Combination effect of ciprofloxacin and gentamicin against clinical isolates of Salmonella enterica serovar typhi with reduced susceptibility to ciprofloxacin.

The present study evaluated the in vitro efficacy of ciprofloxacin (CPFX) in combination with gentamicin (GM) using agar dilution checkerboard method against six blood culture isolates of Salmonella enterica serovar Typhi with CPFX minimum inhibitory concentration (MIC) values of 0.75 - 1.25 microg/ml and GM MIC values of 0.75 - 2 microg/ml. When used in combination, the fractional inhibitory concentration (FIC) values of CPFX and GM for the isolates ranged from 0.008 - 0.032 microg/ml and 0.1 - 0.2 microg/ml, respectively. The range of the FIC index from 0.121 - 0.216 indicated the synergistic effect between CPFX and GM for all the isolates. The time-kill method, which showed a 2.64 log(10) decrease in CFU/ml between the combination and its more active compound, also established synergism between CPFX and GM against one isolate employed in the method. These results may be helpful in making clinical decisions in the treatment of enteric fever due to the infection of multidrug resistant S. enterica serovar Typhi.