Tailed bacteriophages typically induce lysis of infected bacteria to allow efficient escape of the virion progeny to the extracellular milieu. This review summa

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.

Peptidoglycan degrading enzymes are of increasing interest as antibacterial agents, especially against multi-drug resistant pathogens. Herein we present a review about the biological features of virion-associated lysins and endolysins, phage-derived enzymes that have naturally evolved to compromise the bacterial peptidoglycan from without and from within, respectively. These natural features may determine the adaptability of the enzymes to kill bacteria in different environments. Endolysins are by far the most studied group of peptidoglycan-degrading enzymes, with several studies showing that they can exhibit potent antibacterial activity under specific conditions. However, the lytic activity of most endolysins seems to be significantly reduced when tested against actively growing bacteria, something that may be related to fact that these enzymes are naturally designed to degrade the peptidoglycan from within dead cells. This may negatively impact the efficacy of the endolysin in treating some infections in vivo. Here, we present a critical view of the methods commonly used to evaluate in vitro and in vivo the antibacterial performance of PG-degrading enzymes, focusing on the major hurdles concerning in vitro-to-in vivo translation.

Lytic enzymes encoded by bacteriophages have been intensively explored as alternative agents for combating bacterial pathogens in different contexts. The antibacterial character of these enzymes (enzybiotics) results from their degrading activity towards peptidoglycan, an essential component of the bacterial cell wall. In fact, phage lytic products have the capacity to kill target bacteria when added exogenously in the form of recombinant proteins. However, there is also growing recognition that the natural bactericidal activity of these agents can, and sometimes needs to be, substantially improved through manipulation of their functional domains or by equipping them with new functions. In addition, often, native lytic proteins exhibit features that restrict their applicability as effective antibacterials, such as poor solubility or reduced stability. Here, I present an overview of the engineering approaches that can be followed not only to overcome these and other restrictions, but also to generate completely new antibacterial agents with significantly enhanced characteristics. As conventional antibiotics are running short, the remarkable progress in this field opens up the possibility of tailoring efficient enzybiotics to tackle the most menacing bacterial infections.

Bacteriophage SPP1 targets the host cell membrane protein YueB to irreversibly adsorb and infectBacillus subtilis. Interestingly, SPP1 still binds to the surface ofyueBmutants, although in a completely reversible way. We evaluated here the relevance of a reversible step in SPP1 adsorption and identified the receptor(s) involved. We show that reversible adsorption is impaired inB. subtilismutants defective in the glucosylation pathway of teichoic acids or displaying a modified chemical composition of these polymers. The results indicate that glucosylated poly(glycerolphosphate) cell wall teichoic acid is the major target for SPP1 reversible binding. Interaction with this polymer is characterized by a fast adsorption rate showing low-temperature dependence, followed by a rapid establishment of an equilibrium state between adsorbed and free phages. This equilibrium is basically determined by the rate of phage dissociation, which exhibits a strong dependence on temperature compatible with an Arrhenius law. This allowed us to determine an activation energy of 22.6 kcal/mol for phage release. Finally, we show that SPP1 reversible interaction strongly accelerates irreversible binding to YueB. Our results support a model in which fast SPP1 adsorption to and desorption from teichoic acids allows SPP1 to scan the bacterial surface for rapid YueB recognition.

The majority of known bacteriophages have long noncontractile tails ( Siphoviridae ) that serve as a pipeline for genome delivery into the host cytoplasm. The tail extremity distal from the phage head is an adsorption device that recognises the bacterial receptor at the host cell surface. This interaction generates a signal transmitted to the head that leads to DNA release. We have determined structures of the bacteriophage SPP1 tail before and after DNA ejection. The results reveal extensive structural rearrangements in the internal wall of the tail tube. We propose that the adsorption device–receptor interaction triggers a conformational switch that is propagated as a domino‐like cascade along the 1600 Å‐long helical tail structure to reach the head‐to‐tail connector. This leads to opening of the connector culminating in DNA exit from the head into the host cell through the tail tube.