Prediction of the Effects of Inoculum Size on the Antimicrobial Action of Trovafloxacin and Ciprofloxacin against Staphylococcus aureus and Escherichia coli in an In Vitro Dynamic Model

The effect of inoculum size (N(0)) on antimicrobial action has not been extensively studied in in vitro dynamic models. To investigate this effect and its predictability, killing and regrowth kinetics of Staphylococcus aureus and Escherichia coli exposed to monoexponentially decreasing concentrations of trovafloxacin (as a single dose) and ciprofloxacin (two doses at a 12-h interval) were compared at N(0) = 10(6) and 10(9) CFU/ml (S. aureus) and at N(0) = 10(6), 10(7), and 10(9) CFU/ml (E. coli). A series of pharmacokinetic profiles of trovafloxacin and ciprofloxacin with respective half-lives of 9.2 and 4 h were simulated at different ratios of area under the concentration-time curve (AUC) to MIC (in [micrograms × hours/milliliter]/[micrograms/milliliter]): 58 to 466 with trovafloxacin and 116 to 932 with ciprofloxacin for S. aureus and 58 to 233 and 116 to 466 for E. coli, respectively. Although the effect of N(0) was more pronounced for E. coli than for S. aureus, only a minor increase in minimum numbers of surviving bacteria and an almost negligible delay in their regrowth were associated with an increase of the N(0) for both organisms. The N(0)-induced reductions of the intensity of the antimicrobial effect (I(E), area between control growth and the killing-regrowth curves) were also relatively small. However, the N(0) effect could not be eliminated either by simple shifting of the time-kill curves obtained at higher N(0)s by the difference between the higher and lowest N(0) or by operating with I(E)s determined within the N(0)-adopted upper limits of bacterial numbers (I(E)′s). By using multivariate correlation and regression analyses, linear relationships between I(E) and log AUC/MIC and log N(0) related to the respective mean values [(log AUC/MIC)(average) and (log N(0))(average)] were established for both trovafloxacin and ciprofloxacin against each of the strains (r(2) = 0.97 to 0.99). The antimicrobial effect may be accurately predicted at a given AUC/MIC of trovafloxacin or ciprofloxacin and at a given N(0) based on the relationship I(E) = a + b [(log AUC/MIC)/(log AUC/MIC)(average)] − c [(log N(0))/(log N(0))(average)]. Moreover, the relative impacts of AUC/MIC and N(0) on I(E) may be evaluated. Since the c/b ratios for trovafloxacin and ciprofloxacin against E. coli were much lower (0.3 to 0.4) than that for ampicillin-sulbactam as examined previously (1.9), the inoculum effect with the quinolones may be much less pronounced than with the β-lactams. The described approach to the analysis of the inoculum effect in in vitro dynamic models might be useful in studies with other antibiotic classes.