In vitro activity of antimicrobial combinations of stenotrophomonas maltophilia strains by gradient diffusion and checkerboard method

Samples sent from various clinics and services to the Bacteriology Laboratory of Bursa Uludağ University Health Practice and Research Center Medical Microbiology Laboratory Department between 2016-2021 were isolated and Stenotrophomonas maltophilia was identified with MALDI-TOF MS. 20 S. maltophilia strains, 6 of which were trimethoprim+sulfamethoxazole resistant, whose antibiotic susceptibility was studied with automated systems, were included in the study. In the study, the strains; Their susceptibility to trimethoprim+sulfamethoxazole, levofloxacin, and ceftazidime antibiotics and combinations between these antibiotics (trimethoprim+sulfamethoxazole/levofloxacin, trimethoprim+sulfamethoxazole/cefta zidime) were made in vitro using gradient diffusion and checkerboard method synergy tests. Trimethoprim+sulfamethoxazole resistance of the studied S. maltophilia strains was (microdilution and gradient diffusion method) 30% compatible in broth methods; levofloxacin resistance is 40% in microdilution method, 25% in gradient diffusion method; ceftazidime resistance was found to be 80% in the microdilution method and 65% in the gradient diffusion method. In the checkerboard method, trimethoprim+sulfamethoxazole/levofloxacin combination was 5%, trimethoprim+sulfamethoxazole/ceftazidime combination was 40%; In the gradient diffusion method, the combination of trimethoprim+sulfamethoxazole/levofloxacin was 5%, and the combination of trimethoprim+sulfamethoxazole/ceftazidime was 25% synergistic. 1 strain showed antagonism in trimethoprim+sulfamethoxazole/ceftazidime combination. For the 20 strains studied, both combinations were superior to use alone. Since there is no statistically significant difference between checkerboard and gradient diffusion methods for trimethoprim+sulfamethoxazole, levofloxacin and ceftazidime, two methods can be preferred to determine their sensitivities and combinations. Due to the changing resistance rates, each institution should monitor its own antimicrobial resistance rates, and attention should be paid to risk factors and environmental reservoirs in order to reduce transmission. There is a need to study combination tests with methods that provide more standardization in S. maltophilia strains.