Differential expression profiling of proteomes of pathogenic and commensal strains of Staphylococcus aureus using SILAC

Staphylococcus aureus (S. aureus) is the etiological agent of food-borne diseases, skin infections in humans and mastitis in bovines. S. aureus is also known to exist as a commensal on skin, nose and other mucosal surfaces of the host. This symbiotic association is a result of immune dampening or tolerance induced in the host by this pathogen. We proposed the variation in protein expression by commensal and pathogenic strain as an important factor behind the difference in pathogenicity. The identification of differentially expressed proteins was carried out using a quantitative mass spectrometry (MS)-based proteomic approach, known as stable isotope labeling of amino acids in cell culture (SILAC). Four commensal and pathogenic strains each were grown in the SILAC minimal media (RPMI 1640), containing light (12C) and heavy (13C) form of lysine, respectively, until early stationary growth phase. Various protein fractions, including cell wall, membrane and secreted, were extracted from the bacterial cultures and mixed in a 1:1 ratio. The relative abundance of proteins present in light and heavy labeled samples was determined using MS analysis. From a total of 151 differentially expressed proteins, 58 were found to be upregulated in the pathogenic strains. These proteins are involved in a variety of cellular functions, including immune modulation, iron-binding, cellular transport, redox reactions, and metabolic enzymes. The differentially expressed proteins can serve as putative candidates to improve current approach towards development of a vaccine against S. aureus.