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DNA microarrays revealed that expression of ycfR, which encodes a putative outer membrane protein, is significantly induced in Escherichia coli biofilms and is also induced by several stress conditions. We show that deletion of ycfR increased biofilm formation fivefold in the presence of glucose; the glucose effect was corroborated by showing binding of the cyclic AMP receptor protein to the ycfR promoter. It appears that YcfR is a multiple stress resistance protein, since deleting ycfR also rendered the cell more sensitive to acid, heat treatment, hydrogen peroxide, and cadmium. Increased biofilm formation through YcfR due to stress appears to be the result of decreasing indole synthesis, since a mutation in the tnaA gene encoding tryptophanase prevented enhanced biofilm formation upon stress and adding indole prevented enhanced biofilm formation upon stress. Deleting ycfR also affected outer membrane proteins and converted the cell from hydrophilic to hydrophobic, as well as increased cell aggregation fourfold. YcfR seems to be involved in the regulation of E. coli K-12 biofilm formation by decreasing cell aggregation and cell surface adhesion, by influencing the concentration of signal molecules, and by interfering with stress responses. Based on our findings, we propose that this locus be named bhsA, for influencing biofilm through hydrophobicity and stress response. [ABSTRACT FROM AUTHOR]