Antimicrobial, Antivirulence and Probiotic Activities of Protective Bacterial Cultures Against Foodborne Pathogens

Abstract

Despite mandated interventions to improve food safety, foodborne outbreaks continue to occur throughout the world. At the same time, there is increasing consumer interest towards natural and functional foods. Therefore, there is a need for natural preventive controls to enhance food safety. Beneficial bacteria such as protective cultures (PCs) represent a promising strategy for pathogen protection in food and in human hosts. The overall goal of this dissertation research was to determine the antimicrobial, antivirulence and probiotic activities of PCs against foodborne pathogens. The antimicrobial activity of individual and combined PCs was first determined using cell-free supernatants. Overall, PC supernatants were more effective against Listeria monocytogenes compared to Gram-negative pathogens and synergistic combinations of PCs were identified. Cocultures of PCs in raw milk following a cheesemaking time and temperature profile revealed that growth of L. monocytogenes and Staphylococcus aureus was inhibited when cocultured with Lactococcus lactis or Lactiplantibacillus plantarum. Coculture with Hafnia alvei in raw milk inhibited growth of shiga toxin-producing Escherichia coli, Salmonella, and S. aureus. Potential synergistic activity against L. monocytogenes was also observed for the combination Lc. lactis and Pediococcus acidilactici. The ability of PCs to attenuate pathogen virulence in food was then determined. Several PCs reduced enterotoxin production by S. aureus in raw milk and laboratory medium. Pre-exposure of L. monocytogenes to Lb. plantarum in milk reduced the pathogen ability to subsequently survive simulated gastrointestinal conditions. L. monocytogenes invasion and translocation across human Caco-2 cells was also reduced following pre-exposure to Lc. lactis and Lb. plantarum. PCs with demonstrated antimicrobial and antivirulence activities were selected to determine their potential as probiotics to protect against L. monocytogenes infection. Lb. plantarum cultures were resistant to gastrointestinal conditions, while Lc. lactis was sensitive to such conditions. Pretreatment of Caco-2 monolayers with these PCs reduced adhesion, invasion, and translocation of L. monocytogenes across the epithelial barrier, as well as pathogen-induced cytotoxicity, pathogen-mediated tight junction permeability, and pathogen-induced mortality in Caenorhabditis elegans. These findings suggest the potential for PCs to enhance the safety of food products and to exert probiotic effects in the host.