Molecular and phenotypic comparison of two staphylococcus aureus strains associated to severe or mild mastitis in cows

Staphylococcus aureus is an opportunistic pathogen in humans and warm-blooded animals. In milk production, it is notably a major cause of mastitis in dairy ruminants, which induces huge economical loss. In this work, two S. aureus strains isolated from bovine mastitis were compared: RF122 and Newbould305, which reproducibly induce severe and mild mastitis, respectively. We previously sequence-typed RF122 (ST151) and Newbould305 (ST115) and showed that RF122 was more closely related to ovine strains whereas Newbould305 clustered with other bovine strains as determined by Whole Genome PCR Scanning and Comparative Genome Hybridization (J.Bact. 2008. 190(19):6302). The goal of this work was to deeper identify staphylococcal factors that could explain the differences of clinical signs and mastitis acuteness observed for these strains. We thus further compared the two strains, in silico and in vitro, at the genome, proteome and phenotype levels. The genome of S. aureus Newbould305 was sequenced using an Illumina Genome Analyzer. A total of 2752 coding sequences were detected and Newbould305 sequence was compared to that of RF122, which is publicly available. The overall Newbould305 genome shares high similarity with that of RF122. The majority of the genes (n= 1819) were common to both strains. Proteome analysis of supernatant, cell wall and total cell fractions was carried out in conditions mimicking the mastitis context and results lead to an additional characterization at the phenotype level. Efficiency of adhesion and internalisation of both strains were indeed tested and compared in vitro on bovine mammary epithelial cells (MAC-T). The values for adhesion and internalization showed significantly higher rates for Newbould 305 with differences of up to 1 order of magnitude in adhesion and 1.5 orders of magnitude for internalization. However, once internalized, intracellular survival seemed to be identical for both strains. This work provides targets for further investigations, especially to determine whether the differences observed at these initial steps of infection are indeed critical for the development and acuteness of cow mastitis.