Effects of changes in pH, medium, and inoculum size on the in vitro activity of amifloxacin against urinary isolates of Staphylococcus saprophyticus and Escherichia coli.

The in vitro activity of amifloxacin (WIN 49375), a new fluoroquinolone, was compared with the activities of antimicrobial agents that are commonly used for the treatment of urinary tract infection (cinoxacin, cephalexin, gentamicin, amoxicillin, trimethoprim, and trimethoprim-sulfamethoxazole) against 25 strains of Staphylococcus saprophyticus and 28 strains of Escherichia coli. Bacterial strains were isolated from urine specimens of college women with acute urinary tract infections. Bacterial isolates were more susceptible to trimethoprim-sulfamethoxazole, trimethoprim, and amifloxacin than to the other drugs tested. The in vitro activity of amifloxacin against S. saprophyticus had an inverse relation to increases in the pH of the test medium. Changes in the type of culture medium had no effect on the in vitro activity of amifloxacin. There was a direct relationship between increases in inoculum size and the MICs of amifloxacin.