Biofilm Formation by <i>Stenotrophomonas maltophilia</i> : Modulation by Quinolones, Trimethoprim-Sulfamethoxazole, and Ceftazidime

<jats:title>ABSTRACT</jats:title> <jats:p> We investigated the in vitro effects of seven fluoroquinolones (ciprofloxacin, grepafloxacin, levofloxacin, moxifloxacin, norfloxacin, ofloxacin, and rufloxacin), compared to those of trimethoprim-sulfamethoxazole (SXT) and ceftazidime on total biomass and cell viability of <jats:italic>Stenotrophomonas maltophilia</jats:italic> biofilm. <jats:italic>S. maltophilia</jats:italic> attached rapidly to polystyrene, within 2 h of incubation, and then biofilm formation increased over time, reaching maximum growth at 24 h. In the presence of fluoroquinolones at one-half and one-fourth the MIC, biofilm biomass was significantly ( <jats:italic>P</jats:italic> < 0.01) reduced to 55 to 70% and 66 to 76% of original mass, respectively. Ceftazidime and SXT did not exert any activity. Biofilm bacterial viability was significantly reduced by all antibiotics tested at one-half the MIC. At one-fourth the MIC all antibiotics, except levofloxacin, significantly reduced viability. Treatment of preformed biofilms with bactericidal concentrations (500, 100, and 50 μg/ml) of all fluoroquinolones caused, except for norfloxacin, significant reduction of biofilm biomass to 29.5 to 78.8, 64.1 to 83.6, and 70.5 to 82.8% of original mass, respectively. SXT exerted significant activity at 500 μg/ml only. Ceftazidime was completely inactive. Rufloxacin exhibited the highest activity on preformed biofilm viability, significantly decreasing viable counts by 0.6, 5.4, and 17.1% at 500, 100, and 50 μg/ml, respectively. Our results show that (i) subinhibitory (one-half and one-fourth the MIC) concentrations of fluoroquinolones inhibit adherence of <jats:italic>S. maltophilia</jats:italic> to polystyrene and (ii) clinically achievable concentrations (50 and 100 μg/ml) of rufloxacin are able to eradicate preformed <jats:italic>S. maltophilia</jats:italic> biofilm. </jats:p>