The interaction of penicillin and chloramphenicol against meningococci in vitro.

Combinations of penicillin and chloramphenicol are frequently used initially in the treatment of bacterial meningitis. The simultaneous effects against meningococci of these two drugs were examined in vitro in a chemically defined, proteinfree medium. The investigation was performed with different combinations of the antibiotics, including optimal concentrations. In most instances, penicillin and chloramphenicol seemed to have an additive, but not an iso-additive, effect. No antagonism was found, and only one out of nine strains showed indifference.