Foodborne Enterobacteriaceae pathogens, especially Salmonella, still seriously threaten food safety. To establish a foundation for further developing phage- and endolysin-based methods combating these pathogens, in this study, the newly isolated Salmonella-virus-FelixO1 phage BPS15S6 for biocontrol purposes was characterised by genomic bioinformatic analysis, and then its endolysin LyS15S6 was obtained using a prokaryotic expression system, characterised in vitro and evaluated in the antibacterial efficacy. It was shown that BPS15S6 had an 87,609-bp genome with 130 open reading frames and does not appear to carry known lysogeny-associated genes and other damaging genetic determinants and is unlikely to perform generalised transduction. Furthermore, LyS15S6 was determined to possess the high enzymatic activity of 1,001,000 U mg-1 and the broad spectrum of lysing 56 tested Gram-negative strains. The assays of thermostability and optimum pH revealed that LyS15S6 was stable up to 40 °C and more active at pH 7. Notably, we demonstrate that edible ε-poly-L-lysine (EPL) can be used as an outer-membrane permeabiliser to improve the antibacterial performance of endolysins. When combined with 1 μg ml-1 EPL, 2 μM LyS15S6 could cause 3-4 log viable cell reductions of the three tested Enterobacteriaceae pathogens in vitro after 2 h of reaction at 25 °C and 2.56 and 3.14 log reductions of Salmonella ATCC13076 after 15 min of reaction at 25 °C and 2 h of reaction at 8 °C respectively. A new strategy, the combined application of endolysins and edible EPL for combating Enterobacteriaceae pathogens in food, is thus presented in this work.
Keywords: Epsilon-poly-L-lysine; Genome; Glycoside hydrolase; Gram-negative bacteria; Outer membrane permeabiliser.