Comparison of the bactericidal activity of cefotaxime and desacetylcefotaxime alone and in combination against Bacteroides fragilis group organisms.

Through the use of time-kill kinetic studies, the bactericidal activity of cefotaxime (CTX) and desacetylcefotaxime (dCTX) alone and in combination against 18 strains of Bacteroides fragilis group was studied. Each isolate was tested at subinhibitory, inhibitory, and suprainhibitory concentrations of each drug as determined from the MIC values. Overall CTX was more bactericidal than dCTX at each of the three concentration levels tested. The combination of CTX and dCTX showed comparable bactericidal activity to CTX at the subinhibitory and inhibitory concentrations, even though each component was present at only one-half the concentration of CTX alone. At suprainhibitory concentrations, the combination of CTX/dCTX appeared synergistic since the combination with each component at a concentration of 1 x MIC was as bactericidal as CTX at a concentration of 4 x MIC. CTX and dCTX alone and in combination exhibited comparable bactericidal activity against test isolates with high (greater than or equal to 32 micrograms/ml) or low (less than or equal to 16 micrograms/ml) MICs. Thus, in vitro the combination of CTX and its naturally occurring metabolite dCTX interacts to produce an additive or synergistic effect against strains of B. fragilis group. Whether the in vitro testing of the combinations is more relevant to clinical outcome than testing CTX alone needs further study.