Nisin or Chitosan Enhance the Antimicrobial Activity of Ceftiofur Against Antibiotic-Resistant Staphylococcus aureus and Have Antibiofilm Effects

Mastitis is one of the major diseases affecting dairy cattle worldwide. Antibiotic therapy remains the most widely used treatment. However, its effectiveness has been compromised due to the selection of antibiotic-resistant and biofilm-producing pathogenic bacteria. This promotes the search for alternatives that increase the antibacterial and antibiofilm efficacy of antibiotics such ceftiofur (CFT). Nisin (N) and chitosan (CH) may possess these properties. The aim of this study was to evaluate whether N + CFT and CH + CFT combinations enhance the antibacterial activity of the antibiotic on <i>Staphylococcus aureus</i> associated with bovine mastitis, as well as its antibiofilm effect. Two clinical isolates of <i>S. aureus</i> (AMC-43 and AMC-48) and the reference strain ATCC 27543 resistant to CFT were used. Through the microdilution method in 96-well microplates, 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 bacterial growth; however, the CH + CFT mixtures were the most efficient. The crystal violet staining method and live cell plating showed antibiofilm activity in biofilm synthesis and in the reduction in 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.