antibiotics.

Drill 2 more on antibiotics — discovery and how they work

Concept 2 of 4

Antibiotic resistance — β-lactamase

Intuition

Some bacteria survive penicillin because they make an enzyme that chops the drug apart before it can act. That enzyme is β-lactamase (penicillinase) — it breaks the β-lactam ring at the heart of the penicillin molecule, switching the drug off.

Definition

How penicillin resistance works:

Penicillin's killing power lives in its β-lactam ring.
Resistant bacteria produce the enzyme β-lactamase (also called penicillinase), which hydrolyses (breaks) the β-lactam ring, inactivating the drug.
The bacteria do not store the drug in a vacuole, and the enzyme is not 'lactic acid dehydrogenase' — those are distractors.

Mechanism of resistance	Correct?
Degrade penicillin with the enzyme β-lactamase	Yes — the correct mechanismQ β-lactamase breaks the β-lactam ring. NDA 2019.
Store the antibiotic in a vacuole	No — a distractor
Degrade it with lactic acid dehydrogenase	No — wrong enzyme
Penicillin is simply not absorbed	No — a distractor

Practice this conceptself-check · 3 quick reps

Try it yourself

A strain of bacteria is unaffected by penicillin even at high doses. Name the enzyme responsible and the exact part of the penicillin molecule it destroys.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which enzyme makes bacteria resistant to penicillin?
2.
Which part of penicillin does β-lactamase destroy?
3.
Another name for β-lactamase is ___.

From the bank · past-year question

Example 2Microbiology and DiseaseMODERATE

Which one of the following statements regarding Penicillin is correct?

[Q148 · Apr · 2019]

The resistance enzyme is β-lactamase, not 'lactic acid dehydrogenase'

The right enzyme is β-lactamase (penicillinase), which hydrolyses penicillin's β-lactam ring. 'Lactic acid dehydrogenase' is a planted look-alike, and 'stored in a vacuole' is wrong — bacteria destroy the drug, they don't hoard it.

Concept 3 of 4

Useful microbes — Lactobacillus and probiotics

Intuition

Not all microbes cause disease — many are put to work. Lactic acid bacteria turn milk into curd, and live beneficial microbes taken as food are called probiotics. Keep these friendly microbes separate from the disease-causers; the bank mixes them into the same option lists.

Definition

The useful-microbe facts:

Curdling of milk is done by Lactobacillus (Lactic Acid Bacillus), which ferments lactose into lactic acid — acidifying and curdling the milk.
A probiotic is a live microbial food supplement — beneficial bacteria (often Lactobacillus) eaten to improve gut health. It is NOT an antacid, an antibiotic, or 'organic food'.
*Saccharomyces cerevisiae* (yeast) is used in baking and brewing — a useful microbe, but it does not curdle milk.

Useful microbe / term	Role
Lactobacillus (Lactic Acid Bacillus)	Curdles milk — ferments lactose to lactic acidQ The acidification + curdling agent. NDA 2017.
Probiotic	Live microbial food supplement (beneficial bacteria)Q Not an antacid, antibiotic, or organic food. NDA 2017.
Saccharomyces cerevisiae (yeast)	Baking and brewing (fermentation)

Practice this conceptself-check · 3 quick reps

Try it yourself

Match the role to the microbe: (a) curdles milk, (b) a live supplement eaten for gut health. Name the microbe/term for each.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which microbe curdles milk?
2.
A probiotic is a ___.
3.
Which microbe is used in baking and brewing?

From the bank · past-year question

Example 3Microbiology and DiseaseEASY

Which one of the following microbes causes acidification and curding of milk?

[Q105 · Sep · 2017]

A probiotic is live microbes, not an antibiotic

A probiotic is a live microbial food supplement (beneficial bacteria). The look-alike 'antibiotic' is the opposite — a chemical that KILLS microbes. Distractors also offer 'antacid' and 'organic food'; the answer is the live-microbe supplement.

Milk curdles by Lactobacillus, not yeast

Curdling is done by Lactobacillus (lactic acid fermentation). *Saccharomyces cerevisiae* is a yeast used for baking/brewing, and Vibrio/Clostridium are disease-causers — all distractors here.

Drill 1 more on useful microbes — lactobacillus and probiotics

Concept 4 of 4

Worm diseases — elephantiasis

Intuition

Not every parasite is a microbe — some are worms (helminths). Elephantiasis is the marquee worm disease: a thread-like filarial worm blocks the lymph vessels, swelling the limbs.

