Staphylococcus spp. play an important role in the etiology of bovine mastitis. Staphylococcus aureus is considered the most relevant species due to the production of virulence factors such as slime, which is required for biofilm formation. This study aimed to evaluate biofilm production and its possible relation to beta-lactamic resistance in 20 S. aureus isolates from bovine mastitic milk. The isolates were characterized by pheno-genotypic and MALDI TOF-MS assays and tested for genes such as icaA, icaD, bap, agr RNAIII, agr I, agr II, agr III, and agr IV, which are related to slime production and its regulation. Biofilm production in microplates was evaluated considering the intervals determined along the bacterial growth curve. In addition, to determine the most suitable time interval for biofilm analysis, scanning electron microscopy was performed. Furthermore, genes such as mecA and blaZ that are related to beta-lactamic resistance and oxacillin susceptibility were tested. All the studied isolates were biofilm producers and mostly presented icaA and icaD. The Agr type II genes were significantly prevalent. According to the SEM, gradual changes in the bacterial arrangement were observed during biofilm formation along the growth curve phases, and the peak was reached at the stationary phase. In this study, the penicillin resistance was related to the production of beta-lactamase, and the high minimal bactericidal concentration for cefoxitin was possibly associated with biofilm protection. Therefore, further studies are warranted to better understand biofilm formation, possibly contributing to our knowledge about bacterial resistance in vivo.

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Staphylococcus aureus is an important pathogen involved in subclinical bovine mastitis, causing high economic losses for the dairy industry. The successful persistence of this pathogen in the host occurs due to a series of factors associated with colonization ability and the acquisition of virulence factors. This bacterial species carries genetic heterogeneity, and genetically diverse strains characterize the population. Analysis of the genetic variation is an important tool for epidemiological studies. For this study, S. aureus strains were randomly selected by molecular profiling. All strains were originated from the milk of cows of subclinical mastitis on farms in the State of Rio de Janeiro. Strains of S. aureus were profiled using virulence gene analysis profiles, agr and spa typing, Pulsed Field Gel Electrophoresis (PFGE), and Multilocus Sequence Typing (MLST). 47% (8/17) of the tested strains were positive strains for icaA gene; 82.3% (14/17) for icaD gene; 41% (7/17) for fbnA gene; 47% (8/17) positive for fbnB gene; 94% (16/17) for the hlA gene and 70.5% (12/17) for hlB gene. These virulence results generated 11 different profiles. Most strains (58.8% -10/17) were classified as type-II by agr system. spa typing identified seven different spa types. PFGE analysis found extensive genetic heterogeneity and no clones were observed. MLST analysis generated five different types of ST/CC. Considering the results observed at the present study, a high genetic variety of S. aureus strains associated of the presence of different virulence factors justified the absence of clonal strains at the properties evaluated. Besides, that, the definition of S. aureus clonal strain as well dynamic population in not fully understood since there are a limited number of studies in S. aureus associated with bovine mastitis.