Binding domains of Bacillus anthracis phage endolysins recognize cell culture age-related features on the bacterial...

Bacteriolytic enzymes often possess a C-terminal binding domain that recognizes specific motifs on the bacterial surface and a catalytic domain that cleaves covalent linkages within the cell wall peptidoglycan. PlyPH, one such lytic enzyme of bacteriophage origin, has been reported to be highly effective against Bacillus anthracis, and can kill up to 99.99% of the viable bacteria. The bactericidal activity of this enzyme, however, appears to be strongly dependent on the age of the bacterial culture. Although highly bactericidal against cells in the early exponential phase, the enzyme is substantially less effective against stationary phase cells, thus limiting its application in real-world settings. We hypothesized that the binding domain of PlyPH may differ in affinity to cells in different Bacillus growth stages and may be primarily responsible for the age-restricted activity. We therefore employed an in silico approach to identify phage lysins differing in their specificity for the bacterial cell wall. Specifically we focused our attention on Plyβ, an enzyme with improved cell wall-binding ability and age-independent bactericidal activity. Although PlyPH and Plyβ have dissimilar binding domains, their catalytic domains are highly homologous. We characterized the biocatalytic mechanism of Plyβ by identifying the specific bonds cleaved within the cell wall peptidoglycan. Our results provide an example of the diversity of phage endolysins and the opportunity for these biocatalysts to be used for broad-based protection from bacterial pathogens.