Arabinose induces pellicle formation by Vibrio fischeri

RpoN mediates biofilm formation by directly controlling vps gene cluster and c-di-GMP synthetic metabolism in V. alginolyticus

Vibrio alginolyticus is a prevalent pathogen in both humans and marine species, exhibiting high adaptability to various adverse environmental conditions. Our previous studies have shown that ΔrpoN formed three enhanced biofilm types, including spectacular surface-attached biofilm (SB), scattered pellicle biofilm (PB), and colony rugosity. However, the precise mechanism through which rpoN regulates biofilm formation has remained unclear. Based on the critical role of Vibrio exopolysaccharide (VPS) in biofilm formation, several genes related to the production and regulation of VPS were characterized in V. alginolyticus. Our findings from mutant strains indicated that VPS has complete control over the formation of rugose colony morphology and PB, while it only partially contributes to SB formation. Among the four transcriptional regulators of the vps gene cluster, vpsR and VA3545 act as promoters, whereas VA3546 and VA2703 function as repressors. Through transcriptome analysis and c-di-GMP concentration determination, VA0356 and VA3580 which encoded diguanylate cyclase were found to mediate the ΔrpoN biofilm formation. As a central regulator, rpoN governed biofilm formation through two regulatory pathways. Firstly, it directly bound to the upstream region of VA4206 to regulate the expression of the vps gene cluster (VA4206-VA4196). Secondly, it directly and indirectly modulated c-di-GMP synthesis gene VA3580 and VA0356, respectively, thereby affecting c-di-GMP concentration and subsequently influencing the expression of vps transcription activators vpsR and VA3545. Under conditions promoting SB formation, ΔrpoN was unable to thrive below the liquid level due to significantly reduced activities of three catalytic enzymes (ACK, ADH, and ALDH) involved in pyruvate metabolism, but tended to reproduce in air-liquid interface, a high oxygen niche compared to the liquid phase. In conclusion, both exopolysaccharide synthesis and oxygen-related metabolism contributed to ΔrpoN biofilm formation. The role of RpoN-mediated hypoxic metabolism and biofilm formation were crucial for comprehending the colonization and pathogenicity of V. alginolyticus in hosts, providing a novel target for treating V. alginolyticus in aquatic environments and hosts.

A prototrophic suppressor of a Vibrio fischeri D-glutamate auxotroph reveals a member of the periplasmic broad-spectrum racemase family (BsrF)

Although bacterial peptidoglycan (PG) is highly conserved, some natural variations in PG biosynthesis and structure have evolved. Understanding the mechanisms and limits of such variation will inform our understanding of antibiotic resistance, innate immunity, and the evolution of bacteria. We have explored the constraints on PG evolution by blocking essential steps in PG biosynthesis in Vibrio fischeri and then selecting mutants with restored prototrophy. Here, we attempted to select prototrophic suppressors of a D-glutamate auxotrophic murI racD mutant. No suppressors were isolated on unsupplemented lysogeny broth salts (LBS), despite plating >1011 cells, nor were any suppressors generated through mutagenesis with ethyl methanesulfonate. A single suppressor was isolated on LBS supplemented with iso-D-gln, although the iso-D-gln subsequently appeared irrelevant. This suppressor has a genomic amplification formed by the creation of a novel junction that fuses proB to a gene encoding a putative broad-spectrum racemase of V. fischeri, bsrF. An engineered bsrF allele lacking the putative secretion signal (ΔSS-bsrF) also suppressed D-glu auxotrophy, resulting in PG that was indistinguishable from the wild type. The ΔSS-bsrF allele similarly suppressed the D-alanine auxotrophy of an alr mutant and restored prototrophy to a murI alr double mutant auxotrophic for both D-ala and D-glu. The ΔSS-bsrF allele increased resistance to D-cycloserine but had no effect on sensitivity to PG-targeting antibiotics penicillin, ampicillin, or vancomycin. Our work helps define constraints on PG evolution and reveals a periplasmic broad-spectrum racemase in V. fischeri that can be co-opted for PG biosynthesis, with concomitant D-cycloserine resistance.

