Lytic activity of the staphylolytic Twort phage endolysin CHAP domain is enhanced by the SH3b cell wall binding domain

Increases in the prevalence of antibiotic-resistant strains of Staphylococcus aureus have elicited efforts to develop novel antimicrobials to treat these drug-resistant pathogens. One potential treatment repurposes the lytic enzymes produced by bacteriophages as antimicrobials. The phage Twort endolysin (PlyTW) harbors three domains, a cysteine, histidine-dependent amidohydrolases/peptidase domain (CHAP), an amidase-2 domain and a SH3b-5 cell wall binding domain (CBD). Our results indicate that the CHAP domain alone is necessary and sufficient for lysis of live S. aureus, while the amidase-2 domain is insufficient for cell lysis when provided alone. Loss of the CBD results in similar to 10X reduction of enzymatic activity in both turbidity reduction and plate lysis assays compared to the full length protein. Deletion of the amidase-2 domain resulted in a protein (PlyTW Delta 172-373) with lytic activity that exceeded the activity of the full length construct in both the turbidity reduction and plate lysis assays. Addition of Ca2+ enhanced the turbidity reduction activity of both the full length protein and truncation constructs harboring the CHAP domain. Chelation by addition of EDTA or the addition of zinc inhibited the activity of all PlyTW constructs.