Stenotrophomonas maltophilia bloodstream infection in patients with hematologic malignancies: A retrospective study and in vitro activities of antimicrobial combinations

Background: Stenotrophomonas maltophilia causes serious infections in immunocompromised hosts. Here, we analyzed the clinical characteristics of S. maltophilia bloodstream infection (BSI) in patients with hematologic malignancies and evaluated in vitro synergistic effects of antimicrobial combinations. Methods: We retrospectively reviewed all consecutive episodes of S. maltophilia BSIs in adult hematologic patients from June 2009 to May 2014, with in vitro susceptibility and synergy tests using high-throughput bioluminescence assay performed for available clinical isolates. Results: Among 11,004 admissions during 5-year period, 31 cases were identified as S. maltophilia BSIs. The incidence rate of S. maltophilia BSI was 0.134 cases/1,000 patient-days. Overall and attributable mortality of S. maltophilia BSI was 64.5% and 38.7%, respectively. Severe neutropenia (adjusted hazard ratio [HR] 5.24, p =0.013), shock at the onset of BSI (adjusted HR 6.05, p <0.001), and pneumonia (adjusted HR 3.15, p =0.017) were independent risk factors for mortality. In vitro susceptibilities to ceftazidime, levofloxacin, ticarcillin-clavulanic acid (TIM) and trimethoprim-sulfamethoxazole (SXT) were 11.1%, 44.0%, 40.7%, and 88.9%, respectively. MIC₅₀/MIC₉₀ for moxifloxacin and tigecycline were 1/4 mg/L and 4/8 mg/L. The 50% and 90% fractional inhibitory concentrations (FIC⁵⁰/FIC⁹⁰) of clinical isolates against a combination of SXT and TIM were 0.500/0.750. For SXT plus levofloxacin or moxifloxacin, FIC⁵⁰/FIC⁹⁰ were 0.625/1.000 and 0.625/0.625, respectively. Conclusion:S. maltophilia BSIs show high mortality, which is related to severe neutropenia, shock, and S. maltophilia pneumonia. Based upon drug susceptibility testing, the primary treatment of choice for S. maltophilia BSIs should be SXT in hematologic patients, rather than quinolones, with combination therapies including SXT serving as a feasible treatment option.