In Vitro Activities of Antimicrobial Combinations against Clinical Isolates of Stenotrophomonas Maltophilia

Background and Purpose: Stenotrophomonas maltophilia, a major pathogen causing nosocomial infection, is inherently resistant to multiple antimicrobial agents. Evaluation of the effectiveness of recommended therapeutic options for S. maltophilia infections is crucial, particularly in areas with high antimicrobial resistance in this nosocomial pathogen. Methods: The in vitro activities of ceftazidime ( CAZ), ticarcillin- clavulanate (TIM), amikacin (AN), ciprofloxacin (CIP), and trimethoprim- sulfamethoxazole (TMP- SMZ) against 102 clinical isolates of S. maltophilia collected from January 1998 to December 1999 at a university hospital were evaluated. The disk diffusion and agar dilution susceptibilities of individual agents against these isolates were determined concomitantly. Errors between results obtained by the two methods were identified based on the guidelines for Acinetobacter species provided by the National Committee for Clinical Laboratory Standards. Activities of three two-drug combinations (AN+CIP, CAZ+CIP, and TIM+TMP- SMZ) against 32 of these isolates were analyzed using the checkerboard synergy test. Results. Among the agents tested, TMP-SMZ was the most active against S. maltophilia (83.3% susceptible), followed by CIP (63.7%) , CAZ (39.2%), TIM (36.2%), and AN (20.5%). Errors (very major and major) between the results obtained by the disk diffusion and agar dilution methods occurred at a high frequency for AN (15% and 3%), CAZ (8% and 6%), and CIP (3% and 3%). Synergy or partial synergy of antimicrobial agent combinations was detected predominantly with CAZ+CIP (81.3%) and TIM+TMP- SMZ (84.4%) but not with AN+CIP (37.5%). No antagonism was detected with any drug combinations. Conclusion: The dilution method is preferable to the disk diffusion method for susceptibility testing of S. maltophilia isolates, particularly for testing with AN, CAZ, and TIM, which have considerable error rates between the results obtained by the two methods. The findings from the synergy test suggest that TIM+TMP-SMZ and CAZ+CIP combinations are the treatments of choice for infections caused by S. maltophilia