Biofilm-formation by Staphylococcus aureus and Staphylococcus epidermidis isolates from subclinical mastitis in conditions mimicking the udder environment.

Staphylococcus is the genus most commonly isolated from bovine mastitis in many countries. It may express several virulence factors including biofilm formation, which may protect the bacterial community from antimicrobials' action, preventing these compounds from reaching its interior, where they reach subinhibitory concentrations (subMIC). Most biofilm production assays are performed in static conditions, while studies regarding antimicrobial resistance usually do not resemble the udder environment because they are performed at high concentrations. In this study we evaluated the influence of dynamic conditions and media, including Mueller Hinton Broth (MHB) and UHT whole milk (WM), as well as the effect of subMIC concentrations of five different antimicrobial agents on biofilm formation by staphylococci isolated from subclinical mastitis. Results suggest that dynamic conditions and media may influence biofilm formation and revealed that milking simulation may significantly increase biofilm production. Sub-MIC concentrations decrease biofilm formation in MHB but increase in WM, suggesting a protective role of milk against antimicrobial compounds' action. Therefore, in vitro conditions that simulate the udder environment and in vivo conditions should be included as one of the parameters in evaluation of biofilm producing strains, in order to provide more reliable results.