Tetrathionate reduction and production of hydrogen sulfide from thiosulfate

Ethanolamine metabolism through two genetically distinct loci enables Klebsiella pneumoniae to bypass nutritional competition in the gut

Successful microbial colonization of the gastrointestinal (GI) tract hinges on an organism's ability to overcome the intense competition for nutrients in the gut between the host and the resident gut microbiome. Enteric pathogens can exploit ethanolamine (EA) in the gut to bypass nutrient competition. However, Klebsiella pneumoniae (K. pneumoniae) is an asymptomatic gut colonizer and, unlike well-studied enteric pathogens, harbors two genetically distinct ethanolamine utilization (eut) loci. Our investigation uncovered unique roles for each eut locus depending on EA utilization as a carbon or nitrogen source. Murine gut colonization studies demonstrated the necessity of both eut loci in the presence of intact gut microbiota for robust GI colonization by K. pneumoniae. Additionally, while some Escherichia coli gut isolates could metabolize EA, other commensals were incapable, suggesting that EA metabolism likely provides K. pneumoniae a selective advantage in gut colonization. Molecular and bioinformatic analyses unveiled the conservation of two eut loci among K. pneumoniae and a subset of the related taxa in the K. pneumoniae species complex, with the NtrC-RpoN regulatory cascade playing a pivotal role in regulation. These findings identify EA metabolism as a critical driver of K. pneumoniae niche establishment in the gut and propose microbial metabolism as a potential therapeutic avenue to combat K. pneumoniae infections.

Impact of Reactive Sulfur Species on Entamoeba histolytica: Modulating Viability, Motility, and Biofilm Degradation Capacity

Reactive sulfur species (RSS) like hydrogen sulfide (H2S) and cysteine persulfide (Cys-SSH) emerged as key signaling molecules with diverse physiological roles in the body, depending on their concentration and the cellular environment. While it is known that H2S and Cys-SSH are produced by both colonocytes and by the gut microbiota through sulfur metabolism, it remains unknown how these RSS affect amebiasis caused by Entamoeba histolytica, a parasitic protozoan that can be present in the human gastrointestinal tract. This study investigates H2S and Cys-SSH's impact on E. histolytica physiology and explores potential therapeutic implications. Exposing trophozoites to the H2S donor, sodium sulfide (Na2S), or to Cys-SSH led to rapid cytotoxicity. A proteomic analysis of Cys-SSH-challenged trophozoites resulted in the identification of >500 S-sulfurated proteins, which are involved in diverse cellular processes. Functional assessments revealed inhibited protein synthesis, altered cytoskeletal dynamics, and reduced motility in trophozoites treated with Cys-SSH. Notably, cysteine proteases (CPs) were significantly inhibited by S-sulfuration, affecting their bacterial biofilm degradation capacity. Immunofluorescence microscopy confirmed alterations in actin dynamics, corroborating the proteomic findings. Thus, our study reveals how RSS perturbs critical cellular functions in E. histolytica, potentially influencing its pathogenicity and interactions within the gut microbiota. Understanding these molecular mechanisms offers novel insights into amebiasis pathogenesis and unveils potential therapeutic avenues targeting RSS-mediated modifications in parasitic infections.

Interplay between autotrophic and heterotrophic prokaryotic metabolism in the bathypelagic realm revealed by metatranscriptomic analyses

BACKGROUND

Heterotrophic microbes inhabiting the dark ocean largely depend on the settling of organic matter from the sunlit ocean. However, this sinking of organic materials is insufficient to cover their demand for energy and alternative sources such as chemoautotrophy have been proposed. Reduced sulfur compounds, such as thiosulfate, are a potential energy source for both auto- and heterotrophic marine prokaryotes.

METHODS

Seawater samples were collected from Labrador Sea Water (LSW, ~ 2000 m depth) in the North Atlantic and incubated in the dark at in situ temperature unamended, amended with 1 µM thiosulfate, or with 1 µM thiosulfate plus 10 µM glucose and 10 µM acetate (thiosulfate plus dissolved organic matter, DOM). Inorganic carbon fixation was measured in the different treatments and samples for metatranscriptomic analyses were collected after 1 h and 72 h of incubation.

