Slime production and biofilm forming ability by Staphylococcus aureus bovine mastitis isolates

Some Staphylococcus aureus strains, identified as causative agents of mastitis in cattle, exhibit the ability of producing a viscous extracellular polysaccharide layer (slime), which is nowadays considered to be a virulence factor, as it promotes bacterial adhesion onto the mammary epithelial cells and protects bacteria from opsonization and phagocytosis. Some strains of this genus are believed to exist in the form of a biofilm in the udder tissue, which may partly explain frequent therapeutic failures and a chronic course of infection. In this study we investigated the ability of slime production and biofilm formation in 70 subclinical and clinical bovine mastitis isolates S. aureus. Slime production was determined from colony morphology of isolates in Congo red agar. The ability of biofilm formation was assessed in a quantitative assay using a microtiter-plate test involving crystal violet staining and for selected strains, scanning electron microscopy on stainless steel coupons was done. Eight S. aureus isolates (11.42%) formed black colonies of dry consistency, characteristic for slime-producing strains. According to the result of microtiter plate test, 9 isolates (12.86%) were categorized as strong biofilm producers, 21 (30%) as moderate, and 40 (57.14%) as weak biofilm producers. Scanning electron microscopy revealed differences between investigated isolates with respect to their ability to colonize stainless steel surfaces and to form a three-dimensional biofilm structure.