Effect of ampicillin and chloramphenicol against Streptococcus pneumoniae and Neisseria meningitidis.

Antagonism, determined by isobolograms constructed from data from combinations of ampicillin and chloramphenicol at or below the minimal inhibitory or bactericidal concentrations, was observed against 13 clinical isolates of meningococcus and against one isolate of pneumococcus. Synergy occurred against six strains of pneumococcus and three of meningococcus. Additive effects were noted against 14 isolates of pneumococcus and 5 of meningococcus. There was no relationship between the minimal inhibitory or bactericidal concentrations for the isolates and the occurrence of antagonistic, additive, or synergistic effects. These data indicate that ampicillin and chloramphenicol may be antagonistic in vitro against some strains of pneumococcus or meningococcus.