Cefotaxime (CTX) and
desacetylcefotaxime (des-CTX) alone and in combination were tested against anaerobic bacteria collected from clinical
infections from several geographically diverse medical centers. The CTX minimum inhibitory concentration (MIC) inhibiting 50% of tested Bacteroides fragilis strains was in the moderately susceptible range (32 micrograms/ml), but when placed in combination with des-CTX it had a potency compatible to
cefoxitin (MIC50, 8.0 micrograms/ml). Other B. fragilis group species (B. distasonis and B. vulgatus) were also susceptible to CTX and des-CTX alone at the MIC50 level. MIC90 statistics for CTX,
cefoxitin, and
ticarcillin were generally in the resistant range. Synergy studies showed that 80% of tested anaerobes were synergistically killed by the combination of CTX and des-CTX. Most of these strains had their synergy occur at
drug levels that could be achieved in vivo. A large number of the B. thetaiotaomicron strains must be considered resistant to the combination because of the very high levels of des-CTX required to produce synergistic killing. Other drugs routinely used for anaerobic
infections (
clindamycin,
chloramphenicol, and
metronidazole) also had elevated B. thetaiotaomicron MICs. Endemic difference in susceptibility to the
beta-lactam drugs were observed, especially the CTX-des-CTX combination. The combination and other
beta-lactams were most usable for strains isolated from the Portland metropolitan area, were moderately active against those from Cleveland, and were rarely usable on Bacteroides isolates at Northwestern in Chicago. Laboratories are urged to monitor the
cephalosporin and semisynthetic
penicillin in vitro efficacy and not rely on published statistics. Staphylococcus aureus strains were susceptible to CTX alone, but were even more susceptible (two- to fourfold reduction in MICs) when used in combination with des-CTX. These data show CTX to be the most active antistaphylococcal compound among the new
cephalosporins and to be comparable to
cefamandole and
cefuroxime, but superior to the anaerobe-active
cefoxitin.