IMPORTANCE

D-Amino acids are used and produced by organisms across all domains of life, but often, their origins and roles are not well understood. In bacteria, D-ala and D-glu are structural components of the canonical peptidoglycan cell wall and are generated by dedicated racemases Alr and MurI, respectively. The more recent discovery of additional bacterial racemases is broadening our view and deepening our understanding of D-amino acid metabolism. Here, while exploring alternative PG biosynthetic pathways in Vibrio fischeri, we unexpectedly shed light on an unusual racemase, BsrF. Our results illustrate a novel mechanism for the evolution of antibiotic resistance and provide a new avenue for exploring the roles of non-canonical racemases and D-amino acids in bacteria.

Construction of a CRISPR Interference System for Gene Knockdown in Stenotrophomonas maltophilia AGS-1 from Aerobic Granular Sludge

To identify the function of attachment genes involved in biofilm formation in Stenotrophomonas maltophilia AGS-1 isolated from aerobic granular sludge, an effective gene molecular tool is needed. We developed a two-plasmid CRISPRi system in Stenotrophomonas maltophilia AGS-1. One plasmid expressed dCas9 protein with the l-arabinose inducible promoter, and the other plasmid contained the sgRNA cassette complementary to the target gene. Under control of the araC-inducible promoter, this system exhibited little leaky basal expression and highly induced expression that silenced endogenous and exogenous genes with reversible knockdown. This system achieved up to 211-fold suppression for mCherry expression on the nontemplate strand compared to the template strand (91-fold). The utility of the developed CRISPRi platform was also characterized by suppressing the xanA and rpfF genes. The expression of these two genes was rapidly depleted and the adhesion ability decreased, which demonstrated that the modulation of either gene was an important factor for biofilm formation of the AGS-1 strain. The system also tested the ability to simultaneously silence transcriptional suppression of multiple targeted genes, an entire operon, or part of it. Lastly, the use of CRISPRi allowed us to dissect the gene intricacies involved in flagellar biosynthesis. Collectively, these results demonstrated that the CRISPRi system was a simple, feasible, and controllable manipulation system of gene expression in the AGS-1 strain.

L-arabinose affects the growth, biofilm formation, motility, c-di-GMP metabolism, and global gene expression of Vibrio parahaemolyticus

The L-arabinose inducible pBAD vectors are commonly used to turn on and off the expression of specific genes in bacteria. The utilization of certain carbohydrates can influence bacterial growth, virulence factor production, and biofilm formation. Vibrio parahaemolyticus, the causative agent of seafood-associated gastroenteritis, can grow in media with L-arabinose as the sole carbon source. However, the effects of L-arabinose on V. parahaemolyticus physiology have not been investigated. In this study, we show that the growth rate, biofilm formation capacity, capsular polysaccharide production, motility, and c-di-GMP production of V. parahaemolyticus are negatively affected by L-arabinose. RNA-seq data revealed significant changes in the expression levels of 752 genes, accounting for approximately 15.6% of V. parahaemolyticus genes in the presence of L-arabinose. The affected genes included those associated with L-arabinose utilization, major virulence genes, known key biofilm-related genes, and numerous regulatory genes. In the majority of type III secretion system, two genes were upregulated in the presence of L-arabinose, whereas in those of type VI secretion system, two genes were downregulated. Ten putative c-di-GMP metabolism-associated genes were also significantly differentially expressed, which may account for the reduced c-di-GMP levels in the presence of L-arabinose. Most importantly, almost 40 putative regulators were significantly differentially expressed due to the induction by L-arabinose, indicating that the utilization of L-arabinose is strictly regulated by regulatory networks in V. parahaemolyticus. The findings increase the understanding of how L-arabinose affects the physiology of V. parahaemolyticus. Researchers should use caution when considering the use of L-arabinose inducible pBAD vectors in V. parahaemolyticus. IMPORTANCE The data in this study show that L-arabinose negatively affects the growth rate, biofilm formation, capsular polysaccharide production, motility, and c-di-GMP production of V. parahaemolyticus. The data also clarify the gene expression profiles of the bacterium in the presence of L-arabinose. Significantly differentially expressed genes in response to L-arabinose were involved in multiple cellular pathways, including L-arabinose utilization, virulence factor production, biofilm formation, motility, adaptation, and regulation. The collective findings indicate the significant impact of L-arabinose on the physiology of V. parahaemolyticus. There may be similar effects on other species of bacteria. Necessary controls should be established when pBAD vectors must be used for ectopic gene expression.