RESULTS

Amendment of LSW with thiosulfate and thiosulfate plus DOM enhanced prokaryotic inorganic carbon fixation. The energy generated via chemoautotrophy and heterotrophy in the amended prokaryotic communities was used for the biosynthesis of glycogen and phospholipids as storage molecules. The addition of thiosulfate stimulated unclassified bacteria, sulfur-oxidizing Deltaproteobacteria (SAR324 cluster bacteria), Epsilonproteobacteria (Sulfurimonas sp.), and Gammaproteobacteria (SUP05 cluster bacteria), whereas, the amendment with thiosulfate plus DOM stimulated typically copiotrophic Gammaproteobacteria (closely related to Vibrio sp. and Pseudoalteromonas sp.).

CONCLUSIONS

The gene expression pattern of thiosulfate utilizing microbes specifically of genes involved in energy production via sulfur oxidation and coupled to CO2 fixation pathways coincided with the change in the transcriptional profile of the heterotrophic prokaryotic community (genes involved in promoting energy storage), suggesting a fine-tuned metabolic interplay between chemoautotrophic and heterotrophic microbes in the dark ocean. Video Abstract.

Thiosulfate driving bio-reduction mechanisms of scorodite in groundwater environment

Reductive dissolution of scorodite results in the release and migration of arsenic (As) in groundwater. The purpose of this study was to explore the possible abiotic and biotic reduction of scorodite in groundwater environment and the effect of microbial-mediated sulfur cycling on the bio-reduction of scorodite. Microcosm experiments consisting of scorodite with bacterium Citrobacter sp. JH012-1 or free sulfide were carried out to determine the effects of thiosulfate on the mobilization of As/Fe. The results show arsenic release is positively correlated with iron reduction. The arsenate [As(V)] released can agglomerate with Fe(II) on the surface of scorodite to form crystalline parasymplesite, while no parasymplesite was detected in the abiotic reduction of scorodite by sulfide. The reduction of scorodite and As(V) was affected by thiosulfate. When 0.5 mM thiosulfate was added, the Fe(III) reduction rate increased from 32% to 82%, and the As(V) reduction rate rose from 54% to 64%. When the addition of thiosulfate was increased from 0.5 mM to 2 mM and 5 mM, Fe(III) reduction rate added 4% and 8%, and As(V) reduction rate increased 11% and 16%, respectively. In addition, the presence of thiosulfate promoted the scorodite almost completely converting to parasymplesite. Therefore, the effect of microbial-mediated sulfur cycling should be considered in arsenic migration and reduction from scorodite.

A metabolic puzzle: Consumption of C1 compounds and thiosulfate in Hyphomicrobium denitrificans XT

Many obligately heterotrophic methylotrophs oxidize thiosulfate as an additional electron source during growth on C₁ compounds. Although two different pathways of thiosulfate oxidation are implemented in Hyphomicrobium denitrificans X^T, a pronounced negative effect on growth rate is observed when it is cultured in the simultaneous presence of methanol and thiosulfate. In this model organism, periplasmic thiosulfate dehydrogenase TsdA catalyzes formation of the dead-end product tetrathionate. By reverse genetics we verified the second pathway that also starts in the periplasm where SoxXA catalyzes the oxidative fusion of thiosulfate to SoxYZ, from which sulfate is released by SoxB. Sulfane sulfur is then further oxidized in the cytoplasm by the sulfur-oxidizing heterodisulfide reductase-like system (sHdr) which is produced constitutively in a strain lacking the transcriptional repressor sHdrR. When exposed to thiosulfate, the ΔshdrR strain exhibited a strongly reduced growth rate even without thiosulfate in the pre-cultures. When grown on methanol, cells exhibit significantly increased NAD⁺/NADH ratios in the presence of thiosulfate. In contrast, thiosulfate did not exert any negative effect on growth rate or increase NAD⁺ levels during growth on formate. On both C₁ substrates, excretion of up to 0.5 mM sulfite as an intermediate of thiosulfate (2 mM) oxidation was recorded. Sulfite is known to form adducts with pyrroloquinoline quinone, the cofactor of periplasmic methanol dehydrogenase. We rationalize that this causes specific inhibition of methanol degradation in the presence of thiosulfate while formate metabolism in the cytoplasm remains unaffected.

