This dissertation concentrates on the study of the antibacterial potential of bacteriophage-encoded endolysins derived from phages that infect the Gram-positive Bacillus cereus sensu lato group. Bacteriophage-encoded endolysins are peptidoglycan hydrolases that have been identified as important factors in the phage life cycle. Endolysins are encoded by phage late genes during an intracellular infection cycle to lyse the bacterial cell wall from within and allow phage progeny release. Endolysins derived from phages of Gram-positive bacterial hosts are equipped with an enzymatic domain that hydrolyzes conserved covalent bonds in bacterial peptidoglycan, and a cell wall binding domain that ensures proper attachment of endolysins to bacilli. In this study three novel endolysins, PlyP56, PlyN74, and PlyTB40 have been discovered and identified. The biochemical analysis shows that all three endolysins have relatively broad antimicrobial activity against organisms of the B. cereus group with the optimal lytic activity at physiological pH (pH 7.0–8.0), over a broad temperature range (4–55°C), and at low concentrations of NaCl (<50 mM). The domain shuffling engineering studies were undertaken to observe enhancements of bacteriolytic properties of chimeric lysins that retained their specificity to B. cereus species. Finally, these studies have identified a new development in lysis of peptidoglycan of Gram-positive B. cereus group of organisms by phage-encoded endolysins. When grown to stationary phase, bacilli, especially, in overnight cultures become more resistant to lysis despite the evidence that the cell wall domains bind the bacterial surface. In light of these findings, I hypothesize that B. cereus group of species have evolved complex behaviors to interact with phage by modulating surface associated secondary polymers throughout the maturation of the bacilli in order to render them more resistant to the lytic action of phage encoded endolysins, which, contributes to bacterial survival from phage infection.