Abstract: Bacteria are equipped with complex cell surface structures, such as cell walls. How they maintain cell surface integrity through cell wall metabolism during growth and adaptation to unfavorable environmental conditions is still elusive. In the Gram-positive soil bacterium Bacillus subtilis, one extracytoplasmic function (ECF) sigma factor, SigM, is believed to play a primary role in cell surface integrity. Here, we find that expression of CsbB, which is known to be involved in the extracellular stress response, causes constitutive activation of SigM when YfhO, a membrane protein with unknown function, is lost. CsbB has similarity with the well-characterized bactoprenol glucosyltransferase GtrB found in Gram-negative bacteria. Substitution of a single amino acid residue at the putative catalytic site of CsbB abolishes this constitutive activation, and expression of Escherichia coli GtrB in B. subtilis causes similar effects as expression of CsbB, suggesting that SigM is activated by the glycosyltransferase activity of CsbB. A comparison with the Gtr system in Gram-negative bacteria suggests that accumulation of glycosylated bactoprenol catalyzed by CsbB reduces the bactoprenol pool in the absence of YfhO. Reduction of bactoprenol synthesis causes similar effects as expression of CsbB. We propose that it is the shortage of available bactoprenol within a cell that induces SigM activity.