The function of the holin is to create a lesion in the cytoplasmic membrane through which the murein hydrolase passes to gain access to the murein layer.
Lysis of bacterial cell wall is based on the action of two phage encoded enzymes: holins and lysins.
Lysins are phage-encoded murein hydrolases that act on the bacterial host cell wall at the terminal stage of the phage reproduction cycle to release progeny phage.
These enzymes are also known as phage lysozymes, endolysins, or muralytic/mureolytic enzymes. Their action is tightly regulated by holins, by membrane arrest, and by conversion from their inactive state. Their N-terminal catalytic domains are able to target almost every possible bond in the peptidoglycan network, and their corresponding C-terminal cell wall binding domains target the enzymes to their substrate.
Owing to their specificity and high activity, endolysins have been employed for various in vitro and in vivo aims, in food science, in microbial diagnostics, and for treatment of experimental infections.
Clearly, phage lysins represent great tools for use in molecular biology, biotechnology and in medicine, and we are just beginning to tap this potential.There are at least two distinct mechanisms by which phages destroy the cell wall.
Bacteriophages with large genomes use a holin-endolysin system, while bacteriophages with small genomes encode a single lysis protein. Some single protein lysis systems inhibit cell wall synthesis and are thus the phage analogs of antibiotics like penicillin. Sometimes also phage capsid proteins are responsible for lysis.
In gram-positive bacteria, (endo)lysins can also act as exolysins because the pepticoglycan layer of the bacterial cell wall is, in most cases, accessible from the outside. This is not the case for gram-negative bacteria, in which the presence of the outer membrane effectively prevents access by hydrophilic lytic enzymes.
However, when the lipopolysaccharide layer is disrupted (by EDTA,detergents, etc.) cells become sensitive to external murein hydrolases.
Lysin domains can be cloned into non-host bacteria and purified after expression and used for different applications.
Lysins have now been used successfully in animal models to control pathogenic antibiotic resistant bacteria found on mucosal surfaces and in blood. The advantage over antibiotics are their specificity for the pathogen without disturbing the normal flora, the low chance of bacterial resistance to lysins and their ability to kill colonizing pathogens on mucosal surfaces, capabilities that were previously unavailable. Thus, lysins could be an effective anti-infective in an age of mounting antibiotic resistance.
A potential concern in the use of lysins is the development of neutralizing antibodies. Unlike antibiotics, which are small molecules that are generally not immunogenic, enzymes are proteins that stimulate an immune response when delivered both mucosally or systemically. It was found that highly immune serum slows, but does not block, the killing of bacteria by lysins.
Similar to other proteins that are delivered intravenously to animals and humans, phage enzymes have a short half life (ca. 15 min.). However, the action of lysins for bacteria is so rapid, that this might be sufficient time to observe a therapeutic effect.
Because of the specific action of lysins, they offer a unique possibility for the biological control of unwanted bacteria without having any effect on other organisms. The most obvious approach to the use of lysins for the biocontrol of pathogens in food and feed is to directly add purified enzyme to the food or to the raw product. A more elegant and also less expensive alternative is the production and secretion of specific endolysins by fermenting. In this case the cell wall of the host bacterium must be insensitive to the produced lysin. By contrast, some applications aim to cause self-destruction, which is mediated by cells carrying endolysin genes that are able to degrade their own murein.
As bacteria get resistant to phages quite rapidly, they could also become resistant against lysins. However, repeated exposure of several bacteria grown on agar plates to low concentrations of lysins did not lead to the recovery of resistant strains. The cell wall receptor of lysins of S. pneumoniae is choline, a molecule that is essential for pneumococcal viability.
Although not yet proven, it is possible that during interaction of phage and bacteria over the millennia, to avoid becoming trapped inside the host, the binding domain of the lytic enzymes has evolved to target a unique and essential molecule in the cell wall, making resistance to these enzymes a rare event.