Generation and Physiology of Hydrogen Sulfide and Reactive Sulfur Species in Bacteria

Sulfur is not only one of the most abundant elements on the Earth, but it is also essential to all living organisms. As life likely began and evolved in a hydrogen sulfide (H₂S)-rich environment, sulfur metabolism represents an early form of energy generation via various reactions in prokaryotes and has driven the sulfur biogeochemical cycle since. It has long been known that H₂S is toxic to cells at high concentrations, but now this gaseous molecule, at the physiological level, is recognized as a signaling molecule and a regulator of critical biological processes. Recently, many metabolites of H₂S, collectively called reactive sulfur species (RSS), have been gradually appreciated as having similar or divergent regulatory roles compared with H₂S in living organisms, especially mammals. In prokaryotes, even in bacteria, investigations into generation and physiology of RSS remain preliminary and an understanding of the relevant biological processes is still in its infancy. Despite this, recent and exciting advances in the fields are many. Here, we discuss abiotic and biotic generation of H₂S/RSS, sulfur-transforming enzymes and their functioning mechanisms, and their physiological roles as well as the sensing and regulation of H₂S/RSS.

The emergence of the tetrathionate reductase operon in the Escherichia coli/Shigella pan-genome

Escherichia coli pathogenic variants (pathovars) are generally characterized by defined virulence traits and are susceptible to the evolution of hybridized identities due to the considerable plasticity of the E. coli genome. We have isolated a strain from a purified diet intended for research animals that further demonstrates the ability of E. coli to acquire novel genetic elements leading potentially to emergent new pathovars. Utilizing next generation sequencing to obtain a whole genome profile, we report an atypical strain of E. coli, EcoFA807-17, possessing a tetrathionate reductase (ttr) operon, which enables the utilization of tetrathionate as an electron acceptor, thus facilitating respiration in anaerobic environments such as the mammalian gut. The ttr operon is a potent virulence factor for several enteric pathogens, most prominently Salmonella enterica. However, the presence of chromosomally integrated tetrathionate reductase genes does not appear to have been previously reported in wild-type E. coli or Shigella. Accordingly, it is possible that the appearance of this virulence factor may signal the evolution of new mechanisms of pathogenicity in E. coli and Shigella and may potentially alter the effectiveness of existing assays using tetrathionate reductase as a unique marker for the detection of Salmonella enterica.

Role of Micronutrients and Gut Microbiota-Derived Metabolites in COVID-19 Recovery

A balanced and varied diet provides diverse beneficial effects on health, such as adequate micronutrient availability and a gut microbiome in homeostasis. Besides their participation in biochemical processes as cofactors and coenzymes, vitamins and minerals have an immunoregulatory function; meanwhile, gut microbiota and its metabolites coordinate directly and indirectly the cell response through the interaction with the host receptors. Malnourishment is a crucial risk factor for several pathologies, and its involvement during the Coronavirus Disease 2019 pandemic has been reported. This pandemic has caused a significant decline in the worldwide population, especially those with chronic diseases, reduced physical activity, and elder age. Diet and gut microbiota composition are probable causes for this susceptibility, and its supplementation can play a role in reestablishing microbial homeostasis and improving immunity response against Coronavirus Disease 2019 infection and recovery. This study reviews the role of micronutrients and microbiomes in the risk of infection, the severity of disease, and the Coronavirus Disease 2019 sequelae.

