post-antibiotic effect
The post-antibiotic effect, or PAE, is the lingering suppression of bacterial growth that continues even after the drug concentration has fallen below the level that normally inhibits the bug. In plain terms, the antibiotic leaves bacteria stunned: even once the drug has largely cleared, the survivors take a while to shake it off and start dividing again.
The PAE arises because some drugs cause damage that takes time to repair, for example persistent occupation of the ribosome by aminoglycosides, or because a brief exposure has already committed the bacterium to a delayed death. Drugs with a strong PAE, notably aminoglycosides and fluoroquinolones against Gram-negative bacteria, keep working in the gap between doses even when no effective drug is present.
This effect has a direct dosing consequence. For drugs with a long PAE whose killing depends on a high peak concentration, it is rational to give a single large dose once daily, allowing high peaks for maximum kill while letting levels fall between doses, since the PAE covers the trough. By contrast, beta-lactams have little or no PAE against many organisms, so their efficacy depends on keeping the concentration above the MIC for as much of the interval as possible, favouring more frequent or continuous dosing.
Gentamicin is dosed once daily partly because its long post-antibiotic effect keeps suppressing bacteria through the low-concentration trough between doses.
A long PAE permits big, infrequent doses without losing control of the infection.
The PAE is one reason once-daily aminoglycoside dosing is both effective and safer: high peaks maximise kill, while the long drug-free trough reduces toxic accumulation in the kidney and ear.