In vitro antimicrobial activity and susceptibility testing of ofloxacin. Current status

The fluoroquinolone, ofloxacin, exhibits a broad antibacterial spectrum. Based on our data and a review of the literature, ofloxacin inhibited essentially 100 percent of staphylococci, including oxacillin-resistant strains, Haemophilus influenzae, Neisseria spp. and Branhamella catarrhalis. Ninety-five percent of Enterobacteriaceae were susceptible to ofloxacin. Pseudomonas aeruginosa and enterococci were less susceptible: 79 and 63 percent, respectively, were susceptible (minimal inhibitory concentration [MIC], less than or equal to 2.0 micrograms/ml); 15 and 25 percent, respectively, were intermediate (MIC, 4 micrograms/ml); 3 and 6 percent, respectively, were resistant (MIC, more than or equal to 4 micrograms/ml). Clostridium spp., including Clostridium difficile, were resistant to ofloxacin, but other anaerobic species, including the Bacteroides fragilis group (over 90 percent) were either susceptible or intermediate to ofloxacin. Ofloxacin is bactericidal; minimal bactericidal concentrations of ofloxacin rarely exceed the MICs by one doubling concentration. A modest inoculum effect has been observed with ofloxacin: MICs with inocula of 10(7) colony-forming units/ml are often two- to fourfold higher than those with inocula of 5 x 10(5) colony-forming units/ml. Susceptible organisms exposed serially to increasing concentrations of nalidixic acid developed increasing resistance to not only nalidixic acid, but also to all other quinolones, including ofloxacin. Correlations of disk diffusion inhibitory zone diameters using the 5-micrograms ofloxacin disk with ofloxacin MICs have been performed, and the disk diffusion zone diameter breakpoints recommended are: susceptible, greater than or equal to 16 mm; intermediate, 13 to 15 mm; resistant, less than or equal to 12 mm. Quality control parameters also are summarized.