Analysis of Staphylococcus aureus proteins secreted inside infected human epithelial cells.

Staphylococcus aureus, an opportunistic pathogen is able to invade into and persist inside non-professional phagocytic cells. To do so, this bacterium possesses a wide range of secreted virulence factors which enable attachment to the host as well as intracellular survival. Hence, a monitoring of virulence factors specifically produced upon internalization might reveal targets for prevention or therapy of S. aureus infections. However, previous proteome approaches enriching S. aureus from lysed host cells after infection did not cover secreted virulence factors. Therefore, we used density gradient centrifugation and mass spectrometry to identify S. aureus HG001 proteins which were secreted into compartments of infected human bronchial epithelial S9 cells. Because shotgun mass spectrometry revealed only few bacterial proteins amongst 1905 host proteins, we used highly sensitive and selective single reaction monitoring mass spectrometry as an alternative approach and quantified 37 bacterial proteins within the S. aureus containing host cell compartment 2.5 h and 6.5 h post infection. Among them were secreted bacterial virulence factors like lipases, pore forming toxins, and secreted adhesins which are usually hard to detect from infected sample material by proteomics approaches due to their low abundance. S. aureus adapted its proteome to improve its response to oxidative and cell wall stress occurring inside the host, but also, increased the amounts of some adhesins and pore-forming toxins, required for attachment and host cell lysis.