Antibacterial activity and immunomodulatory effects on a bovine mammary epithelial cell line exerted by nisin A-producing Lactococcus lactis strains

Twenty-nine strains of mastitis pathogens were used to study the antibacterial activity of the cell free supernatants (CFSs) of 25 strains of Lactococcus lactis subsp. lactis. Out of the tested strains, only the CFSs of L. lactis LL11 and SL153 were active, inhibiting and killing most of the pathogens. By means of UPLC/HR-MS, they were shown to produce nisin A, a class I bacteriocin. A variable sensitivity to nisin A-containing CFSs was observed among Streptococcus uberis and Enterococcus faecalis strains. Nonetheless, Strep. agalactiae, Strep. uberis and E. faecalis displayed high minimum inhibitory concentration (MIC) values reaching 384 Arbitrary Units/mL (AU/mL). Interestingly, the MIC values and the bactericidal concentrations were almost identical among them for each of the two stains, LL11 and SL153. Staphylococci were on average less sensitive than streptococci, but the two CFSs inhibited and killed, at different dilutions, strains of methicillin-resistant Staphylococcus aureus. The immune response to nisin A-containing CFSs was tested using the bovine mammary epithelial cell line BME-UV1. Application of CFSs did not damage epithelial integrity, as demonstrated by the higher activity of N-acetyl-β-D-glucosaminidase (NAGase) and lysozyme inside the cells, in both treated and control samples. On the other hand, the increase of released NAGase after 15 to 24 h of treatment with LL11 or SL153 live cultures demonstrated an inflammatory response of epithelial cells. Similarly, a significantly higher lysozyme activity was detected in the cells treated with LL11 live culture confirming the stimulation of lysosomal activity. The treatment of epithelial cells with SL153 live culture induced a significant TNF-α down-regulation in the cells, but did not influence IL-8 expression. The control of TNF-α release could be an interesting approach to reduce the symptoms linked to clinical intramammary infections. Due to their antibacterial activity and to the stimulation of lysosomal activity of mammary epithelial cells, the L. lactis strains SL153 and LL11 could be of interest for the development of alternative intramammary treatments to control cow mastitis.