Impact of food matrix on the activity of a phage-derived thermostable modular lytic enzyme against Salmonella in dairy products.

Abstract

Food contamination remains a major challenge for the food industry, including the dairy sector. Rich and complex composition of milk provides an excellent environment for the growth of pathogenic microorganisms, among which the most common bacterial contaminants include enterotoxin-producing Staphylococcus aureus, Listeria monocytogenes, Shiga toxin-producing Escherichia coli (STEC), and Salmonella spp. In this study we characterized the modular lytic enzyme MLE-19 obtained through modular engineering and its potential as an antibacterial agent to protect milk and dairy products against Salmonella enterica ssp. enterica PCM 2266. We demonstrated that MLE-19 with a catalytic domain of the thermostable endolysin Ph2119, exhibited high thermal stability with a Tm of 101.66°C. Furthermore, pre-treatment of MLE-19 under pasteurization conditions (80°C, 10 min) enhanced its anti-Salmonella activity compared with the unheated enzyme (reductions of 1.14 and 0.45 log units compared with the control), highlighting its applicability in the dairy industry. The minimum inhibitory concentration (MIC) of MLE-19 against Salmonella cells was 100 µg/mL, and the presence of 0.5 mM EDTA reduced this value 4-fold (to 25 µg/mL). MLE-19 caused significant decreases in bacterial load at concentrations of 12.5 and 200 µg/mL for planktonic and biofilm cells, respectively. Food matrix represented by milk, yogurt and cottage cheese and external conditions such as temperature and duration had a variable impact on the protective activity of MLE-19. Significant reductions were observed in all food matrices with the most pronounced antibacterial effects in milk, where up to 3 log reduction when storing at 4°C and 20°C were achieved. We propose that the enzyme exhibited greater mobility in milk, enabling more efficient access to and degradation of Salmonella cells. Our findings identify the thermostable MLE-19 as a new promising agent for controlling Salmonella spp. in the dairy industry.