In vitro and in vivo activities of newer fluoroquinolones against Vibrio vulnificus

The MICs of six fluoroquinolones as well as minocycline and cefotaxime for 46 clinical isolates of Vibrio vulnificus were determined by the agar dilution method. All the drugs tested had good activities against all isolates, with the MICs at which 90% of the isolates tested were inhibited (MIC₉₀s) by five of the fluoroquinolones ranging between 0.03 and 0.06 μg/ml. The MIC₉₀ of lomefloxacin, on the other hand, was 0.12 μg/ml. Time-kill studies were conducted with these agents and a clinical strain of V. vulnificus, VV5823. When approximately 5 × 10⁵ CFU of V. vulnificus/ml was incubated with any one of the above-mentioned six fluoroquinolones at concentrations of two times the MIC, there was an inhibitory effect on V. vulnificus that persisted for more than 48 h with no noted regrowth. The efficacies of the fluoroquinolones were further evaluated in vivo in the mouse model of experimental V. vulnificus infection and compared to the efficacy of a combination therapy using cefotaxime plus minocycline. With an inoculum of 1.5 × 10⁷ CFU, 28 (87.5%) of 32 mice in the cefotaxime-minocycline-treated group survived and 29 (91%) of the 32 mice in the moxifloxacin-treated group survived while none of the 32 mice in the control group did. With an inoculum of 3.5 × 10⁷ CFU, the difference in survival rates among groups of 15 mice treated with levofloxacin (13 of 15), moxifloxacin (10 of 15), gatifloxacin (10 of 15), sparfloxacin (11 of 15), ciprofloxacin (12 of 15), or lomefloxacin (10 of 15) was not statistically significant while none of the 15 mice treated with saline survived. We concluded that the newer fluoroquinolones as single agents are as effective as the cefotaxime-minocycline combination in inhibiting V. vulnificus both in vitro and in vivo.