In Vitro Effect on Group A Streptococci of Loracarbef versus Cefadroxil, Cefaclor and Penicillin V

Abstract

The in vitro activity of loracarbef, penicillin V, cefaclor and cefadroxil against log and stationary phase cultures of group A streptococci was compared. MICs and MBCs were determined with the broth dilution method and by a modified agar plate dilution technique where the beta-lactams were inactivated after the MICs were determined allowing inhibited but not killed organisms to grow on further incubation. The MICs of loracarbef and the two cephalosporins were 16–32 times higher than those of penicillin V. In plate dilution the MBC/MIC ratios of all agents were ≤ 2 for log phase cultures. With stationary phase cultures, especially in the broth dilution test, the MBC/MIC ratios of loracarbef and the two cephalosporins were ≥ 32 for a large number of strains.

The phenotype response of stationary phase cultures to beta-lactam antibiotics may not only be related to the physiological status of the streptococci, to the culture conditions and to the beta-lactam under test. The present investigation indicated that the phenotypic response was also an intrinsic property of certain strains.