Objectives: To evaluate, under conditions relevant to cystic fibrosis (CF) lung, the activity of levofloxacin (LVX) against preformed S. maltophilia biofilms and its relation with the expression of multidrug efflux pump SmeDEF conferring resistance to antibiotics belonging to different structural families. Methods: The activity of LVX against both planktonic cells (MIC, MBC) and preformed biofilm (MBEC) of 4 S. maltophilia CF strains was assessed, both under “standard” (CLSI/EUCAST-recommended) and “CF-like” (i.e. in artificial mucin and DNA containing medium, ASM) conditions. LVX was tested against biofilms at concentrations corresponding to achievable sputum levels by aerosolization (100 and 50 mg/l) and serum ones (10 mg/l). Multidrug efflux pump smeD expression was analyzed in planktonic (during log- and stationary-phase) and biofilm cells by quantitative real-time PCR, both in rich-medium and ASM. Results: MBCs ranged from 1 to 4 mg/l (standard) or 4 to 16 mg/l (ASM), while MBECs were > 100 mg/l, with the exception of biofilm produced by Sm144 (MBEC: 100 mg/l). Quantitative expression analysis of smeD showed that “CF-like” conditions reduced expression in log- and stationary-phase, while increased it in biofilm cells. A significant increase in smeD expression was observed during transition from planktonic log-phase to biofilm, regardless of condition considered, with the exception of Sm144 strain. Conclusion: S. maltophilia biofilms are significantly more resistant to LVX compared to planktonic counterparts, regardless of conditions considered. Overexpression of efflux pump SmeDEF is probably involved, particularly in “CF-like” conditions. In vivo smeD expression levels are currently under investigation.
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