"Sharing the matrix" - a cooperative strategy for survival in Salmonella enterica serovar Typhimurium

BACKGROUND

Bacteria in nature live together in communities called biofilms, where they produce a matrix that protects them from hostile environments. The components of this matrix vary among species, with Salmonella enterica serovar Typhimurium (STm- WT) primarily producing curli and cellulose, which are regulated by the master regulator csgD. Interactions between bacteria can be competitive or cooperative, with cooperation more commonly observed among the kin population. This study refers to STm- WT as the generalist which produces all the matrix components and knockout strains that are defective in either curli or cellulose as the specialists, which produces one of the matrix components but not both. We have asked whether two different specialists will cooperate and share matrix components during biofilm formation to match the ability of the generalist which produces both components.

RESULTS

In this study, the response of the specialists and generalist to physical, chemical, and biological stress during biofilm formation is also studied to assess their abilities to cooperate and produce biofilms like the generalist. STm WT colony biofilm which produces both the major biofilm matrix component were protected from stress whereas the non-matrix producer (∆csgD), the cellulose, and curli alone producers ∆csgA, ∆bcsA respectively were affected. During the exposure to various stresses, the majority of killing occurred in ∆csgD. Whereas the co-culture (∆csgA: ∆bcsA) was able to resist stress like that of the STm WT. Phenotypic and morphological characteristics of the colonies were typed using congo red assay and the Influence of matrix on the architecture of biofilms was confirmed by scanning electron microscopy.

CONCLUSION

Our results show that matrix aids in survival during antibiotic, chlorine, and predatory stress. And possible sharing of the matrix is occurring in co-culture, with one counterbalancing the inability of the other when confronted with stress.

Selective Antagonism of Lactiplantibacillus plantarum and Pediococcus acidilactici against Vibrio and Aeromonas in the Bacterial Community of Artemia nauplii

Empiric probiotics are commonly consumed by healthy individuals as a means of disease prevention, pathogen control, etc. However, controversy has existed for a long time regarding the safety and benefits of probiotics. Here, two candidate probiotics, Lactiplantibacillus plantarum and Pediococcus acidilactici, which are antagonistic to Vibrio and Aeromonas species in vitro, were tested on Artemia under in vivo conditions. In the bacterial community of Artemia nauplii, L. plantarum reduced the abundance of the genera Vibrio and Aeromonas and P. acidilactici significantly increased the abundance of Vibrio species in a positive dosage-dependent manner, while higher and lower dosages of P. acidilactici increased and decreased the abundance of the genus Aeromonas, respectively. Based on the liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) analyses of the metabolite of L. plantarum and P. acidilactici, pyruvic acid was used in an in vitro test to explain such selective antagonism; the results showed that pyruvic acid was conducive or suppressive to V. parahaemolyticus and beneficial to A. hydrophila. Collectively, the results of this study demonstrate the selective antagonism of probiotics on the bacterial community composition of aquatic organisms and the associated pathogens. IMPORTANCE Over the last decade, the common preventive method for controlling potential pathogens in aquaculture has been the use of probiotics. However, the mechanisms of probiotics are complicated and mostly undefined. At present, less attention has been paid to the potential risks of probiotic use in aquaculture. Here, we investigated the effects of two candidate probiotics, L. plantarum and P. acidilactici, on the bacterial community of Artemia nauplii and the in vitro interactions between these two candidate probiotics and two pathogens, Vibrio and Aeromonas species. The results demonstrated the selective antagonism of probiotics on the bacterial community composition of an aquatic organism and its associated pathogens. This research contributes to providing a basis and reference for the long-term rational use of probiotics and to reducing the inappropriate use of probiotics in aquaculture.

