Nisin and Chitosan Enhance the Antimicrobial Activity of Ceftiofur Against Antibiotic-Resistant <em>Staphylococcus aureus</em> and Have Anti-Biofilm Effect

Mastitis is one of the major diseases affecting dairy cattle worldwide. Antibiotic therapy remains the most widely used treatment. However, its effectiveness has been com-promised due to the selection of antibiotic-resistant and biofilm-producing pathogenic bacteria. This promotes the search for alternatives that increase the antibacterial and anti-biofilm efficacy of antibiotics such ceftiofur (CFT). Biocompounds that, combined with antibiotics, exhibit these characteristics are gaining attention. Nisin (N) and chi-tosan (CH) may possess these properties. The aim of this study was to evaluate whether N+CFT and CH+CFT enhance the antibacterial activity of the antibiotic on Staphylococcus aureus associated with bovine mastitis, as well as its anti-biofilm effect. Two clinical isolates of S. aureus (AMC-43 and AMC-48) and the reference strain ATCC 27543 re-sistant to CFT were used. The combination of sub-inhibitory concentrations of N (320 µg/mL) and CH (400 µg/mL) with CFT (1, 2, 4, and 8 µg/mL) significantly reduced bac-terial growth (expressed in logarithmic units); however, the CH+CFT mixtures were the most efficient. Anti-biofilm activity was also detected, both in biofilm synthesis and in the reduction of living bacterial cells located inside this preformed structure. These results highlight N and CH as potential agents for the prevention or control of bovine mastitis.