Endolysins or murein hydrolases are hydrolytic enzymes produced by bacteriophages to cleave the host's cell wall during the final stage of the lytic cycle. Whereas globular endolysins are composed of a single enzymatically active domain (EAD), modular endolysins have at least two recognizable modules, often comprising a cell wall binding domain coupled to an EAD. Although such enzymes seem to be rarer, their activity often exceeds that of their globular counterparts. Here, we explored five previously uncharacterized modular endolysins for their expression, purification, and activity against a panel of distinct environmental Gram-negative bacteria. Out of the selected endolysins derived from Enterobacteria-infecting phages, two were soluble and were purified to near homogeneity. Among them, endolysin shared by several Enterobacteria phages, referred to as El1, exhibited a notable bacteriolytic activity not only from within, but also from without the cells. The effects of the studied enzyme on bacterial growth and viability were studied in detail in Escherichia coli. Visual inspection of the treated cells verified that the enzyme could penetrate the E. coli cell membrane when applied exogenously. Interestingly, El1 was active in the absence of ethylenediaminetetraacetic acid (EDTA) against multiple environmental Gram-negative bacteria representing different Gammaproteobacteria orders. Although the exact Gram-negative lysis mechanism of El1 remains unknown, the breadth of its target range suggests El1 as a promising candidate for future studies to scrutinize natural endolysin interactions differences against evolutionary distinct bacteria.
Kazaka et al. (2026) studied this question.