Porous Pellicle Formation of a Filamentous Bacterium, Leptothrix

The bacterium Leptothrix cholodnii generates filaments encased in a sheath comprised of woven nanofibrils. In static liquid culture, L. cholodnii moves toward the air-liquid interface, where it forms porous pellicles. Observations of aggregation at the interface reveal that clusters consisting of only a few bacteria primarily grow by netting free cells. These growing clusters hierarchically enlarge through the random docking of other small clusters. We find that the bacteria swim using their polar flagellum toward the interface, where their sheath assists them in intertwining with others and thereby promotes the formation of small clusters. In contrast, sheathless hydrophobic mutant cells get stuck to the interface. We find that the nanofibril sheath is vital for robust pellicle formation as it lowers cell surface hydrophobicity by 60%, thereby reducing their adsorption and enabling cells to move toward and stick together at the air-liquid interface. IMPORTANCE Efficient and sustainable management of water resources is becoming a fundamental issue for supporting growing populations and for developing economic activity. Fundamental to this management is the treatment of wastewater. Microorganisms are the active component of activated sludge that is employed in the biodegradation process of many wastewater treatment facilities. However, uncontrolled growth of filamentous bacteria such as Sphaerotilus often results in filamentous bulking, lowering the efficiency of water treatment systems. To prevent this undesirable condition, strategies based on a fundamental understanding of the ecology of filamentous bacteria are required. Although the filamentous bacterium Leptothrix cholodnii, which is closely related to Sphaerotilus, is a minor inhabitant of activated sludge, its complete genome sequence is known, making gene manipulation relatively easy. Moreover, L. cholodnii generates porous pellicles under static conditions, which may be a characteristic of filamentous bulking. We show that both swimming motility and nanofibril-mediated air-liquid interface attachment are required for porous pellicle formation. These insights are critical for a better understanding of the characteristics of filamentous bulking and might improve strategies to control activated sludge.

Microbial biofilms: Recent advances and progress in environmental bioremediation

Microbial biofilms are formed by adherence of the bacteria through their secreted polymer matrices. The major constituents of the polymer matrices are extracellular DNAs, proteins, polysaccharides. Biofilms have exhibited a promising role in the area of bioremediation. These activities can be further improved by tuning the parameters like quorum sensing, characteristics of the adhesion surface, and other environmental factors. Organic pollutants have created a global concern because of their long-term toxicity on human, marine, and animal life. These contaminants are not easily degradable and continue to prevail in the environment for an extended period. Biofilms are being used for the remediation of different pollutants, among which organic pollutants have been of significance. The bioremediation of organic contaminants using biofilms is an eco-friendly, cheap, and green process. However, the development of this technology demands knowledge on the mechanism of action of the microbes to form the biofilm, types of specific bacteria or fungi responsible for the degradation of a particular organic compound, and the mechanistic role of the biofilm in the degradation of the pollutants. This review puts forth a comprehensive summary of the role of microbial biofilms in the bioremediation of different environment-threatening organic pollutants.

Identification and analysis of Rap-Phr system in Bacillus cereus 0-9

In this study, eight rap-related genes were found in the Bacillus cereus 0-9 genome; five rap genes were located on chromosomes and three on large plasmids. Five Rap proteins in B. cereus 0-9 were annotated as 'tetratricopeptide repeat proteins'. SMART Server analysis showed that the eight Rap proteins had typical tetrapeptide repeat sequence (TPR) domains. Biofilm assays and crystal violet staining showed that overexpression of the rapp1 and rap5 genes affected the biofilm formation of B. cereus 0-9, and the activities of Rapp1 and Rap5 proteins were inhibited by their corresponding cognate Phr, suggesting that the Rap-Phr quorum sensing (QS) system might also exist in the B. cereus 0-9 strain. In addition, overexpression of rap1 genes inhibited in the extracellular amylase decomposition capacity of B. cereus 0-9. The results of the sporulation assay indicated that overexpression of the eight rap genes inhibited the spore formation of B. cereus 0-9 to varying degrees. These results provide a reference for research on the regulation of the Rap-Phr QS system in B. cereus.

