Lytic transglycosylase repertoire diversity enables daughter cell separation and antibiotic resistance in Escherichia coli under acidic stress

Peptidoglycan (PG) is an indispensable architectural element that imparts physical toughness and rigidity to the bacterial envelope. It is also a dynamic structure that undergoes continuous turnover or autolysis. Escherichia coli possesses redundant autolytic enzymes responsible for PG turnover; however, the rationale behind the existence of numerous autolytic enzymes remains incompletely understood. In this study, we elucidated the physiological roles of MltE and MltC, members of the lytic transglycosylase (LTG) family that catalyze the cleavage of glycosidic bonds between disaccharide subunits within PG strands. MltE and MltC are acidic LTGs that exhibit increased enzymatic activity and protein levels under acidic pH conditions, respectively. Deletion of these two LTGs results in a pronounced growth defect and elevated membrane permeability at acidic pH. Furthermore, these two LTGs are crucial for resistance against various antibiotics, particularly vancomycin. Intriguingly, inactivation of these LTGs induces a chaining morphology, indicative of daughter cell separation defects, only under acidic pH conditions. Simultaneous deletion of PG amidases, the known contributors to daughter cell separation, exacerbates the chaining phenotype at acidic pH. This suggests that the two LTGs may participate in the cleavage of glycan strands between daughter cells that cannot be resolved by PG amidases under acidic pH conditions. Collectively, our findings highlight the role of LTG repertoire diversity in facilitating bacterial survival and antibiotic resistance under stressful conditions. IMPORTANCEOrchestration of peptidoglycan (PG) synthesis and degradation plays a critical role in bacterial growth and development of antibiotic resistance. Bacteria use a diverse array of enzymes involved in PG synthesis and degradation to navigate the challenging conditions of the periplasm. Although Escherichia coli harbors 12 lytic transglycosylases (LTGs) responsible for cleaving glycosidic bonds between disaccharide subunits within PG strands, the physiological significance of their redundancy remains unclear. In this study, we demonstrated the indispensability of two LTGs, MltE and MltC, for cell growth, daughter cell separation, and antibiotic resistance under acidic stress conditions. Our findings highlight the potential significance of MltE and MltC as primary targets for the development of potentiators to augment the antimicrobial efficacy of antibiotics in acidic pH environments. One sentence summaryAcid stress-specific lytic transglycosylases