What is the inoculum effect and how does it affect antibiotic therapy?
The inoculum effect is a phenomenon in which the effectiveness of an antibiotic decreases as the number and/or the duration of the infection increases. This effect is observed with penicillin versus streptococci in both in vitro and animal models. This is because the stationary growth-phase conditions make it less likely that bacterial cell wall active antibiotics such as -lactams and glycopeptides are optimally effective. The inoculum effect also occurs with -lactamase susceptible -lactam antibiotics versus -lactamase producing bacteria, as the denser -lactamase producing bacterial populations may lead to more -lactamase present, making the antibiotic less effective. High inocula are also more likely to have antibiotic-resistant subpopulations that can emerge in the setting of antibiotic therapy. Fluoroquinolones and aminoglycoside antibiotics are less affected by the size of the inoculum because of their different mechanisms of bacterial killing.  