Isocitrate dehydrogenase of Bacillus cereus is involved in biofilm formation

Isocitrate dehydrogenase (IDH), a key enzyme in the TCA cycle, participates in the formation of biofilms in Staphylococcus aureus, but it remains to be clarified whether it is involved in the formation of Bacillus cereus biofilms. In this study, we scanned the genome of B. cereus 0-9 and found a gene encoding isocitrate dehydrogenase (FRY47_22620) named icdH. The IcdH protein was expressed and purified. The enzyme activity assay showed that the protein had IDH activity dependent on NADP⁺, indicating that this gene encoded an IDH. The ΔicdH mutant and its complemented strains were obtained by a homologous recombination strategy, and crystal violet data and CLSM were measured. The results showed that the biofilm yield of the mutant ΔicdH decreased, and the biofilm morphology also changed, while the growth of ΔicdH was not affected. The extracellular pH and citric acid content results showed that the ΔicdH mutant exhibited citric acid accumulation and acidification of the extracellular matrix. In addition, the addition of excess Fe³⁺ restored the biofilm formation of the ΔicdH mutant. It is speculated that IDH in B. cereus may regulate biofilm formation by modulating intracellular redox homeostasis. In addition, we found that the icdH deletion of B. cereus 0-9 could result in a reduced sporulation rate, which was significantly different from sporulation in B. subtilis caused by interruption of the stage I sporulation process due to icdH loss. All the above results provide us with new insights for further research on IDH.

L-arabinose induces the formation of viable non-proliferating spheroplasts in Vibrio cholerae

Vibrio cholerae, the agent of the deadly human disease cholera, propagates as a curved rod-shaped bacterium in warm waters. It is sensitive to cold, but persists in cold waters under the form of viable but non-dividing coccoidal shaped cells. Additionally, V. cholerae is able to form non-proliferating spherical cells in response to cell wall damage. It was recently reported that L-arabinose, a component of the hemicellulose and pectin of terrestrial plants, stops the growth of V. cholerae. Here, we show that L-arabinose induces the formation of spheroplasts that lose the ability to divide and stop growing in volume over time. However, they remain viable and upon removal of L-arabinose they start expanding in volume, form branched structures and give rise to cells with a normal morphology after a few divisions. We further show that WigKR, a histidine kinase/response regulator pair implicated in the induction of a high expression of cell wall synthetic genes, prevents the lysis of the spheroplasts during growth restart. Finally, we show that the physiological perturbations result from the import and catabolic processing of L-arabinose by the V. cholerae homolog of the E. coli galactose transport and catabolic system. Taken together, our results suggest that the formation of non-growing spherical cells is a common response of Vibrios exposed to detrimental conditions. They also permit to define conditions preventing any physiological perturbation of V. cholerae when using L-arabinose to induce gene expression from the tightly regulated promoter of the Escherichia coli araBAD operon.Importance Vibrios among other bacteria form transient cell wall deficient forms as a response to different stresses and revert to proliferating rods when permissive conditions have been restored. Such cellular forms have been associated to antimicrobial tolerance, chronic infections and environmental dispersion.The effect of L-Ara on V. cholerae could provide an easily tractable model to study the ability of Vibrios to form viable reversible spheroplasts. Indeed, the quick transition to spheroplasts and reversion to proliferating rods by addition or removal of L-Ara is ideal to understand the genetic program governing this physiological state and the spatial rearrangements of the cellular machineries during cell shape transitions.

Nonmetabolizable Arabinose Inhibits Vibrio cholerae Growth in M9 Medium with Gluconate as the Sole Carbon Source

The serogroups O1 and O139 of the marine bacterium Vibrio cholerae are responsible for causing cholera in humans. The pentose sugar arabinose is nonmetabolizable by the pathogen and is present in environmental niches as well as in the human intestine. In this study, arabinose-mediated V. cholerae growth interference was assessed in M9 minimal medium containing gluconate as the sole carbon source in the light of Entner-Doudoroff (ED) pathway, an obligatory metabolic route for gluconate utilization. V. cholerae O1 and O139 strains failed to grow in the presence of ≥ 0.3% arabinose in M9 with 0.2% gluconate, but there was no growth inhibition in the presence of arabinose in M9 with 0.2% glucose. Transcriptional analysis of edd and eda, the genes constituting the ED pathway, showed ~100- and ~17-fold increases, respectively, in M9-gluconate. Minor increases of ~4- and ~2-fold for edd and eda, respectively, were noted in AKI medium supplemented with 0.5% arabinose. The observed arabinose-mediated growth inhibition can contribute toward deepening the understanding of altered phenotypes, if any, via complementation/expression studies in V. cholerae with pBAD vectors and arabinose as an inducer.

