Question

3a. Describe two characteristics that antibiotics commonly share using the peptidoglycan-inhibiting drug
penicillin as a specific example.
b. Would you predict that penicillin would be bacteriostatic or bacteriocidal? Would it therefore be an
effective antibiotic for a patient with a poor immune system? Explain why.

Answer both questions thoroughly and completely.

Answer 1

3a) The two characteristics that are shared by antibiotics are:

1) They are selective for the cellular processes they target. Antibiotics may target cell wall synthesis or protein synthesis of any biochemical process in the bacterial cell.

Penicillin is an antibiotic that was discovered by Alexander Fleming. It inhibits the cell wall synthesis in gram-positive bacteria. Penicillin has a highly reactive beta-lactam structure acylates the active site of the transpeptidase enzyme irreversibly. This transpeptidase enzyme is required for cross-linking of peptidoglycan in the final step of cell wall synthesis. Gram-negative bacteria have a thin cell wall of peptidoglycan. Penicillin cannot enter the outer membrane of gram-negative bacteria. Hence, it is only active against gram-positive bacteria. Ampicillin is an analogue of Penicillin that has an additional amino group. This amino group allows ampicillin to enter the outer membrane, allowing ampicillin to be effective against a broader group of bacteria.

2) Antibiotics only target bacterial cell without harming any host cells. Human host cells lack a cell wall. Human cells lack cell wall, as they are animal cells. They only have plasma membrane that surrounds the cell. Plasma membrane is made up of phospholipids, proteins, and carbohydrates. Hence, it lacks peptidoglycan that is targeted by the penicillin antibiotic. Thus, penicillin will not affect the host cells as it lacks the transpeptidase enzyme that is required for cross-linking of peptidoglycan. Gram-positive bacteria have this enzyme and therefore can be killed by Penicillin. Penicillin will destroy the cell wall and causes lysis of the cell.

b) Penicillin prevents cell wall synthesis by inhibiting the transpeptidase enzyme. Hence, it will cause lysis of the bacterial cell. Bacteriocidal antibiotics kill the bacteria while bacteriostatic antibiotics will inhibit growth of the bacteria and not kill them. Thus, they will be bactericidal at high concentrations. However, at low concentration, they will be bacteriostatic.

Bacteriostatic drugs only inhibit the growth of the bacteria. They require the help of the immune system to clear the infection. Bacteriocidal drugs on the other hand will kill the bacteria. Hence, in an immunocompromised host, it is said to be better to give bacteriocidal drugs such as Penicillin. However, Penicillin will also kill the normal flora bacteria, there by affecting innate immune responses in the host. This would be drastic for a patient with poor immune responses.