Activity of moxifloxacin against Bacteroides fragilis and Escherichia coli in an in vitro pharmacokinetic/pharmacodynamic model employing pure and mixed cultures.

The objective of this study was to determine the pharmacodynamic (PD) activity of moxifloxacin against four selected Bacteroides fragilis strains (three strains with low MICs and one strain with a high MIC) and two Escherichia coli strains (one strain with a low MIC and one strain with a high MIC) in a pharmacokinetic (PK) in vitro model in pure cultures as well as in mixed cultures. PK/PD assays of moxifloxacin were carried out with an initial maximum concentration of 4.0 mg l-1 and a half-life of 13 h. The E. coli strain with the low MIC was rapidly killed in both pure and mixed cultures in the in vitro PK/PD model, while the E. coli strain with the high MIC was not killed. None of the B. fragilis strains were rapidly killed in pure or mixed cultures. The bacterial numbers of the B. fragilis strains with low MICs were reduced by about one to two logs after 12 h in pure cultures. The presence of an E. coli strain with a low or a high MIC in the mixed culture reduced this effect even further.