Bacterial outer membrane vesicles (OMVs) are nano‐sized compartments consisting of a lipid bilayer that encapsulates periplasm‐derived, luminal content. OMVs, which pinch off of Gram‐negative bacteria, are now recognized as a generalized secretion pathway which provides a means to transfer cargo to other bacterial cells as well as eukaryotic cells. Compared with other secretion systems, OMVs can transfer a chemically extremely diverse range of cargo, including small molecules, nucleic acids, proteins, and lipids to proximal cells. Although it is well recognized that OMVs can enter and release cargo inside host cells during infection, the mechanisms of host association and uptake are not well understood. This review highlights existing studies focusing on OMV‐host cell interactions and entry mechanisms, and how these entry routes affect cargo processing within the host. It further compares the wide range of methods currently used to dissect uptake mechanisms, and discusses potential sources of discrepancy regarding the mechanism of OMV uptake across different studies.

Summary Vibrio ssp. are associated with infections caused by contaminated food and water. A Type III Secretion System (T3SS2) is a shared feature of all clinical isolates of V. parahaemolyticus and some V. cholerae strains. Despite being responsible for enterotoxicity, no molecular mechanism has been determined for the T3SS2-dependent pathogenicity. Here we show that although Vibrio ssp. are typically thought of as extracellular pathogens, the T3SS2 of Vibrio mediates host cell invasion, vacuole formation and replication of intracellular bacteria. The catalytically active effector VopC is critical for Vibrio T3SS2 mediated invasion. There are other marine bacteria encoding VopC homologues associated with a T3SS and, therefore, we predict that these bacteria will also likely to use T3SS mediated invasion as part of their pathogenesis mechanisms. These findings suggest a new molecular paradigm for Vibrio pathogenicity and modify our view for the roles of T3SS2 effectors translocated during infection.

The initial binding of bacteria to host cells is crucial to the delivery of virulence factors and thus is a key determinant of the pathogen's success. We report a multivalent adhesion molecule (MAM) that enables a wide range of Gram-negative pathogens to establish high-affinity binding to host cells during the early stages of infection. MAM7 binds to the host by engaging in both protein-protein (with fibronectin) and protein-lipid (with phosphatidic acid) interactions with the host cell membrane. We find that MAM7 expression on the outer membrane of a Gram-negative pathogen is necessary for virulence in a nematode infection model and for efficient killing of cultured mammalian host cells. Expression of MAM7 on nonpathogenic strains produced a tool that can be used to impede infection by Gram-negative bacterial pathogens. Targeting or exploiting MAM7 might prove to be important in combating Gram-negative bacterial infections.adhesin | microbiology | bacterial attachment B acterial pathogens have a large repertoire of virulence factors that target and manipulate the host cellular machinery to enable infection. Delivery of effector proteins to the host cytosol by type III, type IV, and type VI secretion systems, as well as delivery of extracellular toxins, is a common strategy used by bacterial pathogens to abrogate the host immune response and alter cellular pathways to the pathogen's advantage (1, 2). Because the secretion of effector and toxin proteins is contact-dependent, the bacteria need to establish tight binding to the host to successfully start an infection. We hypothesize that a common strategy exists across species enabling the pathogen to establish strong initial host binding that is complemented by other species-specific adhesion factors for efficient activation and secretion of virulence factors and toxins. During infection, a variety of adhesion factors are expressed by pathogens to facilitate host-pathogen interactions (3-5). Many of these adhesins are induced during infection and thus likely would not be involved in the initial adhesion of the bacterial pathogen with the host cell.Using bioinformatics, we searched the genome of Vibrio parahaemolyticus, a Gram-negative bacterium that occurs in marine and estuarine environments and can cause shellfish-borne food poisoning, for a constitutively expressed protein that might be involved in the initial binding of bacteria to a host cell (6). We discovered a predicted outer membrane molecule, multivalent adhesion molecule (MAM), which includes a putative transmembrane motif followed by six (MAM6) or seven (MAM7) mammalian cell entry (mce) domains (Fig. 1A and Fig. S1A Unexpectedly, we found that MAM6 or MAM7 is encoded in a wide range of Gram-negative animal pathogens, but not in Grampositive or plant pathogenic bacteria (Fig. 1A and Figs. S1B and S2). In contrast, proteins containing a single mce domain are widespread (Fig. 1A). In Mycobacterium spp. and some Grampositive bacteria, including Rhodococcus spp. and Streptomyces spp., the mce domain occurs ...

The Tol system is a five-protein assembly parasitized by colicins and bacteriophages that helps stabilize the Gramnegative outer membrane (OM). We show that allosteric signalling through the six-bladed b-propeller protein TolB is central to Tol function in Escherichia coli and that this is subverted by colicins such as ColE9 to initiate their OM translocation. Protein-protein interactions with the TolB b-propeller govern two conformational states that are adopted by the distal N-terminal 12 residues of TolB that bind TolA in the inner membrane. ColE9 promotes disorder of this 'TolA box' and recruitment of TolA. In contrast to ColE9, binding of the OM lipoprotein Pal to the same site induces conformational changes that sequester the TolA box to the TolB surface in which it exhibits little or no TolA binding. Our data suggest that Pal is an OFF switch for the Tol assembly, whereas colicins promote an ON state even though mimicking Pal. Comparison of the TolB mechanism to that of vertebrate guanine nucleotide exchange factor RCC1 suggests that allosteric signalling may be more prevalent in b-propeller proteins than currently realized.

Bacterial pathogens use effector proteins to manipulate their hosts to propagate infection. These effectors divert host cell signaling pathways to the benefit of the pathogen and frequently target kinase signaling cascades. Notable pathways that are usurped include the nuclear factor κB (NF-κB), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/Akt, and p21-activated kinase (PAK) pathways. Analyzing the functions of pathogenic effectors and their intersection with host kinase pathways has provided interesting insights into both the mechanisms of virulence and eukaryotic signaling.