Purpose: Our aim was to investigate in vitro biofilm formation by S. maltophilia  and the effects of antibacterial agents used to prevent biofilm formation.
Methods: Two trimethoprim/sulfamethoxazole-resistant S. maltophilia  strains were isolated from the pleural effusion of a patient with cancer. The minimum inhibitory concentrations (MICs) of amikacin, azithromycin, cefoperazone/sulbactam, and tigecycline were determined. The checkerboard method was used to determine the fractional inhibitory concentration indices (FICIs). A crystal violet biofilm assay and confocal laser scanning microscopy (CLSM) were used to observe biofilm formation. In vitro effects of azithromycin combined with tigecycline on biofilms of S. maltophilia  strains were tested.
Results: The two S. maltophilia  isolates were confirmed to produce strong biofilms. Crystal violet biofilm assay and CLSM analysis of S. maltophilia  biofilm were in the initial adhesive stage after 2 h incubation. Biofilm was in the exponential phase of growth at 12 h and reached maximal growth at 36– 48 h. Compared with tigecycline or azithromycin alone, the combination of tigecycline and azithromycin increased the inhibiting effect S. maltophilia  biofilm biomass after incubation for 12 h. Compared with the control group, in almost all strains treated with tigecycline and azithromycin, the biofilm was significantly suppressed significance (P< 0.001). We found that 2x MIC azithromycin combined with 1x MIC tigecycline had the best inhibiting effect against the biofilm, the biofilm inhibition rates of three strains were all over 60%, the biofilm thickness was inhibited from 36.00 ± 4.00 μm to 8.00 μm, from 40.00 μm to 6.67± 2.31 μm, and from 32.00 μm to 13.33 ± 2.31 μm in SMA1, SMA2 and ATCC17666, respectively.
Conclusion: Azithromycin combined with tigecycline inhibited biofilm formation by S. maltophilia . Our study provides an experimental basis for a possible optimal treatment strategy for S. maltophilia  biofilm-related infections.
Keywords: Stenotrophomonas maltophilia , azithromycin, tigecycline, biofilm, trimethoprim/sulfamethoxazole