BACTERIAL WALLS, PEPTIDOGLYCAN HYDROLASES, AUTOLYSINS, AND AUTOLYSIS

Knowledge of the chemistry, ultrastructure, biosynthesis, assembly, an d function of bacterial cell walls has expanded enormously since the o pening of this field of research approximately 40 years ago, primarily by the early work of Milton Salton. It has become abundantly clear th at, in most environments, walls are essential to the survival and grow th of bacteria and in many ways are structurally and functionally uniq ue. A common but not universal feature of bacterial walls is the prese nce 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 wa lls. They are a biologically unique class of macromolecules in that th ey are not linear or even branched macromolecules. Instead they are tw o- or three-dimensional net like polymers that are linked together by three different chemical bonds (glycosidic, amide, and peptide). In ad dition, they contain the D-isomers of some amino acids and therefore m ay possess DL, LD, and no linkages. Furthermore, the exact chemical st ructure 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 mor e important than the exact shape of the bricks used for the construct. Another somewhat unique feature of PGs (and walls) is their final ass embly in situ on the outside of the cellular permeability barrier. A b road variety of bacteria have been shown to possess enzymes that can h ydrolyze bonds in the wall PG. Hydrolysis of a sufficient number of bo nds can result in the weakening of, or serious damage to, the protecti ve properties of the PG. Frequently, a bacterial strain may possess mo re than one PG hydrolase activity. A commonly believed, but as yet unp roven, hypothesis is that PG hydrolases play one or more roles in PG a ssembly and/or surface growth and cell division. At a minimum, such po tentially suicidal activities must be exquisitely well regulated. Curr ently 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 Enter ococcus hirae described by Fontana and collaborators as a genetic elem ent that is involved in the regulation of the synthesis of PBP 5, also is involved in the regulation of several other surface properties. Th ese properties include (1) autolysis rates of exponential phase cells, (2) the retention of this property after cells enter the stationary p hase, (3) lysozyme sensitivity, and (4) the ratio of rhamnose-containi ng wall polysaccharide to PG in the walls. Thus the psr element may be a part of a ''global'' regulation and communication system in E. hira e.