Bacterial walls, peptidoglycan hydrolases, autolysin and autolysis

Knowledge of the chemistry, ultrastructure, biosynthesis, assembly, and function of bacterial cell walls has expanded enormously since the opening of this field of research approximately 40 years ago, primarily by the early work of Milton Saltón. It has become abundantly clear that, in most environments, walls are essential to the survival and growth of bacteria and in many ways are structurally and functionally unique. A common but not universal feature of bacterial walls is the presence of peptidoglycan (PG; murein, or in the case of certain Archae the analogous structure—pseudomurein). PGs are considered to be primarily responsible for the protective and shape-maintaining properties of walls. They are a biologically unique class of macromolecules in that they are not linear or even branched macromolecules. Instead they are two- or three-dimensional net like polymers that are linked together by three different chemical bonds (glycosidic, amide, and peptide). In addition, they contain the D-isomers of some amino acids and therefore may possess dl, ld, and DD linkages. Furthermore, the exact chemical structure of a PG may vary depending on environmental factors, however, retaining the essential protective and shape maintaining properties of the wall. Thus, the overall architectural plan of the wall may be more important than the exact shape of the bricks used for the construct. Another somewhat unique feature of PGs (and walls) is their final assembly in situ on the outside of the cellular permeability barrier. A broad variety of bacteria have been shown to possess enzymes that can hydrolyze bonds in the wall PG. Hydrolysis of a sufficient number of bonds can result in the weakening of, or serious damage to, the protective properties of the PG. Frequently, a bacterial strain may possess more than one PG hydrolase activity. A commonly believed, but as yet unproven, hypothesis is that PG hydrolases play one or more roles in PG assembly and/or surface growth and cell division. At a minimum, such potentially suicidal activities must be exquisitely well regulated. Currently we know little concerning the regulation of these activities, or how they communicate with, and integrate with, chromosome replication, synthesis of cytoplasmic macromolecules, cell growth, and division, although such, probably two-way, communications must occur in growing and dividing cells. Recent data indicate that the psr element in Enterococcus hirae described by Fontana and collaborators as a genetic element that is involved in the regulation of the synthesis of PBP 5, also is involved in the regulation of several other surface properties. These properties include (1) autolysis rates of exponential phase cells, (2) the retention of this property after cells enter the stationary phase, (3) lysozyme sensitivity, and (4) the ratio of rhamnose-containing wall polysaccharide to PG in the walls. Thus the psr element may be a part of a "global" regulation and communication system in E. hirae.