Solved Gate Questions

Antibiotic Resistance Marker that CANNOT be used in a cloning vector in Gram Negative bacteria is? (GATE BT 2010)

Antibiotic Resistance Marker that CANNOT be used in a cloning vector in Gram Negative bacteria is? (GATE BT 2010)

a) Streptomycin b) Ampicillin
c) Vancomycin d) Kanamycin

Correct Answer is (C) Vancomycin

Let’s discuss the mode of action of all given antibiotics to arrive at the correct answer:

1). Streptomycin: It’s an aminoglycoside. Mode of action of all aminoglycosides involves inhibition of protein synthesis by binding to 16S rRNA of 30S subunitĀ  of bacterial ribosomes. It thus interferes with the binding of formyl-methionyl-tRNA to the 30S subunit. This can result in death of bacteria owing to halt of machinery to synthesize crucial proteins needed for survival.

Thus, streptomycin can easily be used as a marker for Gram Negative bacteria as well.

2). Ampicillin: It belongs to penicillin group of beta-lactam antibiotics, and hence works by cell wall synthesis inhibition at final stages (like during binary fission), thereby leading to cell lysis. It can penetrate both Gram-positive and Gram Negative bacteria and has been known to exert it’s affect on most Gram-positive and some Gram-negative bacteria as well.

Note: Did you know, Ampicillin differs structurally from Penicillin in having an extra “amino” group. And it’s this amino group that helps it to penetrate the outer membrane of Gram-negative bacteria.

3). Kanamycin: Kanamycin, like Streptomycin is a Protein synthesis inhibitor (thus globally affective on both Gram negative and positive bacteria). It is reported to act by interacting with the 30S subunit and leading to mistranslations, thereby inhibiting translocation during Protein synthesis.

4). Vancomycin: Vancomycin, the correct answer to this question, actually acts by “Proper Cell Wall Synthesis Inhibition”. Given the fact that, unlike Gram-positive bacteria, Gram-Negative bacteria have myriad of ways for synthesizing their cell walls and difficulty in penetrating a Gram-negative cell wall, Vancomycin has reported to be ineffective on Gram-Negative bacteria (except some Nisserial species). Precisely, it binds to terminal D-alanyl-D-alanine moieties (D-Ala-D-Ala) of the NAM/NAG-peptides, thereby preventing long polymerization of NAM/NAG peptides.

Following is the visual depiction of mechanism of action of Vancomycin, obtained from wikimedia:


One Comment

  1. thanks for the information


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