Which factors contribute to increased antibiotic tolerance in stationary-phase bacterial cells?

• A. Increased metabolic activity and higher growth rate.
• B. Reduced metabolic activity and changes in efflux pumps, with some cells becoming VBNC or forming biofilms.
• C. Increased permeability to antibiotics.
• D. Longer doubling time with higher replication.

Answer: (B)

In stationary-phase cells, antibiotic tolerance rises because growth slows and metabolism is downregulated, plus protective states form. When nutrients are scarce, bacteria enter a low-activity mode so antibiotics that target active processes—like cell wall synthesis, protein production, or DNA replication—are less effective. At the same time, some cells become viable but non-culturable or join biofilms, which create protective environments and slow drug penetration. Changes in efflux pumps can also pump antibiotics out more efficiently, lowering intracellular concentrations. Put together, reduced metabolic activity, altered efflux, and protective states like VBNC and biofilms explain the increased tolerance.

The other scenarios don’t fit the situation: higher metabolic activity and growth would make antibiotics targeting growth processes more lethal, not less; increased permeability would generally increase drug entry and vulnerability; and a longer doubling time with ongoing replication doesn’t capture the protective, dormant-like states that drive tolerance in stationary phase.