An association between milk and slime increases biofilm production by bovine Staphylococcus aureus

Staphylococcus aureus is associated with chronic mastitis in cattle, and disease manifestation is usually refractory to antibiotic therapy. Biofilm production is a key element of S. aureus pathogenesis and may contribute to the treatment failure that is consistently reported by veterinarians. Minas Gerais State is the largest milk-producing state in Brazil, and the characterization of bacterial isolates is an important aspect of disease control for dairy farmers. Here, we investigated the potential of S. aureus isolated from bovine mastitis to produce slime and biofilm in a skim-milk medium and classified the isolates according to their agr type.

Slime was detected using the Congo Red agar (CRA) test in 35.18% (19/54) of the strains; however, 87.04% (47/54) of the strains were considered biofilm-positive based on crystal violet staining. Compared to TSB supplemented with 0.25% glucose, skim milk significantly increased the production of biofilm, but this effect was only observed in slime-producing strains. The bacteria belonged to agr groups I (12/54), II (34/54), III (6/54), and IV (2/54), and bacteria in agr group III were found to be stronger biofilm producers than those in groups I and II. Again, milk had a significant influence only on slime-positive agr I and II isolates, revealing an association between milk and slime.

The present study demonstrated that skim-milk medium and slime production are two factors that together influence biofilm formation by bovine strains of S. aureus. A predominance of bacteria belonging to agr group II was observed, and bacteria from agr group III showed the highest proportion of biofilm producers. The majority of bacteria characterized in this study formed biofilm in milk, which suggests that biofilm formation has an important role in the virulence of S. aureus isolated from bovine intramammary infections.