The Vibrio fischeri-Euprymna scolopes Light Organ Association: Current Ecological Paradigms

Aedes albopictus microbiome derives from environmental sources and partitions across distinct host tissues

The mosquito microbiome consists of a consortium of interacting microorganisms that reside on and within culicid hosts. Mosquitoes acquire most of their microbial diversity from the environment over their life cycle. Once present within the mosquito host, the microbes colonize distinct tissues, and these symbiotic relationships are maintained by immune-related mechanisms, environmental filtering, and trait selection. The processes that govern how environmental microbes assemble across the tissues within mosquitoes remain poorly resolved. We use ecological network analyses to examine how environmental bacteria assemble to form bacteriomes among Aedes albopictus host tissues. Mosquitoes, water, soil, and plant nectar were collected from 20 sites in the Mānoa Valley, Oahu. DNA was extracted and associated bacteriomes were inventoried using Earth Microbiome Project protocols. We find that the bacteriomes of A. albopictus tissues were compositional taxonomic subsets of environmental bacteriomes and suggest that the environmental microbiome serves as a source pool that supports mosquito microbiome diversity. Within the mosquito, the microbiomes of the crop, midgut, Malpighian tubules, and ovaries differed in composition. This microbial diversity partitioned among host tissues formed two specialized modules: one in the crop and midgut, and another in the Malpighian tubules and ovaries. The specialized modules may form based on microbe niche preferences and/or selection of mosquito tissues for specific microbes that aid unique biological functions of the tissue types. A strong niche-driven assembly of tissue-specific microbiotas from the environmental species pool suggests that each tissue has specialized associations with microbes, which derive from host-mediated microbe selection.

Redesigning regulatory components of quorum-sensing system for diverse metabolic control

Quorum sensing (QS) is a ubiquitous cell–cell communication mechanism that can be employed to autonomously and dynamically control metabolic fluxes. However, since the functions of genetic components in the circuits are not fully understood, the developed QS circuits are still less sophisticated for regulating multiple sets of genes or operons in metabolic engineering applications. Here, we discover the regulatory roles of a CRP-binding site and the lux box to −10 region within luxR-luxI intergenic sequence in controlling the lux-type QS promoters. By varying the numbers of the CRP-binding site and redesigning the lux box to −10 site sequence, we create a library of QS variants that possess both high dynamic ranges and low leakiness. These circuits are successfully applied to achieve diverse metabolic control in salicylic acid and 4-hydroxycoumarin biosynthetic pathways in Escherichia coli. This work expands the toolbox for dynamic control of multiple metabolic fluxes under complex metabolic background and presents paradigms to engineer metabolic pathways for high-level synthesis of target products.

Existing quorum sensing (QS) circuits are less sophisticated for regulating multiple sets of genes or operons. Here, the authors redesign the luxR-luxI intergenic sequence of the lux-type QS system and apply it to achieve diverse metabolic control in salicylic acid and 4-hydroxycoumarin biosynthesis in E. coli.

Nocturnal Acidification: A Coordinating Cue in the Euprymna scolopes-Vibrio fischeri Symbiosis

The Vibrio fischeri–Euprymna scolopes symbiosis has become a powerful model for the study of specificity, initiation, and maintenance between beneficial bacteria and their eukaryotic partner. In this invertebrate model system, the bacterial symbionts are acquired every generation from the surrounding seawater by newly hatched squid. These symbionts colonize a specialized internal structure called the light organ, which they inhabit for the remainder of the host’s lifetime. The V. fischeri population grows and ebbs following a diel cycle, with high cell densities at night producing bioluminescence that helps the host avoid predation during its nocturnal activities. Rhythmic timing of the growth of the symbionts and their production of bioluminescence only at night is critical for maintaining the symbiosis. V. fischeri symbionts detect their population densities through a behavior termed quorum-sensing, where they secrete and detect concentrations of autoinducer molecules at high cell density when nocturnal production of bioluminescence begins. In this review, we discuss events that lead up to the nocturnal acidification of the light organ and the cues used for pre-adaptive behaviors that both host and symbiont have evolved. This host–bacterium cross talk is used to coordinate networks of regulatory signals (such as quorum-sensing and bioluminescence) that eventually provide a unique yet stable environment for V. fischeri to thrive and be maintained throughout its life history as a successful partner in this dynamic symbiosis.

Evidence of Genomic Diversification in a Natural Symbiotic Population Within Its Host

Planktonic cells of the luminous marine bacterium Vibrio fischeri establish themselves in the light-emitting organ of each generation of newly hatched Euprymna scolopes bobtail squid. A symbiont population is maintained within the 6 separated crypts of the organ for the ∼9-month life of the host. In the wild, the initial colonization step is typically accomplished by a handful of planktonic V. fischeri cells, leading to a species-specific, but often multi-strain, symbiont population. Within a few hours, the inoculating cells proliferate within the organ’s individual crypts, after which there is evidently no supernumerary colonization. Nevertheless, every day at dawn, the majority of the symbionts is expelled, and the regrowth of the remaining ∼5% of cells provides a daily opportunity for the population to evolve and diverge, thereby increasing its genomic diversity. To begin to understand the extent of this diversification, we characterized the light-organ population of an adult animal. First, we used 16S sequencing to determine that species in the V. fischeri clade were essentially the only ones detectable within a field-caught E. scolopes. Efforts to colonize the host with a minor species that appeared to be identified, V. litoralis, revealed that, although some cells could be imaged within the organ, they were <0.1% of the typical V. fischeri population, and did not persist. Next, we determined the genome sequences of seventy-two isolates from one side of the organ. While all these isolates were associated with one of three clusters of V. fischeri strains, there was considerable genomic diversity within this natural symbiotic population. Comparative analyses revealed a significant difference in both the number and the presence/absence of genes within each cluster; in contrast, there was little accumulation of single-nucleotide polymorphisms. These data suggest that, in nature, the light organ is colonized by a small number of V. fischeri strains that can undergo significant genetic diversification, including by horizontal-gene transfer, over the course of ∼1500 generations of growth in the organ. When the resulting population of symbionts is expelled into seawater, its genomic mix provides the genetic basis for selection during the subsequent environmental dispersal, and transmission to the next host.

The Role of the Microbiota in Regeneration-Associated Processes

The microbiota, the set of microorganisms associated with a particular environment or host, has acquired a prominent role in the study of many physiological and developmental processes. Among these, is the relationship between the microbiota and regenerative processes in various organisms. Here we introduce the concept of the microbiota and its involvement in regeneration-related cellular events. We then review the role of the microbiota in regenerative models that extend from the repair of tissue layers to the regeneration of complete organs or animals. We highlight the role of the microbiota in the digestive tract, since it accounts for a significant percentage of an animal microbiota, and at the same time provides an outstanding system to study microbiota effects on regeneration. Lastly, while this review serves to highlight echinoderms, primarily holothuroids, as models for regeneration studies, it also provides multiple examples of microbiota-related interactions in other processes in different organisms.

