The twin arginine translocation system contributes to symbiotic colonization of Euprymna scolopes by Vibrio fischeri

HbtR, a Heterofunctional Homolog of the Virulence Regulator TcpP, Facilitates the Transition between Symbiotic and Planktonic Lifestyles in Vibrio fischeri

The bioluminescent bacterium Vibrio fischeri forms a mutually beneficial symbiosis with the Hawaiian bobtail squid, Euprymna scolopes, in which the bacteria, housed inside a specialized light organ, produce light used by the squid in its nocturnal activities. Upon hatching, E. scolopes juveniles acquire V. fischeri from the seawater through a complex process that requires, among other factors, chemotaxis by the bacteria along a gradient of N-acetylated sugars into the crypts of the light organ, the niche in which the bacteria reside. Once inside the light organ, V. fischeri transitions into a symbiotic, sessile state in which the quorum-signaling regulator LitR induces luminescence. In this work we show that expression of litR and luminescence are repressed by a homolog of the Vibrio cholerae virulence factor TcpP, which we have named HbtR. Further, we demonstrate that LitR represses genes involved in motility and chemotaxis into the light organ and activates genes required for exopolysaccharide production.IMPORTANCE TcpP homologs are widespread throughout the Vibrio genus; however, the only protein in this family described thus far is a V. cholerae virulence regulator. Here, we show that HbtR, the TcpP homolog in V. fischeri, has both a biological role and regulatory pathway completely unlike those in V. cholerae Through its repression of the quorum-signaling regulator LitR, HbtR affects the expression of genes important for colonization of the E. scolopes light organ. While LitR becomes activated within the crypts and upregulates luminescence and exopolysaccharide genes and downregulates chemotaxis and motility genes, it appears that HbtR, upon expulsion of V. fischeri cells into seawater, reverses this process to aid the switch from a symbiotic to a planktonic state. The possible importance of HbtR to the survival of V. fischeri outside its animal host may have broader implications for the ways in which bacteria transition between often vastly different environmental niches.

Vibrio fischeri metabolism: symbiosis and beyond

Vibrio fischeri is a bioluminescent, Gram-negative marine bacterium that can be found free living and in a mutualistic association with certain squids and fishes. Over the past decades, the study of V. fischeri has led to important discoveries about bioluminescence, quorum sensing, and the mechanisms that underlie beneficial host-microbe interactions. This chapter highlights what has been learned about metabolic pathways in V. fischeri, and how this information contributes to a broader understanding of the role of bacterial metabolism in host colonization by both beneficial and pathogenic bacteria, as well as in the growth and survival of free-living bacteria.

Identification and evaluation of twin-arginine translocase inhibitors

The twin-arginine translocase (TAT) in some bacterial pathogens, including Pseudomonas aeruginosa, Burkholderia pseudomallei, and Mycobacterium tuberculosis, contributes to pathogenesis by translocating extracellular virulence determinants across the inner membrane into the periplasm, thereby allowing access to the Xcp (type II) secretory system for further export in Gram-negative organisms, or directly to the outside surface of the cell, as in M. tuberculosis. TAT-mediated secretion appreciably contributes to virulence in both animal and plant models of bacterial infection. Consequently, TAT function is an attractive target for small-molecular-weight compounds that alone or in conjunction with extant antimicrobial agents could become novel therapeutics. The TAT-transported hemolytic phospholipase C (PlcH) of P. aeruginosa and its multiple orthologs produced by the above pathogens can be detected by an accurate and reproducible colorimetric assay using a synthetic substrate that detects phospholipase C activity. Such an assay could be an effective indicator of TAT function. Using carefully constructed recombinant strains to precisely control the expression of PlcH, we developed a high-throughput screening (HTS) assay to evaluate, in duplicate, >80,000 small-molecular-weight compounds as possible TAT inhibitors. Based on additional TAT-related functional assays, purified PlcH protein inhibition experiments, and repeat experiments of the initial screening assay, 39 compounds were selected from the 122 initial hits. Finally, to evaluate candidate inhibitors for TAT specificity, we developed a TAT titration assay that determines whether inhibition of TAT-mediated secretion can be overcome by increasing the levels of TAT expression. The compounds N-phenyl maleimide and Bay 11-7082 appear to directly affect TAT function based on this approach.

