In vitro activity of moxifloxacin against 179 strains of anaerobic bacteria found in pulmonary infections

The activity of moxifloxacin (BAY 12-8039), a new 8-methoxyquinolone, was d etermined using the NCCLS-approved Wadsworth brucella laked blood agar meth od and compared to the activities of metronidazole, penicillin G, piperacil lin/tazobactam and trovafloxacin. Breakpoints used to define susceptible an d resistant categories were, respectively: less than or equal to 8 and grea ter than or equal to 32 mu g/mL for metronidazole, less than or equal to 2 and greater than or equal to 8 mu g/mL for moxifloxacin and trovafloxacin, less than or equal to 0.5 and greater than or equal to 2 mu g/mL for penici llin G and less than or equal to 32 and greater than or equal to 128 mu g/m L for piperacillin/tazobactam. A total of 179 anaerobic isolates from pulmo nary infections were tested. Piperacillin/tazobactam was the most active an timicrobial, inhibiting 99% of strains at the susceptible breakpoint. Ninet y-seven percent of these isolates were susceptible to moxifloxacin; 96% to trovafloxacin, 89% to metronidazole and 43% to penicillin G. Geometric mean moxifloxacin MIC values for Bacteroides fragilis and the B. fragilis group were 0.5 and 0.8 mu g/mL, respectively. Eighty-eight percent of B, fragili s and 100% of other B. fragilis group species were susceptible to both moxi floxacin and trovafloxacin. All of the strains of B. fragilis and most of t he other B. fragilis group species were resistant to penicillin G. At least 99% of other Bacteroides species, Prevotella, and Fusobacterium strains we re susceptible to moxifloxacin, metronidazole, piperacillin/tazobactam and trovafloxacin (88% were susceptible to trovafloxacin at 2 mu g/mL and all w ere susceptible at 4 mu g/mL). The strains of Clostridium difficile and Clo stridium ramosum found in these specimens were both resistant to penicillin G but susceptible to the other agents. All strains of Peptostreptococcus s pecies were susceptible to all of the agents except penicillin G. Activitie s of the agents against non-spore-forming Gram-positive rods at the interme diate breakpoint were, respectively, moxifloxacin-100%, metronidazole-49%, penicillin G-86%, piperacillin/tazobactam-100%, and trovafloxacin-97%. The promising in vitro activity of moxifloxacin against anaerobic pulmonary iso lates warrants further investigation, including clinical correlation studie s.