Q.78 In a population containing fast and slow growing bacteria, the slow growing bacteria can be
enriched by supplementing the medium with
(A) chloramphenicol (B) penicillin (C) penicillin & chloramphenicol (D) rifampin
Penicillin selectively kills fast-growing bacteria by inhibiting cell wall synthesis during active division, enriching slow-growing populations that aren’t actively dividing. The correct answer is (B) penicillin. [web:previous]
Correct Answer
(B) Penicillin
Mechanism of Selective Enrichment
Fast-growing bacteria divide rapidly, exposing penicillin-binding proteins (PBPs) during peptidoglycan synthesis—beta-lactam antibiotics like penicillin acylate PBPs, halting cross-linking and causing lysis. Slow-growing/persister bacteria have minimal cell wall synthesis, surviving penicillin exposure. After washing out antibiotic, slow-growers dominate. Chloramphenicol/rifampin kill indiscriminately via protein/DNA synthesis inhibition regardless of growth rate.
Option Analysis
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(A) Chloramphenicol: Wrong. Inhibits 50S peptidyl transferase; kills both fast/slow growers by blocking protein synthesis.
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(B) Penicillin: Correct. Cell wall-specific; spares non-dividing slow-growers (classic penicillin enrichment technique).
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(C) Penicillin & chloramphenicol: Wrong. Combo kills via complementary mechanisms; no selective advantage for slow-growers.
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(D) Rifampin: Wrong. RNA polymerase inhibitor; bactericidal regardless of division rate.
Introduction to Slow Growing Bacteria Enrichment
Slow growing bacteria enriched penicillin technique dominates GATE Life Sciences Q.78, exploiting penicillin’s specificity for actively dividing cells. Fast-growers lyse during peptidoglycan synthesis while slow-growers/persisters survive, enabling selective population shifts in mixed cultures.
Penicillin Selection Mechanism
Fast-growers: Active division → PBP exposure → Penicillin binding → Lysis
Slow-growers: Minimal division → Protected PBPs → Survival → Enrichment
Post-wash: Slow-growers dominate (>90% population shift).[web:previous]
Q.78 Antibiotic Comparison
| Antibiotic | Target | Growth Rate Selective? | Enriches Slow-Growers? |
|---|---|---|---|
| Penicillin | Cell wall (PBPs) | Yes | Yes |
| Chloramphenicol | Protein synthesis | No | No |
| Rifampin | RNA polymerase | No | No |
| Combo | Multiple | No | No |
Answer: (B)—only cell wall inhibitor spares non-dividers.
Why Penicillin Excels for Enrichment
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Persister cell phenomenon: 0.01-1% survive via dormancy.
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Classic application: Mycobacterium tuberculosis enrichment from sputum.
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GATE high-yield: Beta-lactams = division-dependent killing.
Exam Preparation Strategy
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Mnemonic: “Pen Kills Proliferators” (fast-dividers die).
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Compare: D-cycloserine (wall analog), bacitracin (lipid carrier)—all division-selective.
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PYQ trap: Chloramphenicol = static (reversible), still kills slow-growers eventually.
Master slow growing bacteria enriched penicillin for perfect microbiology technique questions in competitive exams.
