Question

Penicillin is an antibiotic that inhibits peptidoglycan synthesis. Based on your results and knowledge of cell envelope differences, would penicillin be more effective against S. aureus (purple) or E. coli (red)? Explain your rationale.

Answer 1

Ans. Penicillin is more effective against S.aureus than E.coli.

Reason:- As penicillin is a type of antibiotic which works on the differences in the cell envelope of bacteria by inhibiting peptidoglycan synthesis,due to this it is more effective against Staphylococcus aureus than Escherichia coli.

Actually, S.aureus is gram positive bacteria as in question its purple colour is given which is due to retention of purple crystal violet stain in the thick peptidoglycan layer of its cell wall. E.coli is gram negative bacteria as in question its red colour is given which is due to the the inability of their cell wall to retain colour of crystal violet stain.

In gram positive bacteria peptidoglycan is the major component of cell wall in which a single peptidoglycan chain is cross linked to to other peptidoglycan chains through the action of enzyme DD-transpeptidase. As penicillin belongs to beta-lactam family and it kills S.aureus through binding of the beta-lactam ring to DD-transpeptidase by inhibiting its cross-linking activity,thereby preventing new cell wall formation.As new cell walls are not formed it becomes difficult for S.aureus to cope with the external environmental conditions such as water,molecular pressure,etc which leads to sudden death of S.aureus.

In cell wall of E.coli thin peptidoglycan is surrounded by lipopolysaccharide and protein layer which prevents penicillin from attacking it. Due to difference in structure of cell envelope of E.coli it is difficult for penicillin to bind and thus, peptidoglycan synthesis is not/less inhibited as compared to S.aureus.

Therefore, penicillin is less effective against E.coli than S.aureus.