[Growth curves and killing rate of Haemophilus influenzae exposed to amoxicillin, cefaclor and cefotaxime].

The antibacterial activity of beta-lactams against H. influenzae is difficult to evaluate as a result of the osmotic-pressure-dependent formation of filaments and spheroplasts whose viability is still under debate. We compared growth curves obtained by optic density measurements and by UFC/ml counts for H. influenzae in the presence of amoxicillin, cefaclor and cefotaxime in various concentrations. Only the early response to antibiotics, observed during the first six hours of antibiotic-culture contact, was considered. Results showed, for each of the three antibiotics: an increase in optic density with formation of abnormal organisms that correlated poorly with antibiotic concentrations; a stable number of UFC/ml for more than 90 mn, followed by a slow fall reaching at the most 1.5 Log 10 UFC/ml at the sixth hour. We conclude that amoxicillin, cefaclor and cefotaxime in active concentrations rapidly produce the formation of abnormal organisms with an increase in biomass as a result. These abnormal organisms lose their viability slowly, even with concentrations greater than 50 time the MIC. Cefaclor's MIC and MBC are underestimated when results are read only after 24 hours.