Independent host- and bacterium-based determinants protect a model symbiosis from phage predation

Bacteriophages (phages) are diverse and abundant constituents of microbial communities worldwide, capable of modulating bacterial populations in diverse ways. Here, we describe the phage HNL01, which infects the marine bacterium Vibrio fischeri. We use culture-based approaches to demonstrate that mutations in the exopolysaccharide locus of V. fischeri render this bacterium resistant to infection by HNL01, highlighting the extracellular matrix as a key determinant of HNL01 infection. Additionally, using the natural symbiosis between V. fischeri and the squid Euprymna scolopes, we show that, during colonization, V. fischeri is protected from phages present in the ambient seawater. Taken together, these findings shed light on independent yet synergistic host- and bacterium-based strategies for resisting symbiosis-disrupting phage predation, and we present important implications for understanding these strategies in the context of diverse host-associated microbial ecosystems.

Polyunsaturated fatty acids cause physiological and behavioral changes in Vibrio alginolyticus and Vibrio fischeri

Vibrio alginolyticus and Vibrio (Aliivibrio) fischeri are Gram-negative bacteria found globally in marine environments. During the past decade, studies have shown that certain Gram-negative bacteria, including Vibrio species (cholerae, parahaemolyticus, and vulnificus) are capable of using exogenous polyunsaturated fatty acids (PUFAs) to modify the phospholipids of their membrane. Moreover, exposure to exogenous PUFAs has been shown to affect certain phenotypes that are important factors of virulence. The purpose of this study was to investigate whether V. alginolyticus and V. fischeri are capable of responding to exogenous PUFAs by remodeling their membrane phospholipids and/or altering behaviors associated with virulence. Thin-layer chromatography (TLC) analyses and ultra-performance liquid chromatography-electrospray ionization mass spectrometry (UPLC/ESI-MS) confirmed incorporation of all PUFAs into membrane phosphatidylglycerol and phosphatidylethanolamine. Several growth phenotypes were identified when individual fatty acids were supplied in minimal media and as sole carbon sources. Interestingly, several PUFAs acids inhibited growth of V. fischeri. Significant alterations to membrane permeability were observed depending on fatty acid supplemented. Strikingly, arachidonic acid (20:4) reduced membrane permeability by approximately 35% in both V. alginolyticus and V. fischeri. Biofilm assays indicated that fatty acid influence was dependent on media composition and temperature. All fatty acids caused decreased swimming motility in V. alginolyticus, while only linoleic acid (18:2) significantly increased swimming motility in V. fischeri. In summary, exogenous fatty acids cause a variety of changes in V. alginolyticus and V. fischeri, thus adding these bacteria to a growing list of Gram-negatives that exhibit versatility in fatty acid utilization and highlighting the potential for environmental PUFAs to influence phenotypes associated with planktonic, beneficial, and pathogenic associations.

A lasting symbiosis: how the Hawaiian bobtail squid finds and keeps its bioluminescent bacterial partner

For more than 30 years, the association between the Hawaiian bobtail squid, Euprymna scolopes, and the bioluminescent bacterium Vibrio fischeri has been studied as a model system for understanding the colonization of animal epithelia by symbiotic bacteria. The squid-vibrio light-organ system provides the exquisite resolution only possible with the study of a binary partnership. The impact of this relationship on the partners' biology has been broadly characterized, including their ecology and evolutionary biology as well as the underlying molecular mechanisms of symbiotic dynamics. Much has been learned about the factors that foster initial light-organ colonization, and more recently about the maturation and long-term maintenance of the association. This Review synthesizes the results of recent research on the light-organ association and also describes the development of new horizons for E. scolopes as a model organism that promises to inform biology and biomedicine about the basic nature of host-microorganism interactions.

Tracking the cargo of extracellular symbionts into host tissues with correlated electron microscopy and nanoscale secondary ion mass spectrometry imaging

Extracellular bacterial symbionts communicate biochemically with their hosts to establish niches that foster the partnership. Using quantitative ion microprobe isotopic imaging (nanoscale secondary ion mass spectrometry [NanoSIMS]), we surveyed localization of ¹⁵ N-labelled molecules produced by the bacterium Vibrio fischeri within the cells of the symbiotic organ of its host, the Hawaiian bobtail squid, and compared that with either labelled non-specific species or amino acids. In all cases, two areas of the organ's epithelia were significantly more ¹⁵ N enriched: (a) surface ciliated cells, where environmental symbionts are recruited, and (b) the organ's crypts, where the symbiont population resides in the host. Label enrichment in all cases was strongest inside host cell nuclei, preferentially in the euchromatin regions and the nucleoli. This permissiveness demonstrated that uptake of biomolecules is a general mechanism of the epithelia, but the specific responses to V. fischeri cells recruited to the organ's surface are due to some property exclusive to this species. Similarly, in the organ's deeper crypts, the host responds to common bacterial products that only the specific symbiont can present in that location. The application of NanoSIMS allows the discovery of such distinct modes of downstream signalling dependent on location within the host and provides a unique opportunity to study the microbiogeographical patterns of symbiotic dialogue.

Toxicity assessment of total petroleum hydrocarbons in aquatic environments using the bioluminescent bacterium Aliivibrio fischeri

Toxicity monitoring of environmental pollutants especially petroleum hydrocarbons as priority pollutants is an important environmental issue. This study addresses a rapid, sensitive and cost effective method for the detection of total petroleum hydrocarbons (TPHs) using Aliivibrio fischeri bioluminescence inhibition bioassay. At the first step, the optimum conditions including time, pH and temperature for growth of A. fischeri were determined. Then, two methods were used to evaluate the toxicity of petroleum compounds. In the first method, short-term (15 min) and long-term (16 h) toxicity assays were performed. In the second method luminescence kinetics of A. fischeri was investigated during 24 h. The results demonstrated the most appropriate time for the bacterial growth occurred 16 h after inoculation and optimum temperature and pH were found 25 °C and 7, respectively. Short-term and long-term toxicity did not indicate any toxicity for various concentrations of TPHs (30, 50, 110, 160, 220 mg/L). Considering the luminescence kinetics of A. fischeri the long-term assay was introduced as 6 h. The half maximal effective concentration (EC₅₀) was achieved 1.77 mg/L of TPHs. It is concluded that the luminescence kinetics of A. fischeri can be a valuable approach for assessing toxicity of TPHs in aquatic environments.

Seeing (and Using) the Light: Recent Developments in Bioluminescence Technology

Bioluminescence has long been used to image biological processes in vivo. This technology features luciferase enzymes and luciferin small molecules that produce visible light. Bioluminescent photons can be detected in tissues and live organisms, enabling sensitive and noninvasive readouts on physiological function. Traditional applications have focused on tracking cells and gene expression patterns, but new probes are pushing the frontiers of what can be visualized. The past few years have also seen the merger of bioluminescence with optogenetic platforms. Luciferase-luciferin reactions can drive light-activatable proteins, ultimately triggering signal transduction and other downstream events. This review highlights these and other recent advances in bioluminescence technology, with an emphasis on tool development. We showcase how new luciferins and engineered luciferases are expanding the scope of optical imaging. We also highlight how bioluminescent systems are being leveraged not just for sensing-but also controlling-biological processes.

