(A) Histidine and penicillin
(B) Penicillin but no histidine
(C) Histidine and lysozyme
(D) Lysozyme but no histidine
Penicillin selectively kills growing prototrophic bacteria, allowing histidine auxotrophs to survive on minimal medium. This article explains Q.67 with the correct answer and all options for microbiology exam prep.
Correct Answer
The correct answer is (B) Penicillin but no histidine.
Prototrophs grow on minimal medium lacking histidine and get killed by penicillin, which targets cell wall synthesis in dividing cells. Auxotrophs cannot grow without histidine, remain viable, and form colonies after penicillin removal.
Option Breakdowns
(A) Histidine and penicillin
Histidine allows all cells (prototrophs and auxotrophs) to grow, so penicillin kills everything indiscriminately.
No selective pressure favors auxotroph survival.
(B) Penicillin but no histidine
Minimal medium + penicillin kills only growing prototrophs; non-growing auxotrophs survive.
Standard penicillin enrichment technique for auxotroph isolation.
(C) Histidine and lysozyme
Histidine supports growth of all cells; lysozyme degrades peptidoglycan but requires spheroplasting conditions.
No selective killing of prototrophs specifically.
(D) Lysozyme but no histidine
Auxotrophs and prototrophs both fail to grow without histidine; lysozyme ineffective on non-growing cells.
Lysozyme needs active metabolism or EDTA for efficacy.
| Option | Medium Components | Selection Mechanism | Matches Q.67? |
|---|---|---|---|
| (A) Histidine + penicillin | Complete | Kills all growing cells | No |
| (B) Penicillin, no histidine | Minimal | Kills growing prototrophs only | Yes |
| (C) Histidine + lysozyme | Complete | Non-selective lysis | No |
| (D) Lysozyme, no histidine | Minimal | No growth, no selection | No |