Acid soluble extracellular matrix confers structural stability to marine Bacillus haynesii pellicle biofilms

In natural and engineered settings, bacteria predominantly thrive in biofilms, which are complex microbial communities embedded in a self-produced extracellular polymeric substances (EPSs) matrix. Pellicles are complex macroscopic biofilms floating at air-water interface. Though pellicle formation has been studied in detail in Bacillus subtilis, a soil bacterium, it is not reported in aquatic bacteria, which may use pellicle-growth as survival-strategy. This study shows that Bacillus haynesii isolated from a marine environment forms robust pellicle biofilms at air-water interface. B. haynesii pellicles showed complex architecture, involving dense cell-aggregates with interconnecting thread-like structures in an extracellular matrix. In situ staining by Alcian blue, Concanavalin A and ThioflavinT (ThT), respectively, localized acidic polymers, glycoconjugates and amyloid-like fibers in the pellicle. The pellicle was rigid and not disrupted by common EPS extraction protocols. Hence, a set of reagents and conditions were evaluated for solubilizing the EPS and pellicle. Acetic acid was able to effectively solubilize the structural EPS and pellicle structure. Acid soluble structural EPS contained chemical signatures for both proteins and carbohydrates, as revealed by elemental analysis, Fourier Transform Infrared Spectroscopy and Raman Spectroscopy. Ex situ staining of acid soluble EPS by ThT showed recovery of amyloid-forming proteins from pellicle. Results show that structural stability of the pellicle is mainly conferred by amyloid-like fibers of the EPS matrix. The robust pellicle-growth reported here may represent a survival-strategy in the aquatic bacterium. The findings reported here can support future research on biofilm structure, EPS matrix and its formation, which are critical for understanding how microbes thrive in natural and engineered settings.

Mutagenesis of Vibrio fischeri and Other Marine Bacteria Using Hyperactive Mini-Tn5 Derivatives

Mutagenizing bacterial genomes with selectable transposon insertions is an effective approach for identifying the genes underlying important phenotypes. Specific bacteria may require different tools and methods for effective transposon mutagenesis, and here we describe methods to mutagenize Vibrio fischeri using an engineered mini-Tn5 transposon with synthetic "mosaic" transposon ends. The transposon is delivered by conjugation on a plasmid that cannot replicate in V. fischeri and that encodes a hyperactive transposase outside the transposon itself. The chromosomal location of insertions can be readily identified by cloning and/or PCR-based methods described here. Although developed in V. fischeri, these tools and methods have proven effective in some other bacteria as well.

Discovery of Calcium as a Biofilm-Promoting Signal for Vibrio fischeri Reveals New Phenotypes and Underlying Regulatory Complexity

Vibrio fischeri uses biofilm formation to promote symbiotic colonization of its squid host, Euprymna scolopes Control over biofilm formation is exerted at the level of transcription of the symbiosis polysaccharide (syp) locus by a complex set of two-component regulators. Biofilm formation can be induced by overproduction of the sensor kinase RscS, which requires the activities of the hybrid sensor kinase SypF and the response regulator SypG and is negatively regulated by the sensor kinase BinK. Here, we identify calcium as a signal that promotes biofilm formation by biofilm-competent strains under conditions in which biofilms are not typically observed (growth with shaking). This was true for RscS-overproducing cells as well as for strains in which only the negative regulator binK was deleted. The latter results provided, for the first time, an opportunity to induce and evaluate biofilm formation without regulator overexpression. Using these conditions, we determined that calcium induces both syp-dependent and bacterial cellulose synthesis (bcs)-dependent biofilms at the level of transcription of these loci. The calcium-induced biofilms were dependent on SypF, but SypF's Hpt domain was sufficient for biofilm formation. These data suggested the involvement of another sensor kinase(s) and led to the discovery that both RscS and a previously uncharacterized sensor kinase, HahK, functioned in this pathway. Together, the data presented here reveal both a new signal and biofilm phenotype produced by V. fischeri cells, the coordinate production of two polysaccharides involved in distinct biofilm behaviors, and a new regulator that contributes to control over these processes.IMPORTANCE Biofilms, or communities of surface-attached microorganisms adherent via a matrix that typically includes polysaccharides, are highly resistant to environmental stresses and are thus problematic in the clinic and important to study. Vibrio fischeri forms biofilms to colonize its symbiotic host, making this organism useful for studying biofilms. Biofilm formation depends on the syp polysaccharide locus and its regulators. Here, we identify a signal, calcium, that induces both SYP-PS and cellulose-dependent biofilms. We also identify a new syp regulator, the sensor kinase HahK, and discover a mutant phenotype for the sensor kinase RscS. This work thus reveals a specific biofilm-inducing signal that coordinately controls two polysaccharides, identifies a new regulator, and clarifies the regulatory control over biofilm formation by V. fischeri.

