In vitro activity of azithromycin, newer quinolones and cephalosporins in ciprofloxacin-resistant Salmonella causing enteric fever

The therapeutic alternatives available for use against ciprofloxacin- resistant enteric fever isolates in an endemic area are limited. The antibiotics currently available are the quinolones, third-generation cephalosporins and conventional first-line drugs. In this study, the MICs of various newer drugs were determined for 31 ciprofloxacin-resistant enteric fever isolates (26 Salmonella enterica serovar Typhi and 5 S. enterica serovar Paratyphi A). MICs for ciprofloxacin, ofloxacin, gatifloxacin, levofloxacin, cefotaxime, cefixime, cefepime and azithromycin were determined using Etest strips and the agar dilution method. By Etest, all of the ciprofloxacin-resistant isolates had ciprofloxacin MICs ≥32 μ ml^-1. S. Typhi showed MIC₉₀ values of 0.50, 0.25 and 0.38 μg ml^-1 for cefixime, cefotaxime and cefepime, respectively. For the cephalosporins, a negligible difference in MIC₉₀ and MIC₅₀ values for S. Typhi and S. Paratyphi A was observed. A single isolate of S. Typhi showed a high azithromycin MIC of 64 μg ml^-1. The MIC₉₀ value for azithromycin in S. Typhi and S. Paratyphi was 24 μg ml^-1. Gatifloxacin demonstrated lower resistance (80.8%) compared with the other quinolones (92-100 %) in S. Typhi. The rise in MIC levels of these antimicrobials is a matter for serious concern.