Identifying Potential Therapeutic Targets of Methicillin-resistant Staphylococcus aureus Through in Vivo Proteomic Analysis

Staphylococcus aureus is a leading cause of hospital- and community-acquired infections [1]. It is a versatile pathogen capable of causing a wide spectrum of diseases ranging from minor skin infections to life-threatening diseases, such as endocarditis, osteomyelitis, and sepsis [2]. This versatility is enabled by a large arsenal of virulence factors encoded on the genome [3]. Surface-associated proteins are important virulence factors as they are involved in multiple processes of bacterial pathogenesis, such as mediating adhesion to host cells or sequestering nutrients in nutrient-limiting host environments [4]. Moreover, because of their extracellular localization, surface-associated proteins are attractive targets for both small-molecule antibiotics and large-molecule antibody-based therapeutics. The major classes of surface-associated proteins of S. aureus are sortase-anchored proteins [5, 6] and lipoproteins [7, 8]. To date, the contribution of sortase-anchored proteins to the S. aureus pathogenesis has been extensively studied [9, 10]. In contrast, except for their role as ligands for the mammalian Toll-like receptor 2 [11], the significance of individual lipoproteins for infection has not been well characterized. Several transcriptional analyses have provided insights into how S. aureus regulates the expression of virulence factors under different environmental conditions [12, 13]. However, these data cannot be extrapolated to a complete understanding of the proteome during host infection because protein production can be regulated post-translationally. Detailed knowledge on the protein repertoire of a pathogen during infection is critical not only for elucidating the pathogenesis but also for defining therapeutic targets. Such data, however, are available only for a few human pathogens and comprehensive proteomics study of S. aureus during host infection has not been performed to date [14, 15]. A major challenge in studying the proteome of S. aureus within an intact host has been the recovery of a sufficient number of bacterial cells from infected organs [16, 17]. To circumvent this, alternative approaches have been employed. They include analysing the antibody repertoire of infected subjects to identify antigens produced by S. aureus during host infection [18, 19], or conducting proteomic analysis under in vitro conditions that were thought to mimic the in vivo environment such as low iron concentration [20]. Each approach, however, has limitations. The first approach is only able to detect a subset of proteins that are highly immunogenic during infection, whereas the second approach does not recapitulate the complexity of an in vivo environment. Here we report the first characterization of the S. aureus proteome derived directly from infection of an intact animal. We characterized the surface proteome of the methicillin-resistant s. aureus (MRSA) USA300 strain, the most prevalent MRSA strain causing community-acquired infections in the United States [21, 22], using a murine systemic infection model. Our data contribute to a better understanding of the pathogenesis of MRSA and help in defining potential therapeutic targets.