In vitro antibacterial activity of moxifloxacin against hospital isolates: a multicentre study

To evaluate the in vitro antibacterial activity of moxifloxacinin in comparison to that of other fluoroquinolones (ciprofloxacin, ofloxacin and trovafloxacin).

A total of 2,196 strains was collected in 11 French hospitals in 1998. Minimum inhibitory concentrations (MICs) (mg/L) were determined by agar dilution and agar diffusion was performed with 5-microg discs. Internal quality control was carried out with genetically defined strains.

MIC50s and MIC90s of moxifloxacin against nalidixic acid (NAL)-susceptible Enterobacteriaceae (n = 663) were 0.12 and 0.5. As for other quinolones, the activity of moxifloxacin (4-32) was reduced against NAL-intermediate and NAL-resistant strains (n = 222). MIC50s and MIC90s of moxifloxacin were 2 and 4 for ciprofloxacin-susceptible P. aeruginosa (n = 128); moxifloxacin had no activity against ciprofloxacin-resistant strains (n = 56). The activity of moxifloxacin was maintained against NAL-susceptible A. baumannii (n = 11; 0.032-0.125), but reduced against NAL-resistant strains (n = 30; 16-32). H. influenzae (n = 97) and M. catarrhalis (n = 40) were inhibited by low concentrations (0.03-0.06 and 0.06-0.25, respectively). Moxifloxacin had better activity (0.06-0.12) than other tested quinolones against methicillin-susceptible S. aureus strains (n = 110); ciprofloxacin-resistant strains (n = 85) (2-8) were usually methicillin-resistant. Moxifloxacin was moderately active against enterococci (n = 149) (E. faecalis: 0.5-16; E. faecium: 2-4). Streptococci (n = 194) and pneumococci (n = 136), including 70 penicillin G-intermediate or G-resistant strains, were inhibited by low concentrations (0.25-0.5 for each species). Based on the regression curve, tentative zone diameter breakpoints could be > or =21 and <18 mm for MIC breakpoints of < or =1 and >2 mg/L, respectively.

While retaining activity against Enterobacteriaceae, moxifloxacin was moderately active against P. aeruginosa. Its activity was inferior to that of ciprofloxacin for these species. This study confirmed the comparatively high in vitro activity of moxifloxacin against Gram-positive cocci and other pathogens isolated from community-acquired respiratory tract infections.