SUSCEPTIBILITY OF 539 GRAM-POSITIVE AND GRAM-NEGATIVE ANAEROBES TO NEW AGENTS, INCLUDING RP59500, BIAPENEM, TROSPECTOMYCIN AND PIPERACILLIN TAZOBACTAM

Susceptibilities of 539 Gram-positive and Gram-negative anaerobes were tested by agar dilution against 15 new and existing antimicrobial agents. Organisms included 218 Bacteroides fragilis group strains, 15 non-fragilis group Bacteroides, 130 Porphyromonas/Prevotella, 49 fusobacteria, 50 peptostreptococci, 53 clostridia and 24 Gram-positive non-sporeforming bacilli. Of 412 Gram-negative bacilli, 89% were β-lactamase-positive, while only two of the Gram-positive strains (both clostridia) produced this enzyme. Using established and preliminary breakpoints, all strains were susceptible to biapenem and imipenem (MIC90s 1 mg/L) and chloramphenicol (MIC90 8 mg/L). Only one of all the strains tested (a Cl. innocuum) was resistant (MIC > 4 mg/L) to RP59500; the latter had MIC90 2 mg/L, while 98% of strains were susceptible to trospectomycin (MIC90 16 mg/L). Ninety-nine per cent of strains were susceptible to piperacillin/tazobactam (MIC90 8 mg/L) compared to 86% to piperacillin (MIC90 > 64 mg/L). Corresponding data for ticarcillin/clavulanate versus ticarcillin were 97% susceptible (MIC90 8 mg/L) compared to 83% (MIC90 > 64 mg/L). Enhancement of the β-lactam by the inhibitors was only seen in β-lactamase-producing strains. Amoxycillin and cefoperazone were less often active (36% susceptible, MIC90 > 256 mg/L and 66% susceptible, MIC90 64 mg/L, respectively). Cefoxitin had greater activity than cefotetan (90% susceptible, MIC90 32 mg/L, compared to 72% susceptible, MIC90 > 64 mg/L). Metronidazole was active against 94% of strains (MIC90 4 mg/L). All metronidazole-resistant strains were Gram-positive (75% of non-sporeforming bacilli, 9% of clostridia and 6% of peptostreptococci). Ninety per cent of strains were susceptible to clindamycin (MIC90 4 mg/L). © 1993 The British Society for Antimicrobial Chemotherapy.