Outer membrane vesicles (OMVs) released from Gram-negative bacteria consist of lipids, proteins, lipopolysaccharides and other molecules. OMVs are associated with several biological functions such as horizontal gene transfer, intracellular and intercellular communication, transfer of contents to host cells, and eliciting an immune response in host cells. Although hypotheses have been made concerning the mechanism of biogenesis of these vesicles, research on OMV formation is far from complete. The roles of outer membrane components, bacterial quorum sensing molecules and some specific proteins in OMV biogenesis have been studied. This review discusses the different models that have been proposed for OMV biogenesis, along with details of the biological functions of OMVs and the likely scope of future research.

The outer membrane vesicles (OMVs) from bacteria are known to posses both defensive and protective functions and thus participate in community related functions. In the present study, outer membrane vesicles have been shown to protect the producer bacterium and two other bacterial species from the growth inhibitory effects of some antibiotics. The OMVs isolated from E. coli MG1655 protected the bacteria against membrane-active antibiotics colistin, melittin. The OMVs of E. coli MG1655 could also protect P. aeruginosa NCTC6751 and A. radiodioresistens MMC5 against these membrane-active antibiotics. However, OMVs could not protect any of these bacteria against the other antibiotics ciprofloxacin, streptomycin and trimethoprim. Hence, OMVs appears to protect the bacterial community against membrane-active antibiotics and not other antibiotics, which have different mechanism of actions. The OMVs of E. coli MG1655 sequester the antibiotic colistin, whereas their protein components degrade the antimicrobial peptide melittin. Proteomic analysis of OMVs revealed the presence of proteases and peptidases which appear to be involved in this process. Thus, the protection of bacteria by OMVs against antibiotics is situation dependent and the mechanism differs for different situations. These studies suggest that OMVs of bacteria form a common defense for the bacterial community against specific antibiotics.

Outer membrane vesicles (OMVs) of Gram-negative bacteria form an important aspect of bacterial physiology as they are involved in various functions essential for their survival. The OMVs of the Antarctic bacterium Pseudomonas syringae Lz4W were isolated, and the proteins and lipids they contain were identified. The matrix-assisted laser desorption/ionization time of flight (MALDI-TOF/TOF) analysis revealed that phosphatidylethanolamines and phosphatidylglycerols are the main lipid components. The proteins of these vesicles were identified by separating them by one-dimensional gel electrophoresis and liquid chromatography coupled to electrospray ionization tandem mass spectrometry (ESI-MS/MS). They are composed of outer membrane and periplasmic proteins according to the subcellular localization predictions by Psortb v.3 and Cello V2.5. The functional annotation and gene ontology of these proteins provided hints for various functions attributed to OMVs and suggested a potential mechanism to respond to the extracellular environmental changes. The OMVs were found to protect the producer organism against the membrane active antibiotics colistin and melittin but not from streptomycin. The 1-N-phenylnapthylamine (NPN)-uptake assay revealed that the OMVs protect the bacterium from membrane active antibiotics by scavenging them and also showed that membrane and protein packing of the OMVs was similar to the parent bacterium. The sequestering depends on the composition and organization of lipids and proteins in the OMVs.

Subcellular fractionation of proteins is a preferred method of choice for detection and identification of proteins from complex mixtures such as bacterial cells. To characterize the membrane proteins of the Antarctic bacterium Pseudomonas syringae Lz4W, the membrane fractions were prepared using three different methods, namely Triton X-100 solubilization, sucrose density gradient, and carbonate extraction methods. The proteins were separated on one-dimensional polyacrylamide gels and analyzed using a combination of liquid chromatography-coupled electrospray ionization-MS. The membrane proteins that were prepared by carbonate extraction were separated on twodimensional PAGE in different pI ranges using the detergent 2% amidosulfobetaine (ASB). The proteins were then subjected to matrix-assisted laser desorption ionization-timeof-flight/time-of-flight for analysis and identification. Because the genome sequence of P. syringae Lz4W is not known, the proteins were identified by using the relevant sequence databases of the Pseudomonas sp available at National Centre for Biotechnology Information (NCBI). The sequence identification of some tryptic peptides were validated by de novo sequencing and others by chemical modification and mass spectrometry. The peptide sequences of P. syringae Lz4W were then matched with the sequences of the peptides from different Pseudomonas sp. by similarity search of the proteins from different species using clustal W2 program.