Vibrio fischeri siderophore production drives competitive exclusion during dual-species growth

When two or more bacterial species inhabit a shared niche, often, they must compete for limited nutrients. Iron is an essential nutrient that is especially scarce in the marine environment. Bacteria can use the production, release, and re-uptake of siderophores, small molecule iron chelators, to scavenge iron. Siderophores provide fitness advantages to species that employ them by enhancing iron acquisition, and moreover, by denying iron to competitors incapable of using the siderophore-iron complex. Here, we show that cell-free culture fluids from the marine bacterium Vibrio fischeri ES114 prevent the growth of other vibrio species. Mutagenesis reveals the aerobactin siderophore as the inhibitor. Our analysis reveals a gene, that we name aerE, encodes the aerobactin exporter, and LuxT is a transcriptional activator of aerobactin production. In co-culture, under iron-limiting conditions, aerobactin production allows V. fischeri ES114 to competitively exclude Vibrio harveyi, which does not possess aerobactin production and uptake genes. In contrast, V. fischeri ES114 mutants incapable of aerobactin production lose in competition with V. harveyi. Introduction of iutA, encoding the aerobactin receptor, together with fhuCDB, encoding the aerobactin importer are sufficient to convert V. harveyi into an "aerobactin cheater."

Schizasterid Heart Urchins Host Microorganisms in a Digestive Symbiosis of Mesozoic Origin

Because of their lifestyles, abundance, and feeding habits, infaunal marine deposit feeders have a significant impact on the ocean floor. As these animals also ingest microorganisms associated with their sediment and seawater diet, their digestive tract usually contains a diverse array of bacteria. However, while most of these microorganisms are transients, some may become part of a resident gut microbiome, in particular when sheltered from the main flow of digesta in specialized gut compartments. Here, we provide an in-depth analysis of the structure and contents of the intestinal caecum (IC), a hindgut diverticulum found exclusively in schizasterid heart urchins (Echinoidea: Spatangoida: Schizasteridae). Based on specimens of Brisaster townsendi, in addition to various other schizasterid taxa, our structural characterization of the IC shows that the organ is a highly specialized gut compartment with unique structural properties. Next generation sequencing shows that the IC contains a microbial population composed predominantly of Bacteroidales, Desulfobacterales, and Spirochaetales. The microbiome of this gut compartment is significantly different in composition and lower in diversity than the microbial population in the sediment-filled main digestive tract. Inferences on the function and evolution of the IC and its microbiome suggest that this symbiosis plays a distinct role in host nutrition and that it evolved at least 66 million years ago during the final phase of the Mesozoic.

Optimization of a minimal sample preparation protocol for imaging mass spectrometry of unsectioned juvenile invertebrates

Tissue sections have long been the subject matter for the application of imaging mass spectrometry, but recently the technique has been adapted for many other purposes including bacterial colonies and 3D cell culture. Here, we present a simple preparation method for unsectioned invertebrate tissue without the need for fixing, embedding, or slicing. The protocol was used to successfully prepare a Hawaiian bobtail squid hatchling for analysis, and the resulting data detected ions that correspond to compounds present in the host only during its symbiotic colonization by Vibrio fischeri.

Draft Genome Sequences of Type VI Secretion System-Encoding Vibrio fischeri Strains FQ-A001 and ES401

The type VI secretion system (T6SS) facilitates lethal competition between bacteria through direct contact. Comparative genomics has facilitated the study of these systems in Vibrio fischeri, which colonizes the squid host Euprymna scolopes Here, we report the draft genome sequences of two lethal V. fischeri strains that encode the T6SS, FQ-A001 and ES401.

Natural Strain Variation Reveals Diverse Biofilm Regulation in Squid-Colonizing Vibrio fischeri

The mutualistic symbiont Vibrio fischeri builds a symbiotic biofilm during colonization of squid hosts. Regulation of the exopolysaccharide component, termed Syp, has been examined in strain ES114, where production is controlled by a phosphorelay that includes the inner membrane hybrid histidine kinase RscS. Most strains that lack RscS or encode divergent RscS proteins cannot colonize a squid host unless RscS from a squid symbiont is heterologously expressed. In this study, we examine V. fischeri isolates worldwide to understand the landscape of biofilm regulation during beneficial colonization. We provide a detailed study of three distinct evolutionary groups of V. fischeri and find that while the RscS-Syp biofilm pathway is required in one of the groups, two other groups of squid symbionts require Syp independent of RscS. Mediterranean squid symbionts, including V. fischeri SR5, colonize without an RscS homolog encoded by their genome. Additionally, group A V. fischeri strains, which form a tightly related clade of Hawaii isolates, have a frameshift in rscS and do not require the gene for squid colonization or competitive fitness. These same strains have a frameshift in sypE, and we provide evidence that this group A sypE allele leads to an upregulation in biofilm activity. Thus, this work describes the central importance of Syp biofilm in colonization of diverse isolates and demonstrates that significant evolutionary transitions correspond to regulatory changes in the syp pathway.IMPORTANCE Biofilms are surface-associated, matrix-encased bacterial aggregates that exhibit enhanced protection to antimicrobial agents. Previous work has established the importance of biofilm formation by a strain of luminous Vibrio fischeri bacteria as the bacteria colonize their host, the Hawaiian bobtail squid. In this study, expansion of this work to many natural isolates revealed that biofilm genes are universally required, yet there has been a shuffling of the regulators of those genes. This work provides evidence that even when bacterial behaviors are conserved, dynamic regulation of those behaviors can underlie evolution of the host colonization phenotype. Furthermore, this work emphasizes the importance of investigating natural diversity as we seek to understand molecular mechanisms in bacteria.

Bacterial symbionts use a type VI secretion system to eliminate competitors in their natural host

Competition among cooccurring bacteria can change the structure and function of a microbial community. However, little is known about the molecular mechanisms that impact such interactions in vivo. We used the association between bioluminescent bacteria and their squid host to study how environmentally transmitted bacteria compete for a limited number of host colonization sites. Our work suggests that Vibrio fischeri use a type VI secretion system, acting as a contact-dependent interbacterial “weapon,” to eliminate competing strains from cooccupying sites in the host. This work illuminates a mechanism by which strain-specific differences drive closely related bacteria to engage in lethal battles as they establish a beneficial symbiosis, revealing how genetic variation among potential colonizers directly impacts the spatial structure of the host-associated population.

