Abstract
The initial binding of bacterial cells to a solid surface is a critical and essential step in biofilm formation. In this report we show that stationary-phase cultures of Escherichia coli W3100 (a K-12 strain) can efficiently attach to sand columns when they are grown in Luria broth medium at 28 degrees C in fully aerobic conditions. In contrast, growth in oxygen-limited conditions results in a sharp decrease in adhesion to hydrophilic substrates. We show that the production of lipopolysaccharide (LPS) and of flagella, as well as the transcription of the fliC gene, encoding the major flagellar subunit, increases under oxygen-limited conditions. Inactivation of the global regulatory hns gene counteracts increased production of LPS and flagella in response to anoxia and allows E. coli W3100 to attach to sand columns even when it is grown under oxygen-limited conditions. We propose that increased production of the FliC protein and of LPS in response to oxygen limitation results in the loss of the ability of E. coli W3100 to adhere to hydrophilic surfaces. Indeed, overexpression of the fliC gene results in a decreased adhesion to sand even when W3100 is grown in fully aerobic conditions. Our observations strongly suggest that anoxia is a negative environmental signal for adhesion in E. coli.