Identification of Bacillus subtilis SipW as a bifunctional signal peptidase that controls surfaceadhered biofilm formation

Biofilms of microbial cells encased in an exopolymeric matrix can form on solid surfaces, but how bacteria sense a solid surface and upregulate biofilm genes is largely unknown. We investigated the role of the Bacillus subtilis signal peptidase, SipW, which has a unique role in forming biofilms on a solid surface and is not required at an air-liquid interface. Surprisingly, we found that the signal peptidase activity of SipWwas not required for solid-surface biofilms. Furthermore, a SipWmutant protein was con-structed that lacks the ability to form a solid-surface biofilm but still retains signal peptidase activity. Through genetic and gene expression tests, the non-signal peptidase role of SipWwas found to activate biofilmmatrix genes specifically when cells were on a solid surface. These data provide the first evidence that a signal peptidase is bifunctional and that SipW has a regulatory role in addition to its role as a signal peptidase. Biofilms are a nonmotile community of microbial cells that areencased in an exopolymeric matrix. Biofilms are proposed to be the predominant lifestyle of microbes in nature and to contrib-ute to medically significant infections and biofouling in industrial settings (9, 17, 24, 27). Biofilms can form on solid surfaces, at air-liquid interfaces (i.e., pellicle biofilms), or in liquid as cellular clusters (i.e., flocs) (20, 38). Solid-surface biofilms are of particu-lar interest; they form on teeth, on urinary catheters and cardiac implants, on food industry equipment and drinking water distri