The efficacy of individual and combined commercial protective cultures against Listeria monocytogenes, Salmonella, O157 and non-O157 shiga toxin-producing Escherichia coli in growth medium and raw milk

Abstract

Cheeses made from unpasteurized milk are preferred by many consumers for their complex flavors and aromas among other attributes. However, the potential presence and survival of foodborne pathogens remain a concern. There is, therefore, a need for developing practical and science-based preventive strategies to enhance the safety of these products. The use of commercially produced protective bacterial cultures (PC) can be a practical approach. This study examined the potential of commercially produced PCs when used both individually or in combinations to control Listeria monocytogenes and Salmonella, as well as O157:H7 and non-O157:H7 shiga toxin-producing Escherichia coli (STEC) in growth medium and raw milk. The antimicrobial activity of nine individual PCs and 28 combinations thereof was first determined in growth medium using the cell-free supernatants (CFS). Following the determination of median inhibitory concentrations of individual PCs and their combinations, fractional inhibitory concentration indexes were calculated to classify combinations as synergistic, additive, or antagonistic. Overall, L. monocytogenes was inhibited by lower concentrations of CFS compared to the Gram-negative pathogens. Combinations of PCs with synergistic antimicrobial activity against each pathogen were identified. The efficacy of individual PCs was further determined in raw milk following an incubation time and temperature profile similar to that of cheesemaking and ripening. L. monocytogenes growth was inhibited to counts of ~6 log and ~3 log CFU/mL lower than control when cocultured with PCs of Lactococcus lactis or Lactobacillus plantarum, respectively. Hafnia alvei inhibited the growth of STEC O157, non-O157 STEC, and Salmonella enterica in coculture to ~2 log, ~3.5 log, and ~3 log CFU/mL lower than pathogen controls, respectively. Potential synergistic activity was noted for the combination of Lc. lactis and Pediococcus acidilactici against L. monocytogenes when cocultured in raw milk. This study provides data for the efficacy of commercially produced PCs in controlling the growth of pathogens, suggesting that the use of PCs is a promising strategy for enhancing food safety including the safety of raw milk products such as cheese.