Definition

The worm-disease fact the bank tests:

Elephantiasis (lymphatic filariasis) is caused by Wuchereria bancrofti, a parasitic roundworm (filarial worm) transmitted by mosquitoes (Culex).
It is NOT *Ascaris lumbricoides* (intestinal roundworm), NOT *Culex* (that's the mosquito vector, not the pathogen), and NOT *Fasciola hepatica* (the liver fluke).

Disease	Causal worm	Type
Elephantiasis (filariasis)	Wuchereria bancrofti	Filarial roundwormQ Blocks lymph vessels → limb swelling. Vector is the Culex mosquito. NDA 2017.
Ascariasis	Ascaris lumbricoides	Intestinal roundworm
Fascioliasis	Fasciola hepatica	Liver fluke (flatworm)

Practice this conceptself-check · 3 quick reps

Try it yourself

Elephantiasis is a parasitic disease causing massive limb swelling. Name its causal organism and say what kind of organism it is.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which organism causes elephantiasis?
2.
What kind of organism is Wuchereria?
3.
Which mosquito carries the elephantiasis worm?

From the bank · past-year question

Example 4Microbiology and DiseaseMODERATE

Which one of the following is the scientific name of the causal organism of elephantiasis?

[Q64 · Sep · 2017]

Wuchereria is the pathogen; Culex is only the vector

Elephantiasis is caused by Wuchereria bancrofti (a worm). *Culex* appears as a distractor — but that is the mosquito vector, not the cause. The causal organism is the worm, not the carrier.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (4)

Antibiotics — discovery and how they work3 rows

Fact	Answer
Penicillin was discovered by	Alexander Fleming (1928)Q From the Penicillium mould killing Staphylococcus. NDA 2025.
Are antibiotics obtained from microbes?	Yes — fungi and bacteriaQ 'No antibiotic from any microbe' is the FALSE statement. NDA 2017.
Do antibiotics affect viruses?	No — viruses lack metabolic pathwaysQ Antibiotics target bacterial pathways; viruses have none, so taking antibiotics does NOT cure a viral infection. NDA 2020.

Antibiotics fight bacteria by attacking their metabolism — a virus has none, so antibiotics are useless against viral disease.

Antibiotic resistance — β-lactamase4 rows

Mechanism of resistance	Correct?
Degrade penicillin with the enzyme β-lactamase	Yes — the correct mechanismQ β-lactamase breaks the β-lactam ring. NDA 2019.
Store the antibiotic in a vacuole	No — a distractor
Degrade it with lactic acid dehydrogenase	No — wrong enzyme
Penicillin is simply not absorbed	No — a distractor

Useful microbes — Lactobacillus and probiotics3 rows

Useful microbe / term	Role
Lactobacillus (Lactic Acid Bacillus)	Curdles milk — ferments lactose to lactic acidQ The acidification + curdling agent. NDA 2017.
Probiotic	Live microbial food supplement (beneficial bacteria)Q Not an antacid, antibiotic, or organic food. NDA 2017.
Saccharomyces cerevisiae (yeast)	Baking and brewing (fermentation)

Worm diseases — elephantiasis3 rows

Disease	Causal worm	Type
Elephantiasis (filariasis)	Wuchereria bancrofti	Filarial roundwormQ Blocks lymph vessels → limb swelling. Vector is the Culex mosquito. NDA 2017.
Ascariasis	Ascaris lumbricoides	Intestinal roundworm
Fascioliasis	Fasciola hepatica	Liver fluke (flatworm)

Watch out for (6)

Antibiotics do nothing to viruses
→ Antibiotics — discovery and how they work
Antibiotics DO come from microbes
→ Antibiotics — discovery and how they work
The resistance enzyme is β-lactamase, not 'lactic acid dehydrogenase'
→ Antibiotic resistance — β-lactamase
A probiotic is live microbes, not an antibiotic
→ Useful microbes — Lactobacillus and probiotics
Milk curdles by Lactobacillus, not yeast
→ Useful microbes — Lactobacillus and probiotics
Wuchereria is the pathogen; Culex is only the vector
→ Worm diseases — elephantiasis

Mastery check — 3 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Microbiology and DiseaseMODERATE

Which one of the following statements is correct about effects of antibiotics on viruses?

[Q100 · Apr · 2020]

Example 2Microbiology and DiseaseEASY

The term 'Probiotic' is applied to

[Q104 · Sep · 2017]

Example 3Microbiology and DiseaseEASY

Which one of the following statements about microbes is

\textbf{\text{not}}

correct?

[Q67 · Sep · 2017]

Drill every past-year question on this subtopic

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