Lactococcus lactis: From Probiotic to Pathogen - A Review on its Origin, Diversity, and Pathogenicity

However, due to the abuse of commercial 'L. lactis', it has been isolated from clinical samples, such as the brain of silver carp with Lactococcosis, the liver and spleen of diseased waterfowl, milk samples, and padding from cows with mastitis, as well as blood and urine from patients with endocarditis. This suggests that 'L. lactis' impedes the prosperity of livestock farming and poses a serious threat to human health. Especially in dairy farming, where 'L. lactis' has been used as a probiotic in the past, some studies have found that 'L. lactis' can cause mastitis in cows. However, the lack of understanding of the pathogenesis of mastitis in cows caused by 'L. lactis' has become a new problem. The ability of the mechanisms of 'L. lactis' to evade the defenses of the immune system and thus invade the host, as well as the interconversion of resistance and pathogenicity genes between bacteria, are factors in the failure of antibiotic therapy, making clinical and veterinary treatment more difficult. The main objective of this review is to discuss the origin, natural diversity, probiotic action, and pathogenicity of 'L. lactis' and provide new insights for its future application in different fields.