In vitro synergy of ampicillin and chloramphenicol against gram-negative bacteria.

The activity of ampicillin and chloramphenicol in combination was evaluated against 16 gram-negative bacterial isolates from the cerebrospinal fluid of neonates with meningitis. The combination of antibiotics was synergistic (fractional inhibitory concentration less than 1.0) against 11 of 16 (69%) isolates by agar dilution technique and 12 of 16 (75%) isolates by microbroth dilution technique. In kinetic studies, ampicillin and chloramphenicol together exhibited an increased rate of killing against 9 of 14 isolates. Antagonistic activity between ampicillin and chloramphenicol was not demonstrated in any of the studies. The clinical evaluation of ampicillin and chloramphenicol together in the treatment of gram-negative neonatal meningitis deserves further study.