Activity of difloxacin (A-56619) and A-56620 against clinical anaerobic bacteria in vitro.

We determined the MICs of difloxacin (A-56619) and A-56620 against anaerobic bacteria and assessed the effects of alterations in pH, size of inoculum, addition of human serum, and repeated exposure to subinhibitory levels of antibiotics. We tested for synergism of these drugs with cefoxitin against Bacteroides spp. We found that difloxacin and A-56620 were as active as ciprofloxacin, inhibiting about 90% of B. fragilis (4 micrograms/ml) and other Bacteroides spp. (8 micrograms/ml), A-56620 being more active than difloxacin against several isolates. Increased inoculum size and changes in pH did not alter their MICs much, but human serum and exposure to sub-MICs of antibiotic effected increases in their MICs against Bacteroides spp. Cefoxitin was not synergistic with either difloxacin or A-56620 against B. fragilis.