Exploring the Staphylococcus aureus cell wall for invariant immunodominant targets

Bovine mastitis is the most common livestock disease affecting the dairy industry. It refers to an infection of the mammary gland or udder. This disease can be caused by different bacteria, with Staphylococcus aureus being one of the most common causative agents. Mastitis is usually treated with antibiotics, but a vaccine to prevent this infection would be preferable, because S. aureus rapidly acquires antibiotic resistance. Unfortunately, no anti-staphylococcal vaccine is currently available. Therefore, the present PhD research was aimed at the characterization of potential targets for the development of anti-staphylococcal vaccines that protect not only livestock, but also humans. The approach involved the identification and genetic typing of S. aureus isolates from milk samples of cows suffering from mastitis. The subsequent identification of surface-exposed or cell wall-located proteins from S. aureus mastitis isolates was achieved by proteomics, using cell surface shaving and cell wall protein extraction. Cell surface-exposed antigens were produced and isolated using a Lactococcus lactis-based expression system. Selected purified antigens were individually used for immunization of mice, which elicited strong but non-protective immunoglobulin G (IgG) responses. Finally, several antigen domains were used to map the epitopes recognized by IgGs from immunized mice, colonized humans and cows suffering from mastitis. The results highlight differential species-specific IgG responses against the investigated staphylococcal antigens and their domains. Altogether, the research described in this thesis presents new leads towards the development of vaccines that protect frail individuals from life-threatening staphylococcal diseases, and dairy farmers from the painful losses caused by bovine mastitis