Identification of Protein A and Capsular Structures in Staphylococcus aureus Isolates from Milk of Cows with Subclinical Mastitis

Staphylococcus aureus is a primary etiological agent of subclinical bovine mastitis. While commonly part of the normal microbiota, it can transition into a pathogen through its capacity to produce toxins, invade host tissues, and resist antimicrobial agents. Its virulence is primarily linked to surface-expressed factors such as Protein A and polysaccharide capsules, which facilitate immune evasion by inhibiting phagocytosis. In this study, 28 S. aureus isolates were obtained from preserved biological materials (BBT) and assessed for the presence of Protein A and capsular structures. Protein A was identified using the serum soft agar (SSA) method, and polysaccharide capsules were detected via the salt aggregation test (SAT). Results showed that 25 of the 28 isolates expressed Protein A, while all exhibited capsule formation. These findings suggest that both virulence factors play a crucial role in the pathogenic potential of S. aureus in subclinical mastitis.