How microbiological tests reflect bacterial pathogenesis and host adaptation

Historically, clinical microbiological laboratories have often relied on isolation of pure cultures and phenotypic testing to identify microorganisms. These clinical tests are often based on specific biochemical reactions, growth characteristics, colony morphology, and other physiological aspects. The features used for identification in clinical laboratories are highly conserved and specific for a given group of microbes. We speculate that these features might be the result of evolutionary selection and thus may reflect aspects of the life cycle of the organism and pathogenesis. Indeed, several of the metabolic pathways targeted by diagnostic tests in some cases may represent mechanisms for host colonization or pathogenesis. Examples include, but are not restricted to, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Salmonella enterica, Shigella spp., and enteroinvasive Escherichia coli (EIEC). Here, we provide an overview of how some common tests reflect molecular mechanisms of bacterial pathogenesis.

Mycobacterium tuberculosis H2S Functions as a Sink to Modulate Central Metabolism, Bioenergetics, and Drug Susceptibility

H2S is a potent gasotransmitter in eukaryotes and bacteria. Host-derived H2S has been shown to profoundly alter M. tuberculosis (Mtb) energy metabolism and growth. However, compelling evidence for endogenous production of H2S and its role in Mtb physiology is lacking. We show that multidrug-resistant and drug-susceptible clinical Mtb strains produce H2S, whereas H2S production in non-pathogenic M. smegmatis is barely detectable. We identified Rv3684 (Cds1) as an H2S-producing enzyme in Mtb and show that cds1 disruption reduces, but does not eliminate, H2S production, suggesting the involvement of multiple genes in H2S production. We identified endogenous H2S to be an effector molecule that maintains bioenergetic homeostasis by stimulating respiration primarily via cytochrome bd. Importantly, H2S plays a key role in central metabolism by modulating the balance between oxidative phosphorylation and glycolysis, and it functions as a sink to recycle sulfur atoms back to cysteine to maintain sulfur homeostasis. Lastly, Mtb-generated H2S regulates redox homeostasis and susceptibility to anti-TB drugs clofazimine and rifampicin. These findings reveal previously unknown facets of Mtb physiology and have implications for routine laboratory culturing, understanding drug susceptibility, and improved diagnostics.

The impact of gut microbiota metabolites on cellular bioenergetics and cardiometabolic health

Recent research demonstrates a reciprocal relationship between gut microbiota-derived metabolites and the host in controlling the energy homeostasis in mammals. On the one hand, to thrive, gut bacteria exploit nutrients digested by the host. On the other hand, the host utilizes numerous products of gut bacteria metabolism as a substrate for ATP production in the colon. Finally, bacterial metabolites seep from the gut into the bloodstream and interfere with the host’s cellular bioenergetics machinery. Notably, there is an association between alterations in microbiota composition and the development of metabolic diseases and their cardiovascular complications. Some metabolites, like short-chain fatty acids and trimethylamine, are considered markers of cardiometabolic health. Others, like hydrogen sulfide and nitrite, demonstrate antihypertensive properties. Scientific databases were searched for pre-clinical and clinical studies to summarize current knowledge on the role of gut microbiota metabolites in the regulation of mammalian bioenergetics and discuss their potential involvement in the development of cardiometabolic disorders. Overall, the available data demonstrates that gut bacteria products affect physiological and pathological processes controlling energy and vascular homeostasis. Thus, the modulation of microbiota-derived metabolites may represent a new approach for treating obesity, hypertension and type 2 diabetes.

Interrogation of the perturbed gut microbiota in gouty arthritis patients through in silico metabolic modeling