The alternative oxidase (AOX) gene in Vibrio fischeri is controlled by NsrR and upregulated in response to nitric oxide

Alternative oxidase (AOX) is a respiratory oxidase found in certain eukaryotes and bacteria; however, its role in bacterial physiology is unclear. Exploiting the genetic tractability of the bacterium Vibrio fischeri, we explore the regulation of aox expression and AOX function. Using quantitative PCR and reporter assays, we demonstrate that aox expression is induced in the presence of nitric oxide (NO), and that the NO-responsive regulatory protein NsrR mediates the response. We have identified key amino acid residues important for NsrR function and experimentally confirmed a bioinformatically predicted NsrR binding site upstream of aox. Microrespirometry demonstrated that oxygen consumption by V. fischeri CydAB quinol oxidase is inhibited by NO treatment, whereas oxygen consumption by AOX is less sensitive to NO. NADH oxidation assays using inverted membrane vesicles confirmed that NO directly inhibits CydAB, and that AOX is resistant to NO inhibition. These results indicate a role for V. fischeri AOX in aerobic respiration during NO stress.

Common trends in mutualism revealed by model associations between invertebrates and bacteria

Mutually beneficial interactions between microorganisms and animals are a conserved and ubiquitous feature of biotic systems. In many instances animals, including humans, are dependent on their microbial associates for nutrition, defense, or development. To maintain these vital relationships, animals have evolved processes that ensure faithful transmission of specific microbial symbionts between generations. Elucidating mechanisms of transmission and symbiont specificity has been aided by the study of experimentally tractable invertebrate animals with diverse and highly evolved associations with microorganisms. Here, we review several invertebrate model systems that contribute to our current understanding of symbiont transmission, recognition, and specificity. Although the details of transmission and symbiont selection vary among associations, comparisons of diverse mutualistic associations are revealing a number of common themes, including restriction of symbiont diversity during transmission and glycan-lectin interactions during partner selection and recruitment.

Characterization of two host-specific genes, mannose-sensitive hemagglutinin (mshA) and uridyl phosphate dehydrogenase (UDPDH) that are involved in the Vibrio fischeri-Euprymna tasmanica mutualism

While much has been known about the mutualistic associations between the sepiolid squid Euprymna tasmanica and the luminescent bacterium, Vibrio fischeri, less is known about the connectivity between the microscopic and molecular basis of initial attachment and persistence in the light organ. Here, we examine the possible effects of two symbiotic genes on specificity and biofilm formation of V. fischeri in squid light organs. Uridine diphosphate glucose-6-dehydrogenase (UDPDH) and mannose-sensitive hemagglutinin (mshA) mutants were generated in V. fischeri to determine whether each gene has an effect on host colonization, specificity, and biofilm formation. Both squid light organ colonization assays and transmission electron microscopy confirmed differences in host colonization between wild-type and mutant strains, and also demonstrated the importance of both UDPDH and mshA gene expression for successful light organ colonization. This furthers our understanding of the genetic factors playing important roles in this environmentally transmitted symbiosis.

Symbiotic conversations are revealed under genetic interrogation

The recent development and application of molecular genetics to the symbionts of invertebrate animal species have advanced our knowledge of the biochemical communication that occurs between the host and its bacterial symbionts. In particular, the ability to manipulate these associations experimentally by introducing genetic variants of the symbionts into naive hosts has allowed the discovery of novel colonization mechanisms and factors. In addition, the role of the symbionts in inducing normal host development has been revealed, and its molecular basis described. In this Review, I discuss many of these developments, focusing on what has been discovered in five well-understood model systems.