Bactericidal activities of chloramphenicol and eleven other antibiotics against Salmonella spp

The bactericidal activity of chloramphenicol against 27 strains of Salmonella typhi and 33 strains of S. enteritidis was compared with those of 11 other antibiotics. The geometric mean bactericidal concentrations of chloramphenicol against susceptible strains (36.10 and 43.13 micrograms/ml for S. typhi and S. enteritidis, respectively) far exceeded those of the other 11 antibiotics, with cephalothin having the next highest values (2.67 and 8.66 micrograms/ml) and moxalactam (0.09 and 0.28 micrograms/ml), cefotaxime (0.08 and 0.28 micrograms/ml), ceftriaxone (0.07 and 0.16 micrograms/ml), norfloxacin (0.06 and 0.10 micrograms/ml), and aztreonam (0.05 and 0.20 micrograms/ml) having the lowest values. The results for imipenem (0.24 and 0.81 micrograms/ml) and ceftazidime (0.22 and 0.75 micrograms/ml) were lower than those noted for trimethoprim-sulfamethoxazole (1.20 and 5.56 micrograms/ml), cefamandole (0.62 and 3.29 micrograms/ml), and ampicillin (0.55 and 2.78 micrograms/ml). The MBC of chloramphenicol for some isolates decreased with increased incubation times such that the proportion of susceptible isolates killed by chloramphenicol at concentrations within achievable levels in blood increased from 10% after 24 h to 26% after 48 h of incubation. Although the MBC of the other 11 antibiotics for some isolates were also lowered by prolonged incubation, all 24-h values were within achievable levels in blood. The data indicate that chloramphenicol is not uniformly bacteriostatic against S. typhi and S. enteritidis. The in vivo significance of demonstrating delayed killing by chloramphenicol is, however, uncertain.