Intraspecific competition describes the negative interaction that occurs when different populations of the same species attempt to fill the same niche. Such competition is predicted to occur among host-associated bacteria but has been challenging to study in natural biological systems. Although many bioluminescent Vibrio fischeri strains exist in seawater, only a few strains are found in the light-organ crypts of an individual wild-caught Euprymna scolopes squid, suggesting a possible role for intraspecific competition during early colonization. Using a culture-based assay to investigate the interactions of different V. fischeri strains, we found “lethal” and “nonlethal” isolates that could kill or not kill the well-studied light-organ isolate ES114, respectively. The killing phenotype of these lethal strains required a type VI secretion system (T6SS) encoded in a 50-kb genomic island. Multiple lethal and nonlethal strains could be cultured from the light organs of individual wild-caught adult squid. Although lethal strains eliminate nonlethal strains in vitro, two lethal strains could coexist in interspersed microcolonies that formed in a T6SS-dependent manner. This coexistence was destabilized upon physical mixing, resulting in one lethal strain consistently eliminating the other. When juvenile squid were coinoculated with lethal and nonlethal strains, they occupied different crypts, yet they were observed to coexist within crypts when T6SS function was disrupted. These findings, using a combination of natural isolates and experimental approaches in vitro and in the animal host, reveal the importance of T6SS in spatially separating strains during the establishment of host colonization in a natural symbiosis.

Aptamer-Based Paper Strip Sensor for Detecting Vibrio fischeri

Aptamer-based paper strip sensor for detecting Vibrio fischeri was developed. Our method was based on the aptamer sandwich assay between whole live cells, V. fischeri and DNA aptamer probes. Following 9 rounds of Cell-SELEX and one of the negative-SELEX, V. fischeri Cell Aptamer (VFCA)-02 and -03 were isolated, with the former showing approximately 10-fold greater avidity (in the subnanomolar range) for the target cells when arrayed on a surface. The colorimetric response of a paper sensor based on VFCA-02 was linear in the range of 4 × 10¹ to 4 × 10⁵ CFU/mL of target cell by using scanning reader. The linear regression correlation coefficient ( R²) was 0.9809. This system shows promise for use in aptamer-conjugated gold nanoparticle probes in paper strip format for in-field detection of marine bioindicating bacteria.

Quorum Sensing Gene Regulation by LuxR/HapR Master Regulators in Vibrios

The coordination of group behaviors in bacteria is accomplished via the cell-cell signaling process called quorum sensing. Vibrios have historically been models for studying bacterial communication due to the diverse and remarkable behaviors controlled by quorum sensing in these bacteria, including bioluminescence, type III and type VI secretion, biofilm formation, and motility. Here, we discuss the Vibrio LuxR/HapR family of proteins, the master global transcription factors that direct downstream gene expression in response to changes in cell density. These proteins are structurally similar to TetR transcription factors but exhibit distinct biochemical and genetic features from TetR that determine their regulatory influence on the quorum sensing gene network. We review here the gene groups regulated by LuxR/HapR and quorum sensing and explore the targets that are common and unique among Vibrio species.

Transcriptional characterization of Vibrio fischeri during colonization of juvenile Euprymna scolopes

The marine bacterium Vibrio fischeri is the monospecific symbiont of the Hawaiian bobtail squid, Euprymna scolopes, and the establishment of this association involves a number of signaling pathways and transcriptional responses between both partners. We report here the first full RNA-Seq dataset representing host-associated V. fischeri cells from colonized juvenile E. scolopes, as well as comparative transcriptomes under both laboratory and simulated marine planktonic conditions. These data elucidate the broad transcriptional changes that these bacteria undergo during the early stages of symbiotic colonization. We report several previously undescribed and unexpected transcriptional responses within the early stages of this symbiosis, including gene expression patterns consistent with biochemical stresses inside the host, and metabolic patterns distinct from those reported in associations with adult animals. Integration of these transcriptional data with a recently developed metabolic model of V. fischeri provides us with a clearer picture of the metabolic state of symbionts within the juvenile host, including their possible carbon sources. Taken together, these results expand our understanding of the early stages of the squid-vibrio symbiosis, and more generally inform the transcriptional responses underlying the activities of marine microbes during host colonization.

Host-selected mutations converging on a global regulator drive an adaptive leap towards symbiosis in bacteria

Host immune and physical barriers protect against pathogens but also impede the establishment of essential symbiotic partnerships. To reveal mechanisms by which beneficial organisms adapt to circumvent host defenses, we experimentally evolved ecologically distinct bioluminescent Vibrio fischeri by colonization and growth within the light organs of the squid Euprymna scolopes. Serial squid passaging of bacteria produced eight distinct mutations in the binK sensor kinase gene, which conferred an exceptional selective advantage that could be demonstrated through both empirical and theoretical analysis. Squid-adaptive binK alleles promoted colonization and immune evasion that were mediated by cell-associated matrices including symbiotic polysaccharide (Syp) and cellulose. binK variation also altered quorum sensing, raising the threshold for luminescence induction. Preexisting coordinated regulation of symbiosis traits by BinK presented an efficient solution where altered BinK function was the key to unlock multiple colonization barriers. These results identify a genetic basis for microbial adaptability and underscore the importance of hosts as selective agents that shape emergent symbiont populations.

Quorum Sensing in Marine Microbial Environments

Quorum sensing (QS) is a form of chemical communication used by certain bacteria that regulates a wide range of biogeochemically important bacterial behaviors. Although QS was first observed in a marine bacterium nearly four decades ago, only in the past decade has there been a rise in interest in the role that QS plays in the ocean. It has become clear that QS, regulated by signals such as acylated homoserine lactones (AHLs) or furanosyl-borate diesters [autoinducer-2 (AI-2) molecules], is involved in important processes within the marine carbon cycle, in the health of coral reef ecosystems, and in trophic interactions between a range of eukaryotes and their bacterial associates. The most well-studied QS systems in the ocean occur in surface-attached (biofilm) communities and rely on AHL signaling. AHL-QS is highly sensitive to the chemical and biological makeup of the environment and may respond to anthropogenic change, including ocean acidification and rising sea surface temperatures.

Impact and Influence of the Natural Vibrio-Squid Symbiosis in Understanding Bacterial-Animal Interactions

Animals are colonized by bacteria, and in many cases partners have co-evolved to perform mutually beneficial functions. An exciting and ongoing legacy of the past decade has been an expansion of technology to enable study of natural associations in situ/in vivo. As a result, more symbioses are being examined, and additional details are being revealed for well-studied systems with a focus on the interactions between partners in the native context. With this framing, we review recent literature from the Vibrio fischeri-Euprymna scolopes symbiosis and focus on key studies that have had an impact on understanding bacteria-animal interactions broadly. This is not intended to be a comprehensive review of the system, but rather to focus on particular studies that have excelled at moving from pattern to process in facilitating an understanding of the molecular basis to intriguing observations in the field of host-microbe interactions. In this review we discuss the following topics: processes regulating strain and species specificity; bacterial signaling to host morphogenesis; multiple roles for nitric oxide; flagellar motility and chemotaxis; and efforts to understand unannotated and poorly annotated genes. Overall these studies demonstrate how functional approaches in vivo in a tractable system have provided valuable insight into general principles of microbe-host interactions.

