Effect of method, medium, pH and inoculum on the in-vitro antibacterial activities of fleroxacin and norfloxacin.

The comparative in-vitro activities of fleroxacin and norfloxacin were determined by standard agar dilution and broth micro-dilution methods in four different culture media, varying the pH of the medium (pH 5 to pH 8) and the inoculum size. Four inoculum densities equivalent to approximately 10(3), 10(4), 10(5) and 10(6) colony forming units per well or spot were prepared for each of five reference test strains (Escherichia coli ATCC 25922, E. coli ATCC 35218, Staphylococcus aureus ATCC 25923, S. aureus ATCC 29213 and Pseudomonas aeruginosa ATCC 27853). The antibacterial activity of fleroxacin was similar overall to that of norfloxacin and rather uniform. The influence of the test conditions was moderate. However, the activity of both quinolones was uniformly lower at pH 5 in urine and against a high inoculum of 10(6) cfu/spot or well of S. aureus.