[Post antibiotic, post beta-lactamase inhibitor and post antibiotic sub-MIC effect of ceftriaxone/tazobactam on beta-lactamase-producing Escherichia coli in vitro].

Objective To study post antibiotic effect(PAE),post β-lactamase inhibitor effect(PLIE) and post antibiotic sub-MIC effect(PASME) of ceftriaxone/tazobactam on β-lactamase-producing Escherichia coli in vitro.Methods The minimal inhibitory concentration(MIC) of ceftriaxone/tazobactam against 4 types of β-lactamase producing E.colis strains,was measured by two-fold agar dilution method.The numbers of CFU on plates were counted by micro-inoculation colony counting method.The growth kinetics curves of the bacteria were drawn according to CFU counts,from which the PAE,PLIE and PASME were calculated.Results The MIC of the ceftriaxone/tazobactam combination was eight times more than ceftriaxone alone.No longer PAE(-0.59-0.85 h) was found in the ceftriaxone/tazobactam combination or any of them alone.The PLIE and PASME varied according to the type of β-lactamase but similar results were observed for the strains producing the same type β-lactamase.All PLIEs(0.62-3.22 h) and most PASMEs(0.12-5.61 h) were longer than PAEs.The lower MIC of ceftriaxone the strain had,the longer the PAE,PLIE and PAMSE were.Conclusion The duration of PLIE and PASME may concerned with the type of β-lactamase.With both longer PLIE and PASME,longer dosing interval should be recommended.The PAE,PLIE and PASME provide an important instrument for pharmacodynamic studies of antibiotics,in particular for the design of dosing schedules.