In vitro studies with cefaclor, a new oral cephalosporin.

In vitro studies were performed to evaluate the activity of cefaclor in comparison with cephalexin against 180 clinical isolates. Broth dilution susceptibility tests showed cefaclor to be 4- to 16-fold more active than cephalexin against Streptococcus pneumoniae, Haemophilus influenzae, and cephalothin-susceptible Enterobacteriaceae. Neither drug was highly active against cephalothin-resistant Enterobacteriaceae or methicillin-resistant Staphylococcus aureus. Cefaclor zones with 30-mug disks were generally larger than cephalexin zones, 4 mm larger than cephalothin zones against Enterobacteriaceae, and 6 mm smaller than cephalothin zones against S. aureus. Quantitative kill curves indicated that killing by both cefaclor and cephalexin was slow and often incomplete over a 24-h period. Cefaclor-induced filamentation of gram-negative bacilli was not as extensive as that produced by cephalexin, and some spherule formation did occur. However, cefaclor was significantly more unstable in solution than cephalexin, with a half-life of less than 6 h at 37 degrees C. Thus, results obtained in tests after prolonged incubation may not provide an accurate measure of cefaclor's activity.