Stenotrophomonas maltophilia: Results of Antimicrobial Susceptibility Testing and In Vitro Activity of the Combination of Ceftazidime and Moxifloxacin/Stenotrophomonas maltophilia: Antimikrobik Duyarlilik Testi Sonuclari ve Seftazidimin Moksifloksasinle Kombinasyonunun In Vitro Etkinligi

Objective: Recommended combination for the treatment of serious Stenotrophomonas maltophilia infections is ticarcillin-clavulanate and co-trimoxazole (SXT). However, first agent is not available in our country, and the second component may be a matter of antimicrobial resistance or intolerance. Therefore, we aimed to evaluate antimicrobial susceptibility testing results of S. maltophilia and in vitro activity of ceftazidime and moxifloxacin as a potential therapeutic alternative. Methods: S. maltophilia strains isolated from clinical samples in Infectious Diseases and Clinical Microbiology Laboratory between 1 October 2007-23 November 2017 were included in the study. Disk diffusion antimicrobial susceptibility test results of the strains were evaluated retrospectively. The in vitro activity of combination of ceftazidime and moxifloxacin against 24 of these strains was investigated by Etest[R]. Results: During the study period, 649 S. maltophilia strains were isolated from 649 different patients, and 94%, 93%, 92%, 81%, 60%, 55%, 45%, 41%, 38% of the strains were susceptible to tigecycline, moxifloxacin, SXT, ciprofloxacin, cefoperazone-sulbactam, ceftazidime, netilmicin, gentamicin and amikacin, respectively. For the 24 strains included in the combination study, the moxifloxacin minimal inhibitory concentration [(MIC).sub.50]/[MIC.sub.90] values were defined as 0.064/0.125 [micro]g/mL (MIC range 0.023-4 [micro]g/mL) and ceftazidime [MIC.sub.50]/[MIC.sub.90] values were defined as 32/256 [micro]g/mL (MIC range 1.5-256 [micro]g/mL). According to the results of fractional inhibitor concentration (FIC) index; ceftazidime and moxifloxacin combination displayed synergism, additivity and indifference for 1 (4%), 18 (75%) and 5 (21%) strains, respectively and there was no antagonism. The [FIC.sub.50]/[FIC.sub.90] value is calculated as 0.75/1.01 (FIC range 0.5-1.01). Conclusions: SXT resistance in S. maltophilia strains is below 10%, but it should be closely monitored because of the limited treatment options. Tigecycline and moxifloxacin can be among the treatment alternatives because of their high sensitivity rates similar to SXT. The combination of moxifloxacin plus ceftazidime demonstrated in vitro synergism or additivity against majority of the strains. So, this combination may be used as an alternative for the treatment of patients with S. maltophilia infections if there is resistance or side effects to the antibiotics used as the first option. Key Words: Ste