Virulence properties of nisin-resistant bacteria isolated from raw milk and cheese samples

Nisin is a bacteriocin produced by Lactococcus lactis and it is generally recognised as safe by the U.S. Food and Drug Administration. The study aimed to determine the presence of nisin-resistant Staphylococcus, Enterococcus, and Listeria spp. in 157 raw milk and 125 cheese samples. As a result of the isolation procedure, a total of 282 bacterial strains were isolated. 83/282 strains showed resistance to 300 ppm nisin concentration and they were identified by 16S rRNA sequencing. The two most common species were Enterococcus faecalis and Staphylococcus aureus. In both raw milk and cheese samples, nisin-resistant Listeria could not be detected. Generally, enterococci (59/65) showed higher biofilm formation ability than staphylococci (13/18). 38% (25/65) of enterococci and 88% (16/18) of staphylococci were determined to have protease activity in at least one of Skim Milk Agar, Casein Agar, or Milk Agar. In addition, the most common antibiotic resistance in nisin-resistant enterococci was against chloramphenicol. This study revealed that nisin-resistant staphylococci and enterococci are present in raw milk and cheese. In addition, the presence of some virulence factors such as biofilm production, protease activity, and antibiotic resistance in resistant isolates needs to be drawn attention to. Consumption of nisin-resistant microorganisms with virulence factors through food is a food safety risk.