Comparison of various in vitro susceptibility methods for testing Stenotrophomonas maltophilia

A total of 57 clinical isolates were screened by disk diffusion for a related pharmacodynamic study. Testing was performed using National Committee for Clinical Laboratory Standards guidelines, except that results were interpreted at 16 to 18 h and 48 h. Of the 57 isolates, 19 were randomly chosen for additional comparative susceptibility testing of five methods (disk diffusion, Etest, Alamar colorimetric broth microdilution, Vitek, and MicroScan) and an in-house broth microdilution method. The two diffusion methods (disk and Etest) had the closest correlation. The commercial broth microdilution methods and the in-house microdilution method generated inconsistent results for all agents except trimethoprim-sulfamethoxazole. Vitek compared poorly with both diffusion and microbroth dilution methods. The most significant discrepancies were evident with all methods when the incubation period was extended to 48 h. When results were interpreted at 48 h, the incidence of resistance for all bactericidal agents was approximately double the resistance observed at 16 to 18 h. The bacteriostatic agents, trimethoprim-sulfamethoxazole and doxycycline, demonstrated the greatest in vitro activity and were least influenced by extended incubation with diffusion methods. Because correlative in vivo and in vitro studies have not revealed an effective therapeutic regimen for serious S. maltophilia infections, susceptibility results with all testing methods should be interpreted with caution when choosing therapy for patients with life-threatening infections. Susceptibility testing for this heterogeneous group remains controversial and routine testing, with the possible exception of doxycycline (or minocycline) and trimethoprim-sulfamethoxazole, should be avoided. Our data support that if testing is done with bactericidal agents, consideration should be given to interpretation after 48-h incubation.