CwlQ is required for swarming motility but not flagellar assembly in Bacillus subtilis

Hydrolytic enzymes play an essential role in the remodeling of bacterial peptidoglycan (PG), an extracellular mesh-like structure that retains the membrane in the context of high internal osmotic pressure. Peptidoglycan (PG) integrity must be unfailingly stable to preserve cell integrity but must also be dynamically remodeled for the cell grow, divide and insert macromolecular machines. The flagellum is one such macromolecular machine that transits the PG and the insertion of which is aided by localized activity of a dedicated PG hydrolase in Gram-negative bacteria. To date, there is no known dedicated hydrolase in Gram-positive bacteria for insertion of flagella and here we take a reverse-genetic candidate-gene approach to find that cells mutated for the lytic transglycosylase CwlQ exhibited a severe defect in flagellar dependent swarming motility. We show that CwlQ required its active site to promote swarming, was expressed by the motility sigma factor SigD, and was secreted by the type III secretion system housed inside the flagellum. Nonetheless, cells mutated for CwlQ remained proficient for flagellar biosynthesis even when mutated in combination with four other hydrolases related to motility (LytC, LytD, LytF, and CwlO). The PG hydrolase essential for flagellar synthesis in B. subtilis, if any, remains unknown.