Question

Choose one antibiotic that you would expect to show different effects on Gram-positive and Gram-negative bacteria. Which bacterium is more susceptible? Why do you think this might be the case?

Answer 1

An example of antibiotic that would show different result for gram positive and gram negative bacteria is penicillin. it is a beta lactam antibiotic and active against Gram positive bacteria only. This is because of thick wall made up of peptidoglycan forms the cell wall of Gram positive bacteria. Penicillin acts upon peptidoglycan layer and breaks it. Since the Gram negative bacteria have a thin cell wall with less peptidoglycan layer and also further surrounded by a lipopolysaccharide (LPS) layer, penicillin is unable to reach the peptido glycan layer and damage it. So it has no effect on Gram negative bacteria.

A single peptidoglycan chain is made up of repeating units of N-acetylmuramic acid and N- acetylglucosamine. Each peptidoglycan chain is cross-linked to another polypeptide chain to give a stable, mesh-like structure formation. the enzyme involved in crosslinking is called DD transposase. It is this enzyme on which penicillin acts by attaching it's beta lactam ring to the enzyme and inhibiting its activity. Without the functional enzyme, peptidoglycan crosslinking stops inhibiting peptidoglycan layer formation or restructuring. So the difference in the cell wall structure of Gram positive and negative bacteria makes the Gram positive ones susceptible to the antibiotic.