Nanopods – a new bacterial structure and mechanism for outer membrane vesicles

Nanopods The ability of bacteria to extend their sphere of metabolic influence long distances (microns) from the cell is key to their activity and survival, and is achieved by secretion of small molecules, such as acyl homoserine lactones, which can have broad, regulatory effects on the metabolism of neighboring bacteria, as well as macromolecules, namely enzymes and outer membrane vesicles (OMV), which transmit specific function(s). The latter are unique in that they can encompass a broad range of (macro)molecules, which mediate a variety of processes. For example, OMV can package small molecules for signaling and proteins that effect virulence. OMV-mediated DNA transfer has also been demonstrated. These vesicles are highly versatile as they can be designed for different functions by different organisms, and tasked for different activities by the same organism. Thus OMV are a type of bacterial “Swiss army knife” for projecting extracellular activities and, perhaps reflecting their utility, their production is widespread in proteobacteria. But, despite their prominence, the biology of OMV has been extensively studied only in pathogens, for which these are key vehicles for long distance transmission of virulence factors.

This report describes novel bacterial organelles termed “nanopods” that can project OMV long distances (≥6 µm) from the cell. Nanopod deployment of OMV is independent of diffusion, and thus represents a solution to constraints imposed by partial hydration, a new paradigm in the mechanisms of long distance interaction utilized by bacteria.

 

Nanopods: A New Bacterial Structure and Mechanism for Deployment of Outer Membrane Vesicles. (2011) PLoS ONE 6(6): e20725. doi:10.1371/journal.pone.0020725
Bacterial outer membrane vesicles (OMV) are packets of periplasmic material that, via the proteins and other molecules they contain, project metabolic function into the environment. While OMV production is widespread in proteobacteria, they have been extensively studied only in pathogens, which inhabit fully hydrated environments. However, many (arguably most) bacterial habitats, such as soil, are only partially hydrated. In the latter, water is characteristically distributed as films on soil particles that are, on average thinner, than are typical OMV (ca. ≤10 nm water film vs. 20 to >200 nm OMV;).
We have identified a new bacterial surface structure, termed a “nanopod”, that is a conduit for projecting OMV significant distances (e.g., ≥6 µm) from the cell. Electron cryotomography was used to determine nanopod three-dimensional structure, which revealed chains of vesicles within an undulating, tubular element. By using immunoelectron microscopy, proteomics, heterologous expression and mutagenesis, the tubes were determined to be an assembly of a surface layer protein (NpdA), and the interior structures identified as OMV. Specific metabolic function(s) for nanopods produced by Delftia sp. Cs1-4 are not yet known. However, a connection with phenanthrene degradation is a possibility since nanopod formation was induced by growth on phenanthrene. Orthologs of NpdA were identified in three other genera of the Comamonadaceae family, and all were experimentally verified to form nanopods.
Nanopods are new bacterial organelles, and establish a new paradigm in the mechanisms by which bacteria effect long-distance interactions with their environment. Specifically, they create a pathway through which cells can effectively deploy OMV, and the biological activity these transmit, in a diffusion-independent manner. Nanopods would thus allow environmental bacteria to expand their metabolic sphere of influence in a manner previously unknown for these organisms.

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