Lessons from Digestive-Tract Symbioses Between Bacteria and Invertebrates

In most animals, digestive tracts harbor the greatest number of bacteria in the animal that contribute to its health: by aiding in the digestion of nutrients, provisioning essential nutrients and protecting against colonization by pathogens. Invertebrates have been used to enhance our understanding of metabolic processes and microbe-host interactions owing to experimental advantages. This review describes how advances in DNA sequencing technologies have dramatically altered how researchers investigate microbe-host interactions, including 16S rRNA gene surveys, metagenome experiments, and metatranscriptome studies. Advantages and challenges of each of these approaches are described herein. Hypotheses generated through omics studies can be directly tested using site-directed mutagenesis, and findings from transposon studies and site-directed experiments are presented. Finally, unique structural aspects of invertebrate digestive tracts that contribute to symbiont specificity are presented. The combination of omics approaches with genetics and microscopy allows researchers to move beyond correlations to identify conserved mechanisms of microbe-host interactions.

Dose-response behavior of the bacterium Vibrio fischeri exposed to pharmaceuticals and personal care products

The presence of pharmaceuticals and personal care products (PPCPs) in the environment has become a real and widespread concern in recent years. Therefore, the primary goal of this study was to investigate 20 common and widely used PPCPs to assess their individual and combined effect on an important species in one trophic level, i.e., bacteria. The ecotoxicological effects of PPCPs at two different concentration ranges were determined in the bacterium Vibrio fischeri using Microtox(®) and were statistically analyzed using three models in the GraphPad Prism 6 program for Windows, v.6.03. A four-parameter model best fit the majority of the compounds. The half maximal effective concentration (EC50) of each PPCP was estimated using the best-fitting model and was compared with the results from a recent study. Comparative analysis indicated that most compounds showed the same level of toxicity. Moreover, the stimulatory effects of PPCPs at environmental concentrations (low doses) were assessed. These results indicated that certain compounds have traditional inverted U- or J-shaped dose-response curves, and 55% of them presented a stimulatory effect below the zero effect-concentration point. Effective concentrations of 0 (EC0), 5 (EC5) and 50% (EC50) were calculated for each PPCP as the ecotoxicological points. All compounds that presented narcosis as a mode of toxic action at high doses also exhibited stimulation at low concentrations. The maximum stimulatory effect of a mixture was higher than the highest stimulatory effect of each individually tested compound. Moreover, when the exposure time was increased, the hormetic effect decreased. Hormesis is being increasingly included in dose-response studies because this may have a harmful, beneficial or indifferent effect in an environment. Despite the results obtained in this research, further investigations need to be conducted to elucidate the behavior of PPCPs in aquatic environments.

Antisocial luxO Mutants Provide a Stationary-Phase Survival Advantage in Vibrio fischeri ES114

The squid light organ symbiont Vibrio fischeri controls bioluminescence using two acyl-homoserine lactone pheromone-signaling (PS) systems. The first of these systems to be activated during host colonization, AinS/AinR, produces and responds to N-octanoyl homoserine lactone (C(8)-AHL). We screened activity of a P(ainS)-lacZ transcriptional reporter in a transposon mutant library and found three mutants with decreased reporter activity, low C(8)-AHL output, and other traits consistent with low ainS expression. However, the transposon insertions were unrelated to these phenotypes, and genome resequencing revealed that each mutant had a distinct point mutation in luxO. In the wild type, LuxO is phosphorylated by LuxU and then activates transcription of the small RNA (sRNA) Qrr, which represses ainS indirectly by repressing its activator LitR. The luxO mutants identified here encode LuxU-independent, constitutively active LuxO* proteins. The repeated appearance of these luxO mutants suggested that they had some fitness advantage during construction and/or storage of the transposon mutant library, and we found that luxO* mutants survived better and outcompeted the wild type in prolonged stationary-phase cultures. From such cultures we isolated additional luxO* mutants. In all, we isolated LuxO* allelic variants with the mutations P41L, A91D, F94C, P98L, P98Q, V106A, V106G, T107R, V108G, R114P, L205F, H319R, H324R, and T335I. Based on the current model of the V. fischeri PS circuit, litR knockout mutants should resemble luxO* mutants; however, luxO* mutants outcompeted litR mutants in prolonged culture and had much poorer host colonization competitiveness than is reported for litR mutants, illustrating additional complexities in this regulatory circuit.

IMPORTANCE

Our results provide novel insight into the function of LuxO, which is a key component of pheromone signaling (PS) cascades in several members of the Vibrionaceae. Our results also contribute to an increasingly appreciated aspect of bacterial behavior and evolution whereby mutants that do not respond to a signal from like cells have a selective advantage. In this case, although "antisocial" mutants locked in the PS signal-off mode can outcompete parents, their survival advantage does not require wild-type cells to exploit. Finally, this work strikes a note of caution for those conducting or interpreting experiments in V. fischeri, as it illustrates how pleiotropic mutants could easily and inadvertently be enriched in this bacterium during prolonged culturing.

Characterization of the Vibrio fischeri Fatty Acid Chemoreceptors, VfcB and VfcB2

Bacteria use a wide variety of methyl-accepting chemotaxis proteins (MCPs) to mediate their attraction to or repulsion from different chemical signals in their environment. The bioluminescent marine bacterium Vibrio fischeri is the monospecific symbiont of the Hawaiian bobtail squid, Euprymna scolopes, and encodes a large repertoire of MCPs that are hypothesized to be used during different parts of its complex, multistage lifestyle. Here, we report the initial characterization of two such MCPs from V. fischeri that are responsible for mediating migration toward short- and medium-chain aliphatic (or fatty) acids. These receptors appear to be distributed among only members of the family Vibrionaceae and are likely descended from a receptor that has been lost by the majority of the members of this family. While chemotaxis greatly enhances the efficiency of host colonization by V. fischeri, fatty acids do not appear to be used as a chemical cue during this stage of the symbiosis. This study presents an example of straight-chain fatty acid chemoattraction and contributes to the growing body of characterized MCP-ligand interactions.

Use of Hybridization Chain Reaction-Fluorescent In Situ Hybridization To Track Gene Expression by Both Partners during Initiation of Symbiosis

The establishment of a productive symbiosis between Euprymna scolopes, the Hawaiian bobtail squid, and its luminous bacterial symbiont, Vibrio fischeri, is mediated by transcriptional changes in both partners. A key challenge to unraveling the steps required to successfully initiate this and many other symbiotic associations is characterization of the timing and location of these changes. We report on the adaptation of hybridization chain reaction-fluorescent in situ hybridization (HCR-FISH) to simultaneously probe the spatiotemporal regulation of targeted genes in both E. scolopes and V. fischeri. This method revealed localized, transcriptionally coregulated epithelial cells within the light organ that responded directly to the presence of bacterial cells while, at the same time, provided a sensitive means to directly show regulated gene expression within the symbiont population. Thus, HCR-FISH provides a new approach for characterizing habitat transition in bacteria and for discovering host tissue responses to colonization.

