Sulfamethoxazole-trimethoprim (SXT) is the drug-of-choice in Stenotrophomonas maltophilia caused infections. There has been an increase in resistance to SXT of S. maltophilia over recent years. In this study 30 S. maltophilia clinical isolates resistant to SXT were investigated. Antibiotic susceptibilities for ciprofloxacin, moxifloxacin, levofloxacin, doxycycline, tigecycline, ceftazidime, colistin and chloramphenicol were determined by broth microdilution method. None of the strains were susceptible to ciprofloxacin, tigecycline, ceftazidime or colistin. Only 37% of the isolates were susceptible to levofloxacin or moxifloxacin. Two isolates resistant to all tested antibiotic agents and two others susceptible only to doxycycline were further investigated: susceptibility for combinations of antibiotics was analyzed by checkerboard technique. According to the fractional inhibitory concentration indices calculated, moxifloxacin plus ceftazidime combination was found to be synergistic in each case. Genetic testing revealed the predominance of sul1 gene. Our study concluded that the range of effective antibiotic agents is even more limited in infections caused by SXT-resistant S. maltophilia. In these cases, in vitro synergistic antibiotic combinations could be potential therapeutic options.
Keywords: Stenotrophomonas maltophilia; antibiotic resistance; sulfamethoxazole-trimethoprim resistance; synergism.