Enzymes and Mechanisms Employed by Tailed Bacteriophages to Breach the Bacterial Cell Barriers

Abstract

Monoderm bacteria possess a cell envelope made of a cytoplasmic membrane and a cell wall, whereas diderm bacteria have and extra lipid layer, the outer membrane, covering the cell wall. Both cell types can also produce extracellular protective coats composed of polymeric substances like, for example, polysaccharidic capsules. Many of these structures form a tight physical barrier impenetrable by phage virus particles. Tailed phages evolved strategies/functions to overcome the different layers of the bacterial cell envelope, first to deliver the genetic material to the host cell cytoplasm for virus multiplication, and then to release the virion offspring at the end of the reproductive cycle. There is however a major difference between these two crucial steps of the phage infection cycle: virus entry cannot compromise cell viability, whereas effective virion progeny release requires host cell lysis. Here we present an overview of the viral structures, key protein players and mechanisms underlying phage DNA entry to bacteria, and then escape of the newly-formed virus particles from infected hosts. Understanding the biological context and mode of action of the phage-derived enzymes that compromise the bacterial cell envelope may provide valuable information for their application as antimicrobials.

Keywords: LysB; bacterial cell envelope; cell wall; depolymerase; endolysin; holin; lysin; peptidoglycan; proton motive force; spanin.

Figure 1

Schematic representation of the structure and composition of the bacterial cell envelope in Gram-positive bacteria (a), Gram-negative bacteria (b) and mycobacteria (c). CM, cytoplasmic membrane; CW, cell wall; OM, outer membrane; IMP, inner membrane proteins; PLs, phospholipids; PG, peptidoglycan; AG, arabinogalactan; LP, lipoprotein; CAP, covalently attached protein; LTA, lipoteichoic acids; SCWP, secondary cell wall polymers; WTA, wall teichoic acids; OMP, outer membrane protein; LPS, lipopolysaccharide; MA, mycolic acids; GL, glycolipids; FL, free lipids. The S-layer and capsule are extracellular structures. Branched lipoaraninomannan is not represented in the mycobacterial cell envelope (probably anchored to both the CM and OM).

Figure 2

Basic structure of the bacterial cell wall peptidoglycan (PG). The possible enzymatic activities of PG-degrading enzymes and the bonds they cleave are indicated. m-DAP is found in the peptide chains of the PG of most Gram-negative bacteria, Bacillus spp. and Listeria spp., which present also direct m-DAP-d-Ala bonding between adjacent stem peptides. In most Gram-positive bacteria m-DAP is replaced l-Lys. Cross-linking between this residue and d-Ala of a neighbor peptide chain usually occurs by an interpeptide bridge of variable amino acidic composition (X). The d-Ala residue in light blue may be lost after PG maturation. Carboxypeptidases are rarely produced by bacteriophages. NAG, N-acetylglucosamine; NAM, N-acetylmuramic acid.

Figure 3

Possible actions of phage depolymerases during recognition and penetration of the bacterial cell envelope (Gram-negative bacteria used as example). Depolymerase activity is generically depicted as a pacman symbol. CM, cytoplasmic membrane; PG, cell wall peptidoglycan; OM, outer membrane; LPS, lipopolysaccharide; CA, capsule.

Figure 4

Key features of canonical (a) and non-canonical (b) lysis models. In the canonical model endolysins (c-endolysins) can only gain access to the CW compartment through the holin holes, which need therefore to be large enough to accommodate the enzymes’ size. In non-canonical lysis, host cell machineries (e.g., the bacterial Sec system) export the lytic enzymes (e-endolysins) to the extracytoplasmic space, where they are maintained inactive in association with the CW or the CM. Dissipation of the membrane PMF by the holin holes (canonical holins or pinholins) is an essential requirement for activation of e-endolysins. Recent evidences indicate that holin-mediated PMF collapse may also potentiate the lytic activity of c-endolysins (see text). CM, cytoplasmic membrane; PG, peptidoglycan.