An Expanded Transposon Mutant Library Reveals that Vibrio fischeri δ-Aminolevulinate Auxotrophs Can Colonize Euprymna scolopes

Libraries of defined mutants are valuable research tools but necessarily lack gene knockouts that are lethal under the conditions used in library construction. In this study, we augmented a Vibrio fischeri mutant library generated on a rich medium (LBS, which contains [per liter] 10 g of tryptone, 5 g of yeast extract, 20 g of NaCl, and 50 mM Tris [pH 7.5]) by selecting transposon insertion mutants on supplemented LBS and screening for those unable to grow on LBS. We isolated strains with insertions in alr, glr (murI), glmS, several heme biosynthesis genes, and ftsA, as well as a mutant disrupted 14 bp upstream of ftsQ Mutants with insertions in ftsA or upstream of ftsQ were recovered by addition of Mg²⁺ to LBS, but their cell morphology and motility were affected. The ftsA mutant was more strongly affected and formed cells or chains of cells that appeared to wind back on themselves helically. Growth of mutants with insertions in glmS, alr, or glr was recovered with N-acetylglucosamine (NAG), d-alanine, or d-glutamate, respectively. We hypothesized that NAG, d-alanine, or d-glutamate might be available to V. fischeri in the Euprymna scolopes light organ; however, none of these mutants colonized the host effectively. In contrast, hemA and hemL mutants, which are auxotrophic for δ-aminolevulinate (ALA), colonized at wild-type levels, although mutants later in the heme biosynthetic pathway were severely impaired or unable to colonize. Our findings parallel observations that legume hosts provide Bradyrhizobium symbionts with ALA, but they contrast with virulence phenotypes of hemA mutants in some pathogens. The results further inform our understanding of the symbiotic light organ environment.IMPORTANCE By supplementing a rich yeast-based medium, we were able to recover V. fischeri mutants with insertions in conditionally essential genes, and further characterization of these mutants provided new insights into this bacterium's symbiotic environment. Most notably, we show evidence that the squid host can provide V. fischeri with enough ALA to support its growth in the light organ, paralleling the finding that legumes provide Bradyrhizobium ALA in symbiotic nodules. Taken together, our results show how a simple method of augmenting already rich media can expand the reach and utility of defined mutant libraries.

Cellulose effects on morphology and elasticity of Vibrio fischeri biofilms

Cellulose effects on Vibrio fischeri biofilm morphology were tested for the wild-type and two of its isogenic mutants that either exhibit increased cellulose production or do not produce cellulose at all. Confocal laser scanning microscopy imaging of each biofilm revealed that total sessile volume increases with cellulose expression, but the size of colonies formed with cellulose was smaller, creating a more diffuse biofilm. These morphological differences were not attributed to variations in bacterial deposition, extracellular polymeric substances affinity to the surface or bacterial growth. A positive correlation was found between cellulose expression, Young's (elastic) modulus of the biofilm analyzed with atomic force microscope and shear modulus of the related extracellular polymeric substances layers analyzed with quartz crystal microbalance with dissipation monitoring. Cellulose production also correlated positively with concentrations of extracellular DNA. A significant negative correlation was observed between cellulose expression and rates of diffusion through the extracellular polymeric substances. The difference observed in biofilm morphology is suggested as a combined result of cellulose and likely extracellular DNA (i) increasing biofilm Young's modulus, making shear removal more difficult, and (ii) decreased diffusion rate of nutrients and wastes into and out of the biofilm, which effectively limits colony size.

Effect of biofilm parameters and extracellular polymeric substance composition on polycyclic aromatic hydrocarbon degradation

This study with ten marine isolates demonstrates that the attached phenotypes of the marine bacteria showed significant variation in biofilm architecture and, in turn, biodegradation of PAHs.

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.

