Inactivation of GalU leads to a cell wall-associated polysaccharide defect that reduces the susceptibility of Enterococcus faecalis to bacteriolytic agents.

ABSTRACT

Enterococcal plasmid-encoded bacteriolysin Bac41 is a selective antimicrobial system that is considered to provide a competitive advantage to Enterococcus faecalis cells that carry the Bac41-coding plasmid. The Bac41 effector consists of the secreted proteins BacL₁ and BacA, which attack the cell wall of the target E. faecalis cell to induce bacteriolysis. Here, we demonstrated that galU, which encodes UTP-glucose-1-phosphate uridylyltransferase, is involved in susceptibility to the Bac41 system in E. faecalis. Spontaneous mutants that developed resistance to the antimicrobial effects of BacL₁ and BacA were revealed to carry a truncation deletion of the C-terminal 288–298 a.a. region of the translated GalU protein. This truncation resulted in the depletion of UDP-glucose, leading to a failure to utilize galactose and produce the enterococcal polysaccharide antigen (EPA), which is expressed abundantly on the cell surface of E. faecalis. This cell surface composition defect that resulted from galU or EPA-specific genes caused an abnormal cell morphology, with impaired polarity during cell division and alterations of the limited localization of BacL₁. Interestingly, these mutants conferred reduced susceptibility to beta-lactams besides Bac41, despite their increased susceptibility to other bacteriostatic antimicrobial agents and chemical detergents. These data suggest that a complex mechanism of action underlies lytic killing, as exogenous bacteriolysis induced by lytic bacteriocins or beta-lactams requires an intact cell physiology in E. faecalis.

IMPORTANCE Cell wall-associated polysaccharides of bacteria are involved in various physiological characteristics. Recent studies demonstrated that the cell wall-associated polysaccharide of Enterococcus faecalis is required for susceptibility to bactericidal antibiotic agents. Here, we demonstrated that a galU mutation resulted in resistance to the enterococcal lytic bacteriocin Bac41. The galU homologue is reported to be essential for biosynthesis of species-specific cell wall-associated polysaccharides in other Firmicutes. In E. faecalis, the galU mutant lost the E. faecalis-specific cell wall-associated polysaccharide EPA (enterococcal polysaccharide antigen). The mutant also displayed reduced susceptibility to antibacterial agents and an abnormal cell morphology. We firstly demonstrated that galU was essential for EPA biosynthesis in E. faecalis, and EPA production might underlie susceptibility to lytic bacteriocin and antibiotic agents by undefined mechanism.