Vibrio fischeri-derived outer membrane vesicles trigger host development

Abstract

Outer membrane vesicles (OMV) are critical elements in many host-cell/microbe interactions. Previous studies of the symbiotic association between Euprymna scolopes and Vibrio fischeri had shown that, within 12 h of colonizing crypts deep within the squid’s light organ, the symbionts trigger an irreversible program of tissue development in the host. Here, we report that OMV produced by V. fischeri are powerful contributors to this process. The first detectable host response to the OMV is an increased trafficking of macrophage-like cells called hemocytes into surface epithelial tissues. We showed that exposing the squid to other Vibrio species fails to induce this trafficking; however, addition of a high concentration of their OMV, which can diffuse into the crypts, does. We also provide evidence that tracheal cytotoxin (TCT) release by the symbionts, which can induce hemocyte trafficking, is not part of the OMV cargo, suggesting two distinct mechanisms to induce the same morphogenesis event. By manipulating the timing and localization of OMV signal delivery, we showed that hemocyte trafficking is fully induced only when V. fischeri, the sole species able to reach and grow in the crypts, succeeds in establishing a sustained colonization. Further, our data suggest that the host detection of OMV serves as a symbiotic checkpoint prior to inducing irreversible morphogenesis.

Rotation of Vibrio fischeri Flagella Produces Outer Membrane Vesicles That Induce Host Development

“…This accelerated response is similar to that reported for the experimental addition of purified MAMPs (8) and is consistent with the idea that both OMVs and soluble MAMPs can diffuse rapidly into the crypts. In contrast, the inoculating cells in symbi-ont-colonized animals must first migrate into the crypts and then proliferate before they can induce a response (10). Moreover, by 4 days postcolonization, we observed comparable regression of the ciliated appendages in both symbiont-exposed and OMV-treated light organs relative to aposymbiotic light organs (Fig.…”

“…These two classes of MAMPs have different effects in symbiont-induced morphogenesis: the phosphorylated lipid A of LPS triggers early stage apoptosis, while PG fragments, like tracheal cytotoxin (TCT), activate both hemocyte trafficking and late-stage apoptosis (8). Exposure to OMVs, which contain PG derivatives, has recently been shown to be sufficient to induce hemocyte trafficking in juvenile squid (10). In the current study, we demonstrate that OMVs also present morphogenic LPS, inducing full apoptotic regression of the light organ's ciliated fields (Fig.…”

“…A second method to determine the relative amounts of OMVs in a preparation was by quantification of total lipid and LPS. The total lipid amount was determined by staining with a lipid-binding fluorophore and measuring fluorescence intensity, as described previously (10). Briefly, cell-free supernatants or OMV preparations were incubated with FM4-64 (Life Technologies/Molecular Probes) in dPBS for 10 min at 37°C.…”

“…Thus, the context in which those MAMPs are delivered determines the nature of the response and, ultimately, the outcome of the association. In a previous study, we reported that the addition of purified bacterial outer membrane vesicles (OMVs) to the seawater surrounding juvenile squid was sufficient to trigger PGinducible phenotypes associated with the light organ's irreversible morphogenesis (8,10). The release of OMVs, which frequently contain PG derivatives and display on their surface not only the primary components of LPS (i.e., lipid A and O antigen) but also outer membrane proteins (OMPs), has been hypothesized to facilitate signaling among bacteria and between bacteria and host tissues (11).…”

Caenorhabditis elegans as a Model for Microbiome Research

“…As an example for associations of a type that may lie further toward the center of this range, one can consider the acquisition of beneficial symbionts from a greater environmental diversity, relying on mechanisms permitting partner choice or checking for partner fidelity. This has been shown to occur in the colonization of the bobtail squid's light organ by Vibrio symbionts from the marine environment (Kremer et al, 2013; Aschtgen et al, 2016), as well as in the acquisition of Xenorhabdus gut-residing bacteria by the Steinernema entomoparasitic nematode (Murfin et al, 2015). Figuring out how C. elegans obtains the different members of its characteristic gut microbiota remains to be elucidated.…”