Recent studies have shown perturbed gut microbiota associated with gouty arthritis, a metabolic disease characterized by an imbalance between uric acid production and excretion. To mechanistically investigate altered microbiota metabolism associated with gout disease, 16S rRNA gene amplicon sequence data from stool samples of gout patients and healthy controls were computationally analyzed through bacterial community metabolic models. Patient-specific community models constructed with the metagenomics modeling pipeline, mgPipe, were used to perform k-means clustering of samples according to their metabolic capabilities. The clustering analysis generated statistically significant partitioning of samples into a Bacteroides-dominated, high gout cluster and a Faecalibacterium-elevated, low gout cluster. The high gout cluster was predicted to allow elevated synthesis of the amino acids D-alanine and L-alanine and byproducts of branched-chain amino acid catabolism, while the low gout cluster allowed higher production of butyrate, the sulfur-containing amino acids L-cysteine and L-methionine, and the L-cysteine catabolic product H2S. By expanding the capabilities of mgPipe to provide taxa-level resolution of metabolite exchange rates, acetate, D-lactate and succinate exchanged from Bacteroides to Faecalibacterium were predicted to enhance butyrate production in the low gout cluster. Model predictions suggested that sulfur-containing amino acid metabolism generally and H2S more specifically could be novel gout disease markers.

Cryptic Sulfur Incorporation in Thioangucycline Biosynthesis

Sulfur incorporation into natural products is a critical area of biosynthetic studies. Recently, a subset of sulfur-containing angucyclines has been discovered, and yet, the sulfur incorporation step is poorly understood. In this work, a series of thioether-bridged angucyclines were discovered, and a cryptic epoxide Michael acceptor intermediate was revealed en route to thioangucyclines (TACs) A and B. However, systematic gene deletion of the biosynthetic gene cluster (BGC) by CRISPR/Cas9 could not identify any gene responsible for the conversion of the epoxide intermediate to TACs. Instead, a series of in vitro and in vivo experiments conclusively showed that the conversion is the result of two non-enzymatic steps, possibly mediated by endogenous hydrogen sulfide. Therefore, the TACs are proposed to derive from a detoxification process. These results are expected to contribute to the study of both angucyclines and the utilization of inorganic sulfur in natural product biosynthesis.

Redox loops in anaerobic respiration - The role of the widespread NrfD protein family and associated dimeric redox module

In prokaryotes, the proton or sodium motive force required for ATP synthesis is produced by respiratory complexes that present an ion-pumping mechanism or are involved in redox loops performed by membrane proteins that usually have substrate and quinone-binding sites on opposite sides of the membrane. Some respiratory complexes include a dimeric redox module composed of a quinone-interacting membrane protein of the NrfD family and an iron‑sulfur protein of the NrfC family. The QrcABCD complex of sulfate reducers, which includes the QrcCD module homologous to NrfCD, was recently shown to perform electrogenic quinone reduction providing the first conclusive evidence for energy conservation among this family. Similar redox modules are present in multiple respiratory complexes, which can be associated with electroneutral, energy-driven or electrogenic reactions. This work discusses the presence of the NrfCD/PsrBC dimeric redox module in different bioenergetics contexts and its role in prokaryotic energy conservation mechanisms.

Molybdenum Enzymes and How They Support Virulence in Pathogenic Bacteria

Mononuclear molybdoenzymes are highly versatile catalysts that occur in organisms in all domains of life, where they mediate essential cellular functions such as energy generation and detoxification reactions. Molybdoenzymes are particularly abundant in bacteria, where over 50 distinct types of enzymes have been identified to date. In bacterial pathogens, all aspects of molybdoenzyme biology such as molybdate uptake, cofactor biosynthesis, and function of the enzymes themselves, have been shown to affect fitness in the host as well as virulence. Although current studies are mostly focused on a few key pathogens such as Escherichia coli, Salmonella enterica, Campylobacter jejuni, and Mycobacterium tuberculosis, some common themes for the function and adaptation of the molybdoenzymes to pathogen environmental niches are emerging. Firstly, for many of these enzymes, their role is in supporting bacterial energy generation; and the corresponding pathogen fitness and virulence defects appear to arise from a suboptimally poised metabolic network. Secondly, all substrates converted by virulence-relevant bacterial Mo enzymes belong to classes known to be generated in the host either during inflammation or as part of the host signaling network, with some enzyme groups showing adaptation to the increased conversion of such substrates. Lastly, a specific adaptation to bacterial in-host survival is an emerging link between the regulation of molybdoenzyme expression in bacterial pathogens and the presence of immune system-generated reactive oxygen species. The prevalence of molybdoenzymes in key bacterial pathogens including ESKAPE pathogens, paired with the mounting evidence of their central roles in bacterial fitness during infection, suggest that they could be important future drug targets.

