Characterization and proteomic analysis of outer membrane vesicles from a commensal microbe, Enterobacter cloacae

Outer membrane vesicles (OMVs) are membrane-enclosed spherical entities released by gram-negative bacteria and are important for bacterial survival under stress conditions. There have been numerous studies on OMVs released by gram-negative pathogenic bacteria, but an understanding of the functions and characteristics of the OMVs produced by commensal microbes is still lacking. Enterobacter cloacae is a gram-negative commensal bacterium present in the human gut microbiome, but this organism can also function as an opportunistic pathogen. Understanding the OMV-mediated communication route between bacteria-bacteria or bacteria-host is essential for the determination of the biological functions of the commensal bacterium in the gut and delineating between benign and virulent characteristics. In this study, we have described a proteome of E. cloacae OMVs, which are membrane vesicles in a size range of 20-300 nm. Proteomic analysis showed the presence of membrane-bound proteins, including transporters, receptors, signaling molecules, and protein channels. The physical and proteomic analyses also indicate this bacterium uses two mechanisms for OMV production. This study is one of the few existing descriptions of the proteomic profile of OMVs generated by a commensal Proteobacteria, and the first report of OMVs produced by E. cloacae. Significance: This study prioritizes the importance of understanding the vesicular proteome of the human commensal bacterium, Enterobacter cloacae. We demonstrate for the first time that the gram-negative bacterium E. cloacae ATCC 13047 produces outer membrane vesicles (OMVs). The proteomic analysis showed enrichment of membrane-bound proteins in these vesicles. Understanding the cargo proteins of OMVs will help in exploring the physiological and functional role of these vesicles in the human microbiome and how they assist in the conversion of a bacterium from commensal to pathogen under certain conditions. While EM images reveal vesicles budding from the bacterial surface, the presence of cytoplasmic proteins and genomic DNA within the OMVs indicate that explosive cell lysis is an additional mechanism of biogenesis for these OMVs along with outer membrane blebbing. This research encourages future work on characterizing membrane vesicles produced by commensal bacterial and investigating their role in cell to cell communication.