Establishing a Role for Bacterial Cellulose in Environmental Interactions: Lessons Learned from Diverse Biofilm-Producing Proteobacteria

Bacterial cellulose (BC) serves as a molecular glue to facilitate intra- and inter-domain interactions in nature. Biosynthesis of BC-containing biofilms occurs in a variety of Proteobacteria that inhabit diverse ecological niches. The enzymatic and regulatory systems responsible for the polymerization, exportation, and regulation of BC are equally as diverse. Though the magnitude and environmental consequences of BC production are species-specific, the common role of BC-containing biofilms is to establish close contact with a preferred host to facilitate efficient host-bacteria interactions. Universally, BC aids in attachment, adherence, and subsequent colonization of a substrate. Bi-directional interactions influence host physiology, bacterial physiology, and regulation of BC biosynthesis, primarily through modulation of intracellular bis-(3'→5')-cyclic diguanylate (c-di-GMP) levels. Depending on the circumstance, BC producers exhibit a pathogenic or symbiotic relationship with plant, animal, or fungal hosts. Rhizobiaceae species colonize plant roots, Pseudomonadaceae inhabit the phyllosphere, Acetobacteriaceae associate with sugar-loving insects and inhabit the carposphere, Enterobacteriaceae use fresh produce as vehicles to infect animal hosts, and Vibrionaceae, particularly Aliivibrio fischeri, colonize the light organ of squid. This review will highlight the diversity of the biosynthesis and regulation of BC in nature by discussing various examples of Proteobacteria that use BC-containing biofilms to facilitate host-bacteria interactions. Through discussion of current data we will establish new directions for the elucidation of BC biosynthesis, its regulation and its ecophysiological roles.

Gram-negative bacteria can also form pellicles

There is a growing interest in the bacterial pellicle, a biofilm floating at the air-liquid interface. Pellicles have been well studied in the Gram-positive bacterium Bacillus subtilis, but far less in Gram-negative bacteria, where pellicle studies have mostly focused on matrix components rather than on the regulatory cascades involved. Several Gram-negative bacteria, including pathogenic bacteria, have been shown to be able to form a pellicle under static conditions. Here, we summarize the growing body of knowledge about pellicle formation in Gram-negative bacteria, especially about the components of the pellicle matrix. We also propose that the pellicle is a specific biofilm, and that its formation involves particular processes. Since this lifestyle concerns a growing number of bacteria, its properties undoubtedly deserve further investigation.

Engineering Vibrio fischeri for Inducible Gene Expression

The marine bacterium Vibrio fischeri serves as a model organism for a variety of natural phenomena, including symbiotic host colonization. The ease with which the V. fischeri genome can be manipulated contributes greatly to our ability to identify the factors involved in these phenomena. Here, we have adapted genetic tools for use in V. fischeri to promote our ability to conditionally control the expression of genes of interest. Specifically, we modified the commonly used mini-Tn5 transposon to contain an outward-facing, LacI-repressible/IPTG-inducible promoter, and inserted the lacI gene into the V. fischeri chromosome. Used together, these tools permit the identification and induction of genes that control specific phenotypes. To validate this approach, we identified IPTG-controllable motility mutants. We anticipate that the ability to randomly insert an inducible promoter into the genome of V. fischeri will advance our understanding of various aspects of the physiology of this microbe.

Construction of a tetracycline inducible expression vector and characterization of its use in Vibrio cholerae

We report the construction of a tetracycline inducible expression vector that allows regulated gene expression in the enteric pathogen Vibrio cholerae. The expression vector, named pXB300, contains the tetracycline regulatory elements from Tn10, a multiple cloning site downstream of the tetA promoter and operator sequences, a ColE1 origin of replication, a β-lactamase resistance gene for positive selection, and the hok/sok addiction system for selection in the absence of antibiotic. The function of the tetracycline expression system was demonstrated by cloning lacZ under control of the tetA promoter and quantifying β-galactosidase expression in Escherichia coli and V. cholerae. The utility for pXB300 was documented by complementation of V. cholerae virulence mutants during growth under virulence inducing conditions. The results showed that pXB300 allowed high-level expression of recombinant genes with linear induction in response to the exogenous concentration of the inducer anhydrotetracycline. We further show that pXB300 was reliably maintained in V. cholerae during growth in the absence of antibiotic selection.