Outer-membrane proteomic maps and surface-exposed proteins of Legionella pneumophila using cellular fractionation and fluorescent labelling.

Bacterial surface-associated proteins play crucial roles in host-pathogen interactions and pathogenesis. The identification of these proteins represents an important goal of bacterial proteomics for vaccine development, but also for environmental concerns such as microbial biosensing. Here, we developed such an approach for Legionella pneumophila, a bacterium that causes severe pneumonia. We propose a complementary strategy consisting of (1) a fluorescent labelling of surface-exposed proteins in parallel with (2) a fractionation of the outer-membrane protein extract. These two distinct protein populations were subsequently separated using two-dimensional gel electrophoresis and characterised by mass spectrometry. Within these populations, we found proteins which were expected for the compartments studied, but also a great number of proteins never experimentally described, and also a non-negligible fraction of proteins never described in these fractions. These data provided new routes of inspection for transport and host recognition for Legionella pneumophila. In addition, these results on the membranome and surfaceome show that Legionella in the stationary phase of growth possesses the major determinants to infect host cells.