Importance of inoculum growth phase when using an in vitro pharmacokinetic model to evaluate beta-lactam antibiotics.

The bactericidal activity of cefuroxime, cephaloridine and cephalexin is evaluated in an in vitro model. The inocula are derived from an overnight static culture, or after a pre-incubation period of 1 or 2 hours to allow cell re-growth. The early bactericidal effect of the antibiotics is more evident using pre-incubated cells, especially for Staphylococcus aureus 663. At hour 8, with Escherichia coli 851/E, there is re-growth using the static inoculum, while the antibiotic effect is still evident using the pre-incubated one. The importance arises therefore for considering the phase of growth of the inoculum as a critical parameter when using in vitro models with varying concentrations of beta-lactam antibiotics.