In Vitro Synergy Evaluation of Trimethoprim/Sulfamethoxazole Combined with Levofloxacin and Ceftazidime Against Stenotrophomonas maltophilia: A Comparative Study Using Checkerboard and Gradient Diffusion Methods

Stenotrophomonas maltophilia is a nosocomial pathogen that is resistant to many broad-spectrum antibiotics. This study aimed to evaluate the in vitro synergy of trimethoprim/sulfamethoxazole (SXT) combined with levofloxacin (LEV) or ceftazidime (CAZ) using checkerboard and gradient diffusion methods. Between 2016 and 2021, 20 S. maltophilia strains (five SXT-resistant and 15 SXT-susceptible strains) were collected from various clinical settings. Their susceptibility to SXT, LEV, and CAZ was assessed using both checkerboard and gradient diffusion synergy tests. The gradient diffusion method was performed using commercial strip-based tests (Liofilchem®). It should be noted that the gradient diffusion method has not been standardized by the Clinical and Laboratory Standards Institute (CLSI) for synergy testing and is considered for research purposes only. In the checkerboard method, the SXT + LEV combination showed synergy in one strain and an additive effect in 19 strains; the SXT + CAZ combination exhibited synergy in eight strains and an additive effect in 12 strains. In the gradient diffusion method, the SXT + LEV combination showed synergy in one strain and an additive effect in 19 strains; the SXT + CAZ combination exhibited synergy in five strains, an additive effect in 14 strains, and antagonism in one strain. A correlation between the two methods was observed in 90% of SXT + LEV combinations and 65% of SXT + CAZ combinations. Both checkerboard and gradient diffusion methods yielded similar results, indicating their reliability in determining antibiotic combinations. Given the observed synergy, CAZ combinations may be effective for treating SXT-resistant strains.