Bacterial Cell Enlargement Requires Control of Cell Wall Stiffness Mediated by Peptidoglycan Hydrolases

Abstract

Most bacterial cells are enclosed in a single macromolecule of the cell wall polymer, peptidoglycan, which is required for shape determination and maintenance of viability, while peptidoglycan biosynthesis is an important antibiotic target. It is hypothesized that cellular enlargement requires regional expansion of the cell wall through coordinated insertion and hydrolysis of peptidoglycan. Here, a group of (apparent glucosaminidase) peptidoglycan hydrolases are identified that are together required for cell enlargement and correct cellular morphology of Staphylococcus aureus, demonstrating the overall importance of this enzyme activity. These are Atl, SagA, ScaH, and SagB. The major advance here is the explanation of the observed morphological defects in terms of the mechanical and biochemical properties of peptidoglycan. It was shown that cells lacking groups of these hydrolases have increased surface stiffness and, in the absence of SagB, substantially increased glycan chain length. This indicates that, beyond their established roles (for example in cell separation), some hydrolases enable cellular enlargement by making peptidoglycan easier to stretch, providing the first direct evidence demonstrating that cellular enlargement occurs via modulation of the mechanical properties of peptidoglycan.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

“…Protein synthesis inhibitors induce cell wall thickening by stopping enlargement of the cell wall surface [188,189], possibly by inhibiting synthesis of the peptidoglycan hydrolases needed to loosen the expanding walls [189,194]. This, in the absence of any accompanying decrease in synthesis or incorporation of peptidoglycan precursor, leads to thickening of the existing cell wall [188].…”

SagB Glucosaminidase Is a Determinant of Staphylococcus aureus Glycan Chain Length, Antibiotic Susceptibility, and Protein Secretion

“…These data suggest that although Atl, SagA, and ScaH exhibit roles distinctive from the role of SagB, further genetic loss of glucosaminidase activity aggravates the sagB mutant phenotype. Of note, Wheeler and colleagues, studying S. aureus SH1000, a laboratory strain, reported that expression of glucosaminidase was essential for S. aureus growth and that this requirement was mostly dependent on sagB (36). Changes in the structure of the cell envelope have been described to affect bacterial susceptibility to antibiotics (50).…”

“…aureus Newman variants lacking four glucosaminidase genesatl, sagAB, and scaH-are viable and, compared to the wild type, replicate at a slightly reduced rate. In contrast, the S. aureus SH1000 atl sagA scaH mutant cannot replicate without sagB expression (36). Unlike SH1000, a laboratory strain that has been cured of bacteriophages (36), S. aureus Newman is lysogenized by four different phages, two of which encode muralytic enzymes with predicted N-acetylglucosaminidase activity (Fig.…”

“…In contrast, the S. aureus SH1000 atl sagA scaH mutant cannot replicate without sagB expression (36). Unlike SH1000, a laboratory strain that has been cured of bacteriophages (36), S. aureus Newman is lysogenized by four different phages, two of which encode muralytic enzymes with predicted N-acetylglucosaminidase activity (Fig. 1A) (43).…”

“…Further, sagB mutants display growth and cell-morphological defects that are caused by the exaggerated lengths of peptidoglycan strands, and purified SagB cleaves glycan strands to generate the physiological structure of S. aureus cell wall. During the preparation of the manuscript, Wheeler et al reported on S. aureus SH1000 sagB mutants and associated defects in bacterial growth and peptidoglycan structure without analyzing protein secretion or antibiotic resistance (36).…”