H2S and reactive sulfur signaling at the host-bacterial pathogen interface

Bacterial pathogens that cause invasive disease in the vertebrate host must adapt to host efforts to cripple their viability. Major host insults are reactive oxygen and reactive nitrogen species as well as cellular stress induced by antibiotics. Hydrogen sulfide (H2S) is emerging as an important player in cytoprotection against these stressors, which may well be attributed to downstream more oxidized sulfur species termed reactive sulfur species (RSS). In this review, we summarize recent work that suggests that H2S/RSS impacts bacterial survival in infected cells and animals. We discuss the mechanisms of biogenesis and clearance of RSS in the context of a bacterial H2S/RSS homeostasis model and the bacterial transcriptional regulatory proteins that act as "sensors" of cellular RSS that maintain H2S/RSS homeostasis. In addition, we cover fluorescence imaging- and MS-based approaches used to detect and quantify RSS in bacterial cells. Last, we discuss proteome persulfidation (S-sulfuration) as a potential mediator of H2S/RSS signaling in bacteria in the context of the writer-reader-eraser paradigm, and progress toward ascribing regulatory significance to this widespread post-translational modification.

Malate-Dependent Carbon Utilization Enhances Central Metabolism and Contributes to Biological Fitness of Laribacter hongkongensis via CRP Regulation

Metabolic adaptation in various environmental niches is crucial for bacterial extracellular survival and intracellular replication during infection. However, the metabolism of carbon/nitrogen sources and related regulatory mechanisms in Laribacter hongkongensis, an asaccharolytic bacterium associated with invasive infections and gastroenteritis, are still unknown. In the present study, we demonstrated that malate can be exploited as a preferred carbon source of L. hongkongensis. Using RNA-sequencing, we compared the transcription profiles of L. hongkongensis cultivated with or without malate supplementation, and observed that malate utilization significantly inhibits the use of alternative carbon sources while enhancing respiratory chain as well as central carbon, sulfur, and urease-mediated nitrogen metabolisms. The tight connection among these important metabolic pathways indicates that L. hongkongensis is capable of integrating information from different metabolism branches to coordinate the expression of metabolic genes and thereby adapt to environmental changing. Furthermore, we identified that a transcription factor, CRP, is repressed by malate-mediated metabolism while negatively regulating the effect of malate on these central metabolic pathways. Remarkably, CRP also responds to various environmental stresses, influences the expression of other transcription factors, and contributes to the biological fitness of L. hongkongensis. The regulatory network and cross-regulation enables the bacteria to make the appropriate metabolic responses and environmental adaptation.

The Biogeochemical Sulfur Cycle of Marine Sediments

Microbial dissimilatory sulfate reduction to sulfide is a predominant terminal pathway of organic matter mineralization in the anoxic seabed. Chemical or microbial oxidation of the produced sulfide establishes a complex network of pathways in the sulfur cycle, leading to intermediate sulfur species and partly back to sulfate. The intermediates include elemental sulfur, polysulfides, thiosulfate, and sulfite, which are all substrates for further microbial oxidation, reduction or disproportionation. New microbiological discoveries, such as long-distance electron transfer through sulfide oxidizing cable bacteria, add to the complexity. Isotope exchange reactions play an important role for the stable isotope geochemistry and for the experimental study of sulfur transformations using radiotracers. Microbially catalyzed processes are partly reversible whereby the back-reaction affects our interpretation of radiotracer experiments and provides a mechanism for isotope fractionation. We here review the progress and current status in our understanding of the sulfur cycle in the seabed with respect to its microbial ecology, biogeochemistry, and isotope geochemistry.

