Characterization of Lactococcus lactis isolates from bovine mastitis

Lactococcus lactis is a widely used mesophilic dairy starter and has been included in the Qualified Presumption of Safety (QPS) list of the European Food Safety Authority. However, it is increasingly found as the cause of human or animal infections, such as bovine mastitis, probably due to the improvement of the identification of the infective microorganisms. Since there are some grounds to suspect that at least certain variants of L. lactis may cause animal or human diseases, it is important to properly identify the differences between the strains associated with infections and the safe starter strains. Bovine mastitis isolates and dairy starter strains were genotypically characterized and clustered by the 16S rRNA gene sequence and RAPD-PCR fingerprint patterns, and phenotypically characterized by their tolerance to different stress conditions typically found in the intestinal tract of mammals, the carbohydrate- and antibiotic resistance profile, as well as the in vitro adhesion capacity to udder epithelial cells. Genotypically, there were no differences between the mastitis isolates and the dairy starter strains. However, there were clear phenotypic distinctions between mastitis isolates and typical starter strains, the former showing an increased tolerance to temperature, lysozyme, bile salts, pH and antibiotics, as well as improved carbohydrate fermentation capacity, and in vitro adhesion to udder epithelial cells. Although these differences might not be considered as actual virulence factors, they improve the ability of the strain to survive in the body of homeothermic animals and, interestingly, are also typical properties associated with potential probiotic strains.