Bacterial cell-wall recycling

Abstract

Many Gram-negative and Gram-positive bacteria recycle a significant proportion of the peptidoglycan components of their cell walls during their growth and septation. In many--and quite possibly all--bacteria, the peptidoglycan fragments are recovered and recycled. Although cell-wall recycling is beneficial for the recovery of resources, it also serves as a mechanism to detect cell-wall-targeting antibiotics and to regulate resistance mechanisms. In several Gram-negative pathogens, anhydro-MurNAc-peptide cell-wall fragments regulate AmpC β-lactamase induction. In some Gram-positive organisms, short peptides derived from the cell wall regulate the induction of both β-lactamase and β-lactam-resistant penicillin-binding proteins. The involvement of peptidoglycan recycling with resistance regulation suggests that inhibitors of the enzymes involved in the recycling might synergize with cell-wall-targeted antibiotics. Indeed, such inhibitors improve the potency of β-lactams in vitro against inducible AmpC β-lactamase-producing bacteria. We describe the key steps of cell-wall remodeling and recycling, the regulation of resistance mechanisms by cell-wall recycling, and recent advances toward the discovery of cell-wall-recycling inhibitors.

Figure 1

Simple representations of cell-wall biosynthesis, recycling, and turnover in Gram-negative and Gram-positive bacteria. The biosynthesis of all precursors for the peptidoglycan occurs in the bacterial cytoplasm. The final intermediate, Lipid II, is translocated into the periplasm for de novo peptidoglycan polymer synthesis as catalyzed by the bifunctional penicillin-binding protein enzymes (PBPs). Remodeling of the peptidoglycan polymer occurs during both growth and septation, liberating peptidoglycan fragments called muropeptides. Peptidoglycan cell wall recycling refers to the recovery and removal of these muropeptides to the cytoplasm, for integration into the biosynthesis of the peptidoglycan precursors. Peptidoglycan cell wall turnover refers to the loss of muropeptides from the bacterium to its media. Muropeptides lost to turnover may re-enter the periplasm for recycling. Notwithstanding the clear distinction between recycling and turnover, within the literature the two terms often are used interchangeably. In this review we preserve the distinction.

Figure 2

Summary of selected steps in cell-wall biosynthesis (left-hand side) and recycling (periplasm) in Gram-negative bacteria and regulation of β-lactamase (AmpC) production by cell-wall precursors (2) and anhydromuropeptide fragments (8). While the individual enzymes of the recycling pathways are most often isolated from E. coli, this organism does not possess a complete AmpC β-lactamase induction system. Studies of β-lactamase induction in E. coli use plasmids with ampC and ampR genes from an inducible bacterial strain.

Figure 3

Examples of peptidoglycan structural modifications.

Figure 4

Hydrolytic cleavage sites in E. coli peptidoglycan for periplasmic carboxypeptidases and endopeptidases.^,

Figure 5

Lytic transglycosylase (LT) reactivities toward the peptidoglycan. (A) Cleavage of a synthetic peptidoglycan fragment by MltB of E. coli with formation of anhydromuropeptide 6d. (B) Mechanism proposed for the LT reaction. (C) Inhibitors of LTs.

Figure 6

Muropeptide cleavage by the N-acetylglucosaminidase NagZ. (A) The double-displacement mechanism for the NagZ-catalyzed hydrolysis of GlcNAc-MurNAc disaccharides. (B) NagZ inhibitors.

Figure 7

anhMurNAc-peptide substrates for AmpD and MurNAc-peptide non-substrates.^,

Figure 8

Summary of selected steps in cell-wall biosynthesis and recycling in the Gram-positive organism Bacillus subtilis incorporating mechanistic features proposed by Amoroso et al. for BlaI inactivation in Bacillus licheniformis (green arrows) by the dipeptide γ-D-Glu-m-DAP (m-DAP = meso-1,6-diaminopimelate).

Figure 9

The bla system of S. aureus and B. licheniformis and the mec system of methicillin-resistant S. aureus.