In vitro synergism between cefotaxime and minocycline against Vibrio vulnificus

We conducted time-kill studies to evaluate the inhibitory activities of either cefotaxime or minocycline alone and the two drugs in combination against a clinical strain of Vibrio vulnificus. The MICs of cefotaxime and minocycline were 0.03 and 0.06 μg/ml, respectively. When approximately 5 x 10⁵ CFU of V. vulnificus per ml was incubated with cefotaxime at 0.03 or 0.05 μg/ml, the bacterial growth was inhibited during the initial 2 and 8 h, respectively. Thereafter, V. vulnificus regrew and the level of growth reached that of the control. Within the dose range of less than five times the MIC, the duration of the inhibitory effect of cefotaxime was proportional to its concentration. When minocycline at 0.015, 0.03, 0.045, and 0.06 μg/ml was used to evaluate the inhibitory effect, a similar trend was observed. Either antibiotic at a concentration of five times the MIC or greater prevented the regrowth of V. vulnificus for at least 48 h. When cefotaxime at 0.05 μg/ml and minocycline at 0.045 μg/ml were combined in the same culture, the inhibitory effect against V. vulnificus persisted for more than 48 h, with no regrowth noted. The use of a combination of these two antibiotics resulted in the reduction of growth by 6 orders of magnitude compared to the use of either of the two antibiotics alone, and the number of surviving organisms in the presence of the antibiotics combined was approximately 3 orders of magnitude less than that in the starting inoculum. We conclude that cefotaxime and minocycline acted synergistically in inhibiting V. vulnificus in vitro.