Identification, Expression and Activity of Candidate Nitrite Reductases From Orange Beggiatoaceae, Guaymas Basin

Orange filamentous Beggiatoaceae form massive microbial mats on hydrothermal sediments in Guaymas Basin; these bacteria are considered to oxidize sulfide with nitrate and nitrite as electron acceptors. From a previously analyzed genome of an orange Beggiatoaceae filament, three candidate genes for enzymes with nitrite-reducing function - an orange octaheme cytochrome, a nirS nitrite reductase, and a nitrite/tetrathionate-reducing octaheme cytochrome - were cloned and expressed in Escherichia coli. The expressed and purified orange cytochrome showed reduced nitrite-reducing activity compared to the multifunctional native protein obtained from microbial mats. The nirS gene product showed in vitro but no in-gel nitrite-reducing activity; and the nitrite/tetrathionate-reducing octaheme cytochrome was capable of reducing both nitrite and tetrathionate in vitro. Phylogenetic analysis shows that the orange Beggiatoaceae nirS, in contrast to the other candidate nitrite reductases, does not form monophyletic lineages with its counterparts in other large sulfur-oxidizing bacteria, and most likely represents a recent acquisition by lateral gene transfer. The nitrite/tetrathionate-reducing enzyme of the orange Beggiatoaceae is related to nitrite- and tetrathionate reductases harbored predominantly by Gammaproteobacteria, including obligate endosymbionts of hydrothermal vent tubeworms. Thus, the orange Guaymas Basin Beggiatoaceae have a repertoire of at least three different functional enzymes for nitrite reduction. By demonstrating the unusual diversity of enzymes with a potential role in nitrite reduction, we show that bacteria in highly dynamic, sulfide-rich hydrothermal vent habitats adapt to these conditions that usually prohibit nitrate and nitrite reduction. In the case of the orange Guaymas Beggiatoaceae, classical denitrification appears to be replaced by different multifunctional enzymes for nitrite and tetrathionate reduction; the resulting ecophysiological flexibility provides a new key to the dominance of these Beggiatoaceae in hydrothermal hot spots.

Unprecedented Microbial Conversion of Biliverdin into Bilirubin-10-sulfonate

Biliverdin (BV) possesses antioxidant and anti-inflammatory properties, with previous reports identifying protection against oxidant and inflammatory injury in animal models. Recent reports indicate that intra-duodenal administration of BV results in the formation of an uncharacterised metabolite, which is potently absorbed into the blood and excreted into the bile. This compound may be responsible for protection against inflammatory responses. This study aimed to identify novel, enterally-derived BV metabolites and determine the source of their metabolic transformation. Rat duodena and bacterial cultures of Citrobacter youngae were treated with BV and subsequently analysed via high performance liquid chromatography/high resolution tandem mass spectrometry to identify and characterise metabolites of BV. A highly abundant metabolite was detected in duodenal wash and bacterial culture supernatants with a 663.215 m/z (3 ppm mass accuracy) and a composition of C₃₃N₄O₉H₃₆S, which conformed to the predicted structure of bilirubin-10-sulfonate (BRS) and possessed a λ_max of 440 nm. Bilirubin-10-sulfonate was then synthesized for comparative LCMS/MS analysis and matched with that of the biologically formed BV metabolite. This report confirms the formation of a previously undocumented metabolite of BV in mammals, indicating that a new metabolic pathway likely exists for BV metabolism requiring enteric bacteria, Citrobacter youngae. These data may have important implications with regard to understanding and harnessing the therapeutic efficacy of oral BV administration.

Contribution of the Cpx envelope stress system to metabolism and virulence regulation in Salmonella enterica serovar Typhimurium

The Cpx-envelope stress system regulates the expression of virulence factors in many Gram-negative pathogens. In Salmonella enterica serovar Typhimurium deletion of the sensor kinase CpxA but not of the response regulator CpxR results in the down regulation of the key regulator for invasion, HilA encoded by the Salmonella pathogenicity island 1 (SPI-1). Here, we provide evidence that cpxA deletion interferes with dephosphorylation of CpxR resulting in increased levels of active CpxR and consequently in misregulation of target genes. 14 potential operons were identified to be under direct control of CpxR. These include the virulence determinants ecotin, the omptin PgtE, and the SPI-2 regulator SsrB. The Tat-system and the PocR regulator that together promote anaerobic respiration of tetrathionate on 1,2-propanediol are also under direct CpxR control. Notably, 1,2-propanediol represses hilA expression. Thus, our work demonstrates for the first time the involvement of the Cpx system in a complex network mediating metabolism and virulence function.

