In vitro comparison of ampicillin-chloramphenicol and ampicillin-cefotaxime against 284 Haemophilus isolates

Since November 1982 at the Sainte-Justine Hospital in Montreal, ampicillin and cefotaxime were used in association as initial treatment (greater than or equal to 48 h) for childhood bacterial meningitis. In this report is described the in vitro interaction of the new regimen in comparison with that of the previous ampicillin-chloramphenicol combination against 284 Haemophilus isolates. Among the 156 ampicillin-susceptible, beta-lactamase-negative isolates, synergy was detected in 13 with ampicillin-cefotaxime, and antagonism was detected in only 1; in contrast, synergy was found in only 2 strains with ampicillin-chloramphenicol, and antagonism was found in 15. These differences were statistically significant (P less than 0.01). Such significant differences were not observed among the 128 ampicillin-resistant, beta-lactamase-positive Haemophilus isolates. The synergy of ampicillin-cefotaxime did not contribute to a decrease of the MIC of cefotaxime for 90% of isolates tested, whereas the antagonism of ampicillin-chloramphenicol did not contribute to increase the MIC of ampicillin for 90% of isolates tested.