Antimicrobial susceptibility and combination testing of invasive Stenotrophomonas maltophilia isolates

Abstract

Background: Treatment of invasive Stenotrophomonas maltophilia infections is difficult due to this organism's inherent multidrug resistance and increasing resistance to trimethoprim–sulfamethoxazole via acquisition of the sul genes. Methods: In vitro antibiotic susceptibility testing was performed using a customized broth microdilution panel. Combination testing for tigecycline with anti-Stenotrophomonas agents (i.e. colistin, ticarcillin–clavulanate, ceftazidime, and levofloxacin) was done using the cross Etest method. Genotyping was done using automated repetitive PCR. Results: A total of 90 patients with invasive S. maltophilia infections included: (79%) adults, and 21% children or infants [6/12 (50%) cases occurred in infants aged ≤ 1 year.]. S. maltophilia isolates were recovered from blood (69%), lower respiratory (21%) or other sites (CSF, peritoneal fluid) (11%). Seventeen percent of the isolates were SXT-R, and also demonstrated multi-drug resistant to two or more antibiotic classes. Minocycline, tigecycline and colistin had the best in vitro activities. The combination testing of tigecycline and colistin gave the best results; 12 isolates were tested and synergy occurred in 3 isolates while an additional 7 isolates showed additive results. Conclusions: We recommend further evaluation with killing assays and clinical studies to evaluate the effectiveness of tigecycline and colistin combination for invasive S. maltophilia infections.

Keywords::

• Antimicrobial susceptibility
• combination testing
• invasive Stenotrophomonas maltophilia

Acknowledgements

This study was supported by a peer-reviewed grant from Calgary Laboratory Services. Tigecycline Etest strips were provided by Pfizer Canada. Barbara Chow assisted with the DiversiLab strain similarity analyses.

Declaration of interest: DLC and JDDP have previously received research funds from Merck, Pfizer, and Astra Zeneca. GP and TL have no conflicts of interest.