Staphylococcus aureus chronic intramammary infections in dairy cows: Pathogen-specific characteristics

Staphylococcus aureus is one of the main causes of mastitis in dairy cows worldwide. The most common form of the disease is characterized by chronic subclinical infection with sporadic clinical episodes that can persist throughout the life of the animal. Once infection evolves to chronicity, the bacteriological cure rate following classic antibiotic therapy is low and persistently infected cows become the main reservoir of the organism in a dairy herd. The establishment and persistence of S. aureus intramammary infection (IMI) has been associated with certain characteristics of the pathogen that allow it to evade the immune response or induce a deficient immune response that is unable to eliminate the pathogen. The main pathogen characteristics that have been suggested to be linked to S. aureus persistence in the bovine mammary gland (MG), include the capacity to invade cells and/or survive intracellularly and to form biofilms, the capsular polysaccharides (CP) production, the ability to form small colony variants (SCVs), the accessory gene regulator ( agr ) type of the strain and the capacity to acquire antibiotic resistance. This review summarizes current knowledge about the most relevant pathogen-specific characteristics that are associated with the establishment and chronicity of S. aureus IMI. A better understanding of these pathogen characteristics will allow for the development of intervention strategies aimed at improving different aspects of the control of this disease, including diagnostic methods, therapeutics and modulation of the host immune response.