Core principles of bacterial autoinducer systems

SUMMARY

Autoinduction (AI), the response to self-produced chemical signals, is widespread in the bacterial world. This process controls vastly different target functions, such as luminescence, nutrient acquisition, and biofilm formation, in different ways and integrates additional environmental and physiological cues. This diversity raises questions about unifying principles that underlie all AI systems. Here, we suggest that such core principles exist. We argue that the general purpose of AI systems is the homeostatic control of costly cooperative behaviors, including, but not limited to, secreted public goods. First, costly behaviors require preassessment of their efficiency by cheaper AI signals, which we encapsulate in a hybrid "push-pull" model. The "push" factors cell density, diffusion, and spatial clustering determine when a behavior becomes effective. The relative importance of each factor depends on each species' individual ecological context and life history. In turn, "pull" factors, often stress cues that reduce the activation threshold, determine the cellular demand for the target behavior. Second, control is homeostatic because AI systems, either themselves or through accessory mechanisms, not only initiate but also maintain the efficiency of target behaviors. Third, AI-controlled behaviors, even seemingly noncooperative ones, are generally cooperative in nature, when interpreted in the appropriate ecological context. The escape of individual cells from biofilms, for example, may be viewed as an altruistic behavior that increases the fitness of the resident population by reducing starvation stress. The framework proposed here helps appropriately categorize AI-controlled behaviors and allows for a deeper understanding of their ecological and evolutionary functions.

Signaling between two interacting sensor kinases promotes biofilms and colonization by a bacterial symbiont

Cells acclimate to fluctuating environments by utilizing sensory circuits. One common sensory pathway used by bacteria is two-component signaling (TCS), composed of an environmental sensor [the sensor kinase (SK)] and a cognate, intracellular effector [the response regulator (RR)]. The squid symbiont Vibrio fischeri uses an elaborate TCS phosphorelay containing a hybrid SK, RscS, and two RRs, SypE and SypG, to control biofilm formation and host colonization. Here, we found that another hybrid SK, SypF, was essential for biofilms by functioning downstream of RscS to directly control SypE and SypG. Surprisingly, although wild-type SypF functioned as an SK in vitro, this activity was dispensable for colonization. In fact, only a single non-enzymatic domain within SypF, the HPt domain, was critical in vivo. Remarkably, this domain within SypF interacted with RscS to permit a bypass of RscS's own HPt domain and SypF's enzymatic function. This represents the first in vivo example of a functional SK that exploits the enzymatic activity of another SK, an adaptation that demonstrates the elegant plasticity in the arrangement of TCS regulators.

Global discovery of colonization determinants in the squid symbiont Vibrio fischeri

Animal epithelial tissue becomes reproducibly colonized by specific environmental bacteria. The bacteria (microbiota) perform critical functions for the host's tissue development, immune system development, and nutrition; yet the processes by which bacterial diversity in the environment is selected to assemble the correct communities in the host are unclear. To understand the molecular determinants of microbiota selection, we examined colonization of a simplified model in which the light organ of Euprymna scolopes squid is colonized exclusively by Vibrio fischeri bacteria. We applied high-throughput insertion sequencing to identify which bacterial genes are required during host colonization. A library of over 41,000 unique transposon insertions was analyzed before and after colonization of 1,500 squid hatchlings. Mutants that were reproducibly depleted following squid colonization represented 380 genes, including 37 that encode known colonization factors. Validation of select mutants in defined competitions against the wild-type strain identified nine mutants that exhibited a reproducible colonization defect. Some of the colonization factors identified included genes predicted to influence copper regulation and secretion. Other mutants exhibited defects in biofilm development, which is required for aggregation in host mucus and initiation of colonization. Biofilm formation in culture and in vivo was abolished in a strain lacking the cytoplasmic chaperone DnaJ, suggesting an important role for protein quality control during the elaboration of bacterial biofilm in the context of an intact host immune system. Overall these data suggest that cellular stress responses and biofilm regulation are critical processes underlying the reproducible colonization of animal hosts by specific microbial symbionts.

Comparison of Microtox and Xenoassay light as a near real time river monitoring assay for heavy metals

Luminescence-based assays for toxicants such as Microtox, ToxAlert, and Biotox have been used extensively worldwide. However, the use of these assays in near real time conditions is limited due to nonoptimal assay temperature for the tropical climate. An isolate that exhibits a high luminescence activity in a broad range of temperatures was successfully isolated from the mackerel, Rastrelliger kanagurta. This isolate was tentatively identified as Photobacterium sp. strain MIE, based on partial 16S rDNA molecular phylogeny. Optimum conditions that support high bioluminescence activity occurred between 24 and 30°C, with pH 5.5 to 7.5, 10 to 20 g/L of sodium chloride, 30 to 50 g/L of tryptone, and 4 g/L of glycerol as the carbon source. Assessment of near real time capability of this bacterial system, Xenoassay light to monitor heavy metals from a contaminated river running through the Juru River Basin shows near real time capability with assaying time of less than 30 minutes per samples. Samples returned to the lab were tested with a standard Microtox assay using Vibrio fishceri. Similar results were obtained to Xenoassay light that show temporal variation of copper concentration. Thus, this strain is suitable for near real time river monitoring of toxicants especially in the tropics.

Growth, bioluminescence and shoal behavior hormetic responses to inorganic and/or organic chemicals: a review

A biphasic dose response, termed hormesis, is characterized by beneficial effects of a chemical at a low dose and harmful effects at a high dose. This biphasic dose response phenomenon has the potential to strongly alter toxicology in a broad range. The present review focuses on the progress of research into hormetic responses in terms of growth (in plants, birds, algae and humans), bioluminescence, and shoal behavior as end points. The paper describes how both inorganic and organic chemicals at a low dose show stimulatory responses while at higher doses are inhibitory. The article highlights how factors such as symbiosis, density-dependent factors, time, and contrasting environmental factors (availability of nutrients, temperature, light, etc.) affect both the range and amplitude of hormetic responses. Furthermore, the possible underlying mechanisms are also discussed and we suggest that, for every end point, different hormetic mechanisms may exist. The occurrences of varying interacting receptor systems or receptor systems affecting the assessment of hormesis for each endpoint are discussed. The present review suggests that a hormetic model should be adopted for toxicological evaluations instead of the older threshold and linear non-threshold models.

