Definitions of antibacterial interactions in animal infection models.

Combination of antibiotics in order to achieve antimicrobial synergy is often necessary in the treatment of infections due to resistant bacterial strains. Therefore, several in-vitro test systems have been developed with the purpose of predicting in-vivo action of antibiotics, and the fractional inhibitory concentration (FIC) index has been used to interpret results obtained in different test systems. Using these systems it seems that only antibiotic synergy in vitro is predictive of the results of treatment. It is therefore of interest to have an in-vivo test system that makes it possible to describe antibiotic interaction in detail. Animal infection models such as the foreign body model system enable the measurement of many parameters at the site of infection, such as bactericidal effect (BE) and antibiotic concentrations. A new calculation of drug interaction is suggested in which the measurements used are the BE and the time during which the MIC is exceeded, for the individual drugs and the combination. This calculation enables us to penetrate into observed antibiotic efficacy in vivo to find out whether an observed high BE is due to real synergy or simply to optimal pharmacokinetics of antibiotics at the site of infection.