What is the inoculum effect?
The inoculum effect is the effect of high bacterial densities on antimicrobial activity, where certain groups of antimicrobials commonly used to treat IE such as -lactams and glycopeptides can be less active against highly dense bacterial populations. The effective minimum inhibitory concentration (MIC) at the site of infection with bacterial densities of 108 colony-forming units per 1 g tissue can be much higher than anticipated by in vitro susceptibility tests that use a standard inoculum (105.5 colony-forming units per milliliter). In addition, bacteria that are otherwise killed at low densities by bactericidal antibiotics such as penicillins can be relatively resistant to or tolerant of their bactericidal effect in dense populations.