The ability to form biofilms is a recognized trait of Stenotrophomonas maltophilia, but the extent of its clinical relevance is still unclear. The present multicenter prospective study (ANSELM) aims at investigating the association between biofilm formation and clinical outcomes of S. maltophilia infections. One hundred and nine isolates were collected from various geographical origins and stratified according to their clinical relevance. Biofilm formation was evaluated by the microtiter plate assay and correlated with microbiological and clinical data from the associated strains. Antibiotic susceptibility of the planktonic cells was tested by the disk diffusion technique, while antibiotic activity against mature biofilms was spectrophotometrically assessed. Most strains (91.7%) were able to form biofilm, although bloodborne strains produced biofilm amounts significantly higher than strains causing hospital- rather than community-acquired infections, and those recognized as "definite" pathogens. Biofilm formation efficiency was positively correlated with mechanical ventilation (p = 0.032), whereas a negative relationship was found with antibiotic resistance (r2 = 0.107; p < 0.001), specifically in the case of the pathogenic strains. Mature S. maltophilia biofilms were markedly more resistant (up to 128 times) to cotrimoxazole and levofloxacin compared with their planktonic counterparts, especially in the case of bloodborne strains. Our findings indicate that biofilm formation by S. maltophilia is obviously a contributing factor in the pathogenesis of infections, especially in deep ones, thus warranting additional studies with larger cohort of patients and isolates.

Department of Biotechnological and Applied Clinical Sciences Health and Environmental Sciences University of L'Aquila 67100 L'Aquila Italy

Department of Medical Microbiology 2nd Faculty of Medicine and Motol University Hospital Charles University 15006 Prague Czech Republic

Institute of Microbiology and Immunology Faculty of Medicine University of Belgrade 11000 Belgrade Serbia

Laboratory of Clinical Microbiology Department of Medical Oral and Biotechnological Sciences G d'Annunzio University of Chieti Pescara 66100 Chieti Italy Operative Unit of Clinical Microbiology Center for Advanced Studies and Technology G d'Annunzio University of Chieti Pescara 66100 Chieti Italy

Laboratory of Cystic Fibrosis Microbiology Bambino Gesù Children's Hospital IRCCS 00165 Rome Italy

Microbiology and Parasitology Service University Hospital of Gran Canaria Dr Negrín 35001 Las Palmas de Gran Canaria Spain

Microbiology Clinical Science Department Faculty of Health Sciences University of Las Palmas de Gran Canaria 35001 Las Palmas de Gran Canaria Spain

University Freiburg Medical Center Institute for Infection Prevention and Hospital Epidemiology 79098 Freiburg Germany

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