Associations and dynamics of Vibrionaceae in the environment, from the genus to the population level

The Vibrionaceae, which encompasses several potential pathogens, including V. cholerae, the causative agent of cholera, and V. vulnificus, the deadliest seafood-borne pathogen, are a well-studied family of marine bacteria that thrive in diverse habitats. To elucidate the environmental conditions under which vibrios proliferate, numerous studies have examined correlations with bulk environmental variables—e.g., temperature, salinity, nitrogen, and phosphate—and association with potential host organisms. However, how meaningful these environmental associations are remains unclear because data are fragmented across studies with variable sampling and analysis methods. Here, we synthesize findings about Vibrio correlations and physical associations using a framework of increasingly fine environmental and taxonomic scales, to better understand their dynamics in the wild. We first conduct a meta-analysis to determine trends with respect to bulk water environmental variables, and find that while temperature and salinity are generally strongly predictive correlates, other parameters are inconsistent and overall patterns depend on taxonomic resolution. Based on the hypothesis that dynamics may better correlate with more narrowly defined niches, we review evidence for specific association with plants, algae, zooplankton, and animals. We find that Vibrio are attached to many organisms, though evidence for enrichment compared to the water column is often lacking. Additionally, contrary to the notion that they flourish predominantly while attached, Vibrio can have, at least temporarily, a free-living lifestyle and even engage in massive blooms. Fine-scale sampling from the water column has enabled identification of such lifestyle preferences for ecologically cohesive populations, and future efforts will benefit from similar analysis at fine genetic and environmental sampling scales to describe the conditions, habitats, and resources shaping Vibrio dynamics.

The first engagement of partners in the Euprymna scolopes-Vibrio fischeri symbiosis is a two-step process initiated by a few environmental symbiont cells

We studied the Euprymna scolopes-Vibrio fischeri symbiosis to characterize, in vivo and in real time, the transition between the bacterial partner's free-living and symbiotic life styles. Previous studies using high inocula demonstrated that environmental V. fischeri cells aggregate during a 3 h period in host-shed mucus along the light organ's superficial ciliated epithelia. Under lower inoculum conditions, similar to the levels of symbiont cells in the environment, this interaction induces haemocyte trafficking into these tissues. Here, in experiments simulating natural conditions, microscopy revealed that at 3 h following first exposure, only ∼ 5 V. fischeri cells aggregated on the organ surface. These cells associated with host cilia and induced haemocyte trafficking. Symbiont viability was essential and mutants defective in symbiosis initiation and/or production of certain surface features, including the Mam7 protein, which is implicated in host cell attachment of V. cholerae, associated normally with host cilia. Studies with exopolysaccharide mutants, which are defective in aggregation, suggest a two-step process of V. fischeri cell engagement: association with host cilia followed by aggregation, i.e. host cell-symbiont interaction with subsequent symbiont-symbiont cell interaction. Taken together, these data provide a new model of early partner engagement, a complex model of host-symbiont interaction with exquisite sensitivity.

Exploring host-microbiota interactions in animal models and humans

The animal and bacterial kingdoms have coevolved and coadapted in response to environmental selective pressures over hundreds of millions of years. The meta'omics revolution in both sequencing and its analytic pipelines is fostering an explosion of interest in how the gut microbiome impacts physiology and propensity to disease. Gut microbiome studies are inherently interdisciplinary, drawing on approaches and technical skill sets from the biomedical sciences, ecology, and computational biology. Central to unraveling the complex biology of environment, genetics, and microbiome interaction in human health and disease is a deeper understanding of the symbiosis between animals and bacteria. Experimental model systems, including mice, fish, insects, and the Hawaiian bobtail squid, continue to provide critical insight into how host-microbiota homeostasis is constructed and maintained. Here we consider how model systems are influencing current understanding of host-microbiota interactions and explore recent human microbiome studies.

Knowing your friends: invertebrate innate immunity fosters beneficial bacterial symbioses

The innate immune system is present in all animals and is a crucial first line of defence against pathogens. However, animals also harbour large numbers of beneficial microorganisms that can be housed in the digestive tract, in specialized organs or on tissue surfaces. Although invertebrates lack conventional antibody-based immunity, they are capable of eliminating pathogens and, perhaps more importantly, discriminating them from other microorganisms. This Review examines the interactions between the innate immune systems of several model invertebrates and the symbionts of these organisms, and addresses the central question of how these long-lived and specific associations are established and maintained.

Predation response of Vibrio fischeri biofilms to bacterivorus protists

Vibrio fischeri proliferates in a sessile, stable community known as a biofilm, which is one alternative survival strategy of its life cycle. Although this survival strategy provides adequate protection from abiotic factors, marine biofilms are still susceptible to grazing by bacteria-consuming protozoa. Subsequently, grazing pressure can be controlled by certain defense mechanisms that confer higher biofilm antipredator fitness. In the present work, we hypothesized that V. fischeri exhibits an antipredator fitness behavior while forming biofilms. Different predators representing commonly found species in aquatic populations were examined, including the flagellates Rhynchomonas nasuta and Neobodo designis (early biofilm feeders) and the ciliate Tetrahymena pyriformis (late biofilm grazer). V. fischeri biofilms included isolates from both seawater and squid hosts (Euprymna and Sepiola species). Our results demonstrate inhibition of predation by biofilms, specifically, isolates from seawater. Additionally, antiprotozoan behavior was observed to be higher in late biofilms, particularly toward the ciliate T. pyriformis; however, inhibitory effects were found to be widespread among all isolates tested. These results provide an alternative explanation for the adaptive advantage and persistence of V. fischeri biofilms and provide an important contribution to the understanding of defensive mechanisms that exist in the out-of-host environment.

Robust synchronization analysis in nonlinear stochastic cellular networks with time-varying delays, intracellular perturbations and intercellular noise

Naturally, a cellular network consisted of a large amount of interacting cells is complex. These cells have to be synchronized in order to emerge their phenomena for some biological purposes. However, the inherently stochastic intra and intercellular interactions are noisy and delayed from biochemical processes. In this study, a robust synchronization scheme is proposed for a nonlinear stochastic time-delay coupled cellular network (TdCCN) in spite of the time-varying process delay and intracellular parameter perturbations. Furthermore, a nonlinear stochastic noise filtering ability is also investigated for this synchronized TdCCN against stochastic intercellular and environmental disturbances. Since it is very difficult to solve a robust synchronization problem with the Hamilton-Jacobi inequality (HJI) matrix, a linear matrix inequality (LMI) is employed to solve this problem via the help of a global linearization method. Through this robust synchronization analysis, we can gain a more systemic insight into not only the robust synchronizability but also the noise filtering ability of TdCCN under time-varying process delays, intracellular perturbations and intercellular disturbances. The measures of robustness and noise filtering ability of a synchronized TdCCN have potential application to the designs of neuron transmitters, on-time mass production of biochemical molecules, and synthetic biology. Finally, a benchmark of robust synchronization design in Escherichia coli repressilators is given to confirm the effectiveness of the proposed methods.

