Lysins, also known as endolysins or murein hydrolases, are hydrolytic enzymes produced by bacteriophages in order to cleave the host's cell wall during the final stage of the lytic cycle.

[3] Similarly, not all bacteriophages synthesize lysins: some small single-stranded DNA and RNA phages produce membrane proteins that activate the host's autolytic mechanisms such as autolysins.

[4] Lysins were first used therapeutically in 2001 by the Fischetti lab (see below) and are now being used as antibacterial agents due to their high effectiveness and specificity in comparison with antibiotics, which are susceptible to bacterial resistance.

As is the case for autolysins, early confusion around the cleavage specificity of these individual enzymes has led to some misattributions of the name "lysozyme" to proteins without this activity.

Theoretically, due to the catalytic properties of phage lysins, a single enzyme would be sufficient to kill the host bacterium by cleaving the necessary number of bonds, even though this has yet to be proven.

[5] The work by Loessner et al suggests that cleavage is typically achieved by the joint action of multiple lysin molecules at a local region of the host's cell wall.

In order to solve such a problem, phage viruses synthesize another protein called holin which binds to the cell membrane and makes holes in it (hence its name), allowing lysins to reach the peptidoglycan matrix.

Unlike most antibiotics, proteins are prone to antibody recognition and binding, which means that lysins could be ineffective when treating bacterial infections or even dangerous, potentially leading to a systemic immune response or a cytokine storm.

[16] Phage lysins have been successfully tested in animal models to control pathogenic antibiotic-resistant bacteria found on mucous membranes and in blood.