How Bacteria Respond to Material Stiffness during Attachment: A Role of <i>Escherichia coli</i> Flagellar Motility

Material stiffness has been shown to have potent effects on bacterial attachment and biofilm formation, but the mechanism is still unknown. In this study, response to material stiffness by <i>Escherichia coli</i> during attachment was investigated with biofilm assays and cell tracking using the Automated Contour-base Tracking for in Vitro Environments (ACT<i>IV</i>E) computational algorithm. By comparing the movement of <i>E. coli</i> cells attached on poly­(dimethylsiloxane) (PDMS) surfaces of different Young’s moduli (0.1 and 2.6 MPa, prepared by controlling the degree of cross-linking) using ACT<i>IV</i>E, attached cells on stiff surfaces were found more motile during early stage biofilm formation than those on soft surfaces. To investigate if motility is important to bacterial response to material stiffness, we compared <i>E. coli</i> RP437 and its isogenic mutants of flagellar motor (<i>motB</i>) and synthesis of flagella (<i>fliC</i>) and type I fimbriae (<i>fimA</i>) for attachment on 0.1 and 2.6 MPa PDMS surfaces. The <i>motB</i> mutant exhibited defects in response to PDMS stiffness (based on cell counting and tracking with ACT<i>IV</i>E), which was recovered by complementing the <i>motB</i> gene. Unlike <i>motB</i> results, mutants of <i>fliC</i> and <i>fimA</i> did not show significant defects on both face-up and face-down surfaces. Collectively, these findings suggest that <i>E. coli</i> cells can actively respond to material stiffness during biofilm formation, and <i>motB</i> is involved in this response