Outer membrane vesicle

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Outer membrane vesicles (OMVs) are vesicles released from the outer membranes of Gram-negative bacteria. While Gram-positive bacteria release vesicles as well, those vesicles fall under the broader category of bacterial membrane vesicles (MVs). OMVs were the first MVs to be discovered, and are distinguished from outer inner membrane vesicles (OIMVs), which are gram-negative bacterial vesicles containing portions of both the outer and inner bacterial membrane.^[1] Outer membrane vesicles were first discovered and characterized using transmission-electron microscopy^[2] by Indian Scientist Prof. Smriti Narayan Chatterjee and J. Das in 1966-67.^[3][4] OMVs are ascribed the functionality to provide a manner to communicate among themselves, with other microorganisms in their environment and with the host. These vesicles are involved in trafficking bacterial cell signaling biochemicals, which may include DNA, RNA, proteins, endotoxins and allied virulence molecules. This communication happens in microbial cultures in oceans,^[5] inside animals, plants and even inside the human body.^[6]

Gram-negative bacteria deploy their periplasm to secrete OMVs for trafficking bacterial biochemicals to target cells in their environment. OMVs also can carry endotoxic lipopolysaccharide that may contribute to disease processes in their host.^[7][8] This mechanism imparts a variety of benefits like, long-distance delivery of bacterial secretory cargo with minimized hydrolytic degradation and extra-cellular dilution, also supplemented with other supportive molecules (e.g., virulence factors) to accomplish a specific job and yet, keeping a safe-distance from the defense arsenal of the targeted cells. Biochemical signals trafficked by OMVs may vary largely during 'war and peace' situations. In 'complacent' bacterial colonies, OMVs may be used to carry DNA to 'related' microbes for genetic transformations, and also translocate cell signaling molecules for quorum sensing and biofilm formation. During 'challenge' from other cell types around, OMVs may be preferred to carry degradation and subversion enzymes. Likewise, OMVs may contain more of invasion proteins at the host–pathogen interface (Fig. 1). It is expected that environmental factors around the secretory microbes are responsible for inducing these bacteria to synthesize and secrete specifically-enriched OMVs, physiologically suiting the immediate task. Thus, bacterial OMVs, being strong immunomodulators,^[9] can be manipulated for their immunogenic contents and utilized as potent pathogen-free vaccines^[10] for immunizing humans and animals against threatening infections. VA-MENGOC-BC and Bexsero against meningitis are currently the only OMV vaccines approved in the US, though an OMV vaccine for gonorrhea is seeking approval.^[11][12]