What mechanism is responsible for the tolerance phenotype in bacteria against certain penicillins?

• A. Overexpression of beta-lactamases
• B. Deficiency of cell wall autolysins
• C. Increased efflux pumps
• D. Altered penicillin-binding proteins

Answer: (B)

Penicillins kill many bacteria not only by stopping cell wall synthesis, but also by triggering autolysins to break down the wall and cause cell lysis. If autolysins are deficient, the drug can still inhibit wall assembly, but the autolytic attack—and thus rapid lysis—doesn’t occur as effectively. The cells survive exposure, showing a tolerance phenotype rather than outright resistance.

Other mechanisms would either inactivate the drug (beta-lactamases), pump it out (efflux), or alter the drug’s target (penicillin-binding proteins) to stop the drug from working entirely—these fit resistance better than tolerance, which is about surviving antibiotic action without being killed.