Discrepancies between various methods in susceptibility testing and epidemiological analysis of Stenotrophomonas maltophilia clinical isolates.

The susceptibility of 25 Stenotrophomonas maltophilia (S. maltophilia) clinical isolates to four different antimicrobials (trimethoprim/sulfomethoxazole, piperacillin/tazobactam, ceftazidime, ciprofloxacin) were investigated by disk diffusion, E-test and commercial Sensititre and PASCO broth microdilution techniques. Discrepancies between the results of broth microdilution and the other methods studied were characterized as very major, major and minor errors. Using the broth microdilution as the reference method, 24% of the isolates were found susceptible to trimethoprim/ sulfamethoxazole, 24% to ceftazidime, 0% to piperacillin/tazobactam and 12% to ciprofloxacin. Good correlation was observed between the two broth microdilution Sensititre and PASCO for all antibiotics tested. Disc diffusion and E-test generated inconsistent results for all agents except trimethoprim/sulfamethoxazole. A great genomic diversity was demonstrated within the S. maltophilia strains tested. Although our results confirm that trimethoprim-sulfamethoxazole had some in vitro activity against S. maltophilia, further clinical studies are necessary to evaluate the clinical efficacy of these compounds for the treatment of S. maltophilia infections, since no randomized controlled trials have been carried out and no correlation between the clinical response and susceptibility testing results has been reported. Furthermore, the high genomic diversity observed in the S. maltophilia strains indicates the need for careful epidemiological evaluation especially in nosocomial outbreaks.