An optimized mouse thigh infection model for enterococci and its impact on antimicrobial pharmacodynamics

Abstract

Negligible in vivo growth of enterococci and high-level dispersion of data have led to inaccurate estimations of antibiotic pharmacodynamics (PD). Here we improved an in vivo model apt for PD studies by optimizing the in vitro culture conditions for enterococci. The PD of vancomycin (VAN), ampicillin-sulbactam (SAM), and piperacillin-tazobactam (TZP) against enterococci were determined in vivo, comparing the following different conditions of inoculum preparation: aerobiosis, aerobiosis plus mucin, and anaerobiosis plus mucin. Drug exposure was expressed as the ratio of the area under the concentration-time curve for the free, unbound fraction of the drug to the MIC (fAUC/MIC) (VAN) or the time in a 24-h period that the drug concentration for the free, unbound fraction exceeded the MIC under steady-state pharmacokinetic conditions (fT(>MIC)) (SAM and TZP) and linked to the change in log10 CFU/thigh. Only anaerobiosis plus mucin enhanced the in vivo growth, yielding significant PD parameters with all antibiotics. In conclusion, robust in vivo growth of enterococci was crucial for better determining the PD of tested antibacterial agents, and this was achieved by optimizing the procedure for preparing the inoculum.

FIG 1

Impacts of atmosphere of incubation, mucin supplementation, and immune status on the in vivo growth of E. faecalis ATCC 29212. In all panels, solid lines represent significant and valid regressions using Gompertz's equation, and dotted lines indicate that the regression did not fit or failed the presumptions of normality or homoscedasticity (see Table S1 in the supplemental material). (a) Growth in neutropenic mice infected with inocula prepared under conditions of anaerobiosis (dotted gray line) or anaerobiosis plus mucin (solid black line). (b) Growth in neutropenic mice infected with inocula prepared under conditions of aerobiosis (dotted gray line) or aerobiosis plus mucin (black dotted line). (c) In vivo growth curves for immunocompetent animals infected with inocula prepared under conditions of anaerobiosis plus mucin (solid black line) or anaerobiosis (gray interrupted line). In panel c, data points at 26 h (black) and 14 and 26 h (gray) overlap.

FIG 2

Pharmacodynamics of vancomycin (a), piperacillin-tazobactam (b), and ampicillin-sulbactam (c) in the following groups: aerobiosis (gray triangles and lines), aerobiosis plus mucin (red circles and lines), and anaerobiosis plus mucin (black squares and lines). For the aerobiosis group (gray triangles), no antibiotic fit Hill's equation in terms of fAUC/MIC (VAN) or fT_>MIC (SAM and TZP) against E. faecium ATCC 19434 or E. faecalis ATCC 29212.