Bactericidal effect of levofloxacin on strains with equal susceptibility in an in vitro urinary bladder model

The in vitro bactericidal effects of therapeutic concentrations of levofloxacin (LVFX) on clinical isolates of Pseudomonas aeruginosa and Enterococcus faecalis were assessed. Starting from 10(7) CFU/ml, the bacterial count was determined serially during the exposure to LVFX present at concentrations automatically simulated to its clinically achievable urinary levels at the recommended dosage (100 mg 3 times daily). An isolate of P. aeruginosa for which the MIC of LVFX was 16 microgram/ml was completely eradicated after 10 h of exposure to LVFX. On the other hand, 40 h of exposure to LVFX were required for complete eradication of an isolate of E. faecalis with equal susceptibility, although the bacterial count was rapidly reduced to 10(2) CFU/ml in the early phase of the exposure. It was concluded that LVFX exerts a more potent bactericidal effect on P. aeruginosa than on E. faecalis despite a similar susceptibility of these organisms to LVFX.