Comparative Kill and Growth Rates Determined with Cefdinir and Cefaclor and with Streptococcus pneumoniae and 1-Lactamase- Producing Haemophilus influenzae

The relationship between the growth rate and the kill rate was used to evaluate and to compare the in vitro bactericidal activities of cefdinir, a new oral cephalosporin, and cefaclor against Streptococcus pneumoniae and,-lactamase-producing strains of Haemophilus influenzae. These frequently encountered pathogens of com-munity-acquired respiratory tract infections are usually susceptible to both drugs. The MIC ranges for cefdinir and cefaclor were, respectively, 0.03 to 0.06 and 0.25 to 0.5,ug/ml for S. pneumoniae and 0.25 and 4 to 8,ug/mi for H. influenzae. The colony counts (CFU per milliliter) measured after 6 h of exposure to a range of antibiotic concentrations in broth were plotted against the colony count of the control culture over the same period, of time. Higher kill rates versus bacterial growth rates were noted for S. pneumoniae for both drugs (positive balance). Conversely, lower kill rates versus growth rates were noted for H. influenzae for both drugs (negative balance). In conclusion, the bactericidal activities of both drugs against S. pneumoniae and H. influenzae were similar when expressed by the relationship between the growth rate and the kill rate at 6 h, but cefdinir was more active at lower concentrations. The evaluation of the bactericidal activities of antibiotics against different bacterial species remains an unresolved problem in clinical microbiology. There is considerable controversy about the frequency of antibiotic tolerance (17