Contribution of rapid evolution of the luxR-luxI intergenic region to the diverse bioluminescence outputs of Vibrio fischeri strains isolated from different environments

Vibrio fischeri serves as a valuable model of bacterial bioluminescence, its regulation, and its functional significance. Light output varies more than 10,000-fold in wild-type isolates from different environments, yet dim and bright strains have similar organization of the light-producing lux genes, with the activator-encoding luxR divergently transcribed from luxICDABEG. By comparing the genomes of bright strain MJ11 and the dimmer ES114, we found that the lux region has diverged more than most shared orthologs, including those flanking lux. Divergence was particularly high in the intergenic sequence between luxR and luxI. Analysis of the intergenic lux region from 18 V. fischeri strains revealed that, with one exception, sequence divergence essentially mirrored strain phylogeny but with relatively high substitution rates. The bases conserved among intergenic luxR-luxI sequences included binding sites for known regulators, such as LuxR and ArcA, and bases of unknown significance, including a striking palindromic repeat. By using this collection of diverse luxR-luxI regions, we found that expression of P(luxI)-lacZ but not P(luxR)-lacZ transcriptional reporters correlated with the luminescence output of the strains from which the promoters originated. We also found that exchange of a small stretch of the luxI-luxR intergenic region between two strains largely reversed their relative brightness. Our results show that the luxR-luxI intergenic region contributes significantly to the variable luminescence output among V. fischeri strains isolated from different environments, although other elements of strain backgrounds also contribute. Moreover, the lux system appears to have evolved relatively rapidly, suggesting unknown environment-specific selective pressures.

Phylogeographical patterns among Mediterranean sepiolid squids and their Vibrio symbionts: environment drives specificity among sympatric species

Bobtail squid from the genera Sepiola and Rondeletiola (Cephalopoda: Sepiolidae) form mutualistic associations with luminous Gram-negative bacteria (Gammaproteobacteria: Vibrionaceae) from the genera Vibrio and Photobacterium. Symbiotic bacteria proliferate inside a bilobed light organ until they are actively expelled by the host into the surrounding environment on a diel basis. This event results in a dynamic symbiont population with the potential to establish the symbiosis with newly hatched sterile (axenic) juvenile sepiolids. In this study, we examined the genetic diversity found in populations of sympatric sepiolid squid species and their symbionts by the use of nested clade analysis with multiple gene analyses. Variation found in the distribution of different species of symbiotic bacteria suggests a strong influence of abiotic factors in the local environment, affecting bacterial distribution among sympatric populations of hosts. These abiotic factors include temperature differences incurred by a shallow thermocline, as well as a lack of strong coastal water movement accompanied by seasonal temperature changes in overlapping niches. Host populations are stable and do not appear to have a significant role in the formation of symbiont populations relative to their distribution across the Mediterranean Sea. Additionally, all squid species examined (Sepiola affinis, S. robusta, S. ligulata, S. intermedia, and Rondeletiola minor) are genetically distinct from one another regardless of location and demonstrate very little intraspecific variation within species. These findings suggest that physical boundaries and distance in relation to population size, and not host specificity, are important factors in limiting or defining gene flow within sympatric marine squids and their associated bacterial symbionts in the Mediterranean Sea.

Natural transformation of Vibrio fischeri requires tfoX and tfoY

Recent evidence has indicated that natural genetic transformation occurs in Vibrio cholerae, and that it requires both induction by chitin oligosaccharides, like chitohexaose, and expression of a putative regulatory gene designated tfoX. Using sequence and phylogenetic analyses we have found two tfoX paralogues in all sequenced genomes of the genus Vibrio. Like V. cholerae, when grown in chitohexaose, cells of V. fischeri are able to take up and incorporate exogenous DNA. Chitohexaose-independent transformation by V. fischeri was observed when tfoX was present in multicopy. The second tfoX paralogue, designated tfoY, is also required for efficient transformation in V. fischeri, but is not functionally identical to tfoX. Natural transformation of V. fischeri facilitates rapid transfer of mutations across strains, and provides a highly useful tool for experimental genetic manipulation in this species. The presence of chitin-induced competence in several vibrios highlights the potential for a conserved mechanism of genetic exchange across this family of environmentally important marine bacteria.

A complex journey: transmission of microbial symbionts

The perpetuation of symbioses through host generations relies on symbiont transmission. Horizontally transmitted symbionts are taken up from the environment anew by each host generation, and vertically transmitted symbionts are most often transferred through the female germ line. Mixed modes also exist. In this Review we describe the journey of symbionts from the initial contact to their final residence. We provide an overview of the molecular mechanisms that mediate symbiont attraction and accumulation, interpartner recognition and selection, as well as symbiont confrontation with the host immune system. We also discuss how the two main transmission modes shape the evolution of the symbiotic partners.

An intricate network of regulators controls biofilm formation and colonization by Vibrio fischeri

The initial encounter between a microbe and its host can dictate the success of the interaction, be it symbiosis or pathogenesis. This is the case, for example, in the symbiosis between the bacterium Vibrio fischeri and the squid Euprymna scolopes, which proceeds via a biofilm-like bacterial aggregation, followed by entry and growth. A key regulator, the sensor kinase RscS, is critical for symbiotic biofilm formation and colonization. When introduced into a fish symbiont strain that naturally lacks the rscS gene and cannot colonize squid, RscS permits colonization, thereby extending the host range of these bacteria. RscS controls biofilm formation by inducing transcription of the symbiosis polysaccharide (syp) gene locus. Transcription of syp also requires the sigma(54)-dependent activator SypG, which functions downstream of RscS. In addition to these regulators, SypE, a response regulator that lacks an apparent DNA binding domain, exerts both positive and negative control over biofilm formation. The putative sensor kinase SypF and the putative response regulator VpsR, both of which contribute to control of cellulose production, also influence biofilm formation. The wealth of regulators and the correlation between biofilm formation and colonization adds to the already considerable utility of the V. fischeri-E. scolopes model system.

Role of NtrC-regulated exopolysaccharides in the biofilm formation and pathogenic interaction of Vibrio vulnificus

Vibrio vulnificus has been shown to require a global transcription factor, NtrC for mature biofilm development via controlling the biosyntheses of lipopolysaccharide and exopolysaccharide (EPS). Biofilm formation and EPS production were dramatically increased in a medium including a tricarboxylic acid cycle-intermediate as a carbon source. These phenotypes required functional NtrC and were abolished by the addition of ammonium chloride. During the initial stage of biofilm formation, both expression of the ntrC gene and the cellular content of NtrC protein increased. Thus, the regulatory roles of NtrC in EPS biosynthesis were studied with three gene clusters for EPS biosyntheses. Transcriptions of the three clusters were positively controlled by NtrC and showed maximal expression at the early stage of biofilm development. Mutants deficient in one of the genes (VV1_2661, VV2_1579 and VV1_2305) in each cluster showed decreased production of EPS, attenuated ability to form biofilm and lowered cytoadherence to human epithelial cells. However, mutations in VV2_1579 and VV1_2305 resulted in lower cytotoxicity to human cells and mortality to mice than the mutation in VV1_2661. These results demonstrate that NtrC-regulated EPS are crucial in biofilm formation of V. vulnificus, and some EPS components play important roles in interacting with hosts.