The application of the lytic domain of endolysin from Staphylococcus aureus bacteriophage in milk

Staphylococcus aureus is a widespread foodborne pathogen that threatens human health. In particular, multidrug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) are emerging problems in modern health care, food safety, and animal health, which require the development of new antimicrobials to replace overused conventional antibiotics. Dairy products can potentially act as vehicles for the transmission of S. aureus and other antibiotic-resistant strains from the farm into the general human population, and should be controlled during the production and storage process. Recently, bacteriophage endolysins, which degrade the cell wall that is indispensable for bacteria, have been deemed promising antimicrobial agents. In this study, one endolysin, LysGH15, demonstrated prominent antimicrobial efficacy against S. aureus, as did its catalytic domain, cysteine, histidine-dependent amidohydrolase/peptidases (CHAP)_LysGH15 alone. The LysGH15 and CHAP_LysGH15 exhibited different characteristics in one MRSA strain (MRSA 2701), reaching the highest activity under different conditions (35°C and pH 6.0 for LysGH15, 40°C and pH 9.0 for CHAP_LysGH15). A difference in the sensitivity of LysGH15 and CHAP_LysGH15 to NaCl concentration was found, where the lytic activity of LysGH15 depends strongly on its binding domain's binding capacity, which is positively correlated with the NaCl concentration, whereas the CHAP_LysGH15 activity showed a negative correlation with the NaCl concentration. When the NaCl concentration was 450 mM, the lytic activity of LysGH15 reached its peak, whereas the lytic activity of CHAP_LysGH15 was the highest in the absence of NaCl. The difference in NaCl sensitivity between LysGH15 and CHAP_LysGH15 may be due to the sensitivity of the SH3b binding protein of LysGH15 to NaCl. The CHAP_LysGH15 was tested as a biopreservative in whole and skim milk and exerted effective control against S. aureus (declined by approximately 2.5 log₁₀ cfu/mL when incubated at 4°C for 8 h), which suggests promise for application in dairy products.