Distribution of virulence factors, determinants of antibiotic resistance and molecular fingerprinting of Salmonella species isolated from cattle and beef samples: suggestive evidence of animal-to-meat contamination

In this study, three hundred presumptive Salmonella strains isolated from cattle faeces and raw beef samples were subjected to both preliminary and confirmatory tests specific for Salmonella. PCR assays revealed that 100%, 20% and 26.7% of the isolates were positive for 16S rRNA, fliC and fljB gene fragments, respectively. Large proportions (62.4 to 94.3%) of these isolates were multiple antibiotic resistant (MAR) strains that were resistant to three or more antibiotics belonging to different classes. MAR phenotypes Ab1, Ab2, Ab3, Ab7, Ab8, Ab9, Ab26 and Ab27 were dominant among the isolates. Cluster analysis of antibiotic inhibition zone diameter data revealed two major clusters (clusters 1 and 2), and each cluster contained two sub-clusters (1A, 1B, 2A and 2B). PCR data revealed that 27.1% and 30.7% of the isolates possessed the spvC and invA virulent genes, respectively. There was a significant correlation between the possession of MAR phenotypes and virulent gene determinants. Analysis of restriction fragment length polymorphism (RFLP) of 16S rRNA gene fragments using EcoRI and HaeIII showed that large proportions of isolates from beef and cattle faeces produced similar genetic fingerprints. From these results, it is suggested that Salmonella species in cattle are transmitted to beef and, therefore, the consumption of undercooked beef could pose severe health complications on consumers. These findings provide baseline data that could be of great epidemiological importance and, thus, the need to utilise more sensitive typing tools in determining the genetic relatedness of isolates from different sources.

BigR is a sulfide sensor that regulates a sulfur transferase/dioxygenase required for aerobic respiration of plant bacteria under sulfide stress

To cope with toxic levels of H2S, the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens employ the bigR operon to oxidize H2S into sulfite. The bigR operon is regulated by the transcriptional repressor BigR and it encodes a bifunctional sulfur transferase (ST) and sulfur dioxygenase (SDO) enzyme, Blh, required for H2S oxidation and bacterial growth under hypoxia. However, how Blh operates to enhance bacterial survival under hypoxia and how BigR is deactivated to derepress operon transcription is unknown. Here, we show that the ST and SDO activities of Blh are in vitro coupled and necessary to oxidize sulfide into sulfite, and that Blh is critical to maintain the oxygen flux during A. tumefaciens respiration when oxygen becomes limited to cells. We also show that H2S and polysulfides inactivate BigR leading to operon transcription. Moreover, we show that sulfite, which is produced by Blh in the ST and SDO reactions, is toxic to Citrus sinensis and that X. fastidiosa-infected plants accumulate sulfite and higher transcript levels of sulfite detoxification enzymes, suggesting that they are under sulfite stress. These results indicate that BigR acts as a sulfide sensor in the H2S oxidation mechanism that allows pathogens to colonize plant tissues where oxygen is a limiting factor.

Turnover Rates of Intermediate Sulfur Species (Sx2-, S0, S2O32-, S4O62-, SO32-) in Anoxic Freshwater and Sediments

The microbial reduction of sulfate to sulfide coupled to organic matter oxidation followed by the transformation of sulfide back to sulfate drives a dynamic sulfur cycle in a variety of environments. The oxidative part of the sulfur cycle in particular is difficult to constrain because the eight electron oxidation of sulfide to sulfate occurs stepwise via a suite of biological and chemical pathways and produces a wide variety of intermediates (S x 2 - , S0, S2O 3 2 - , S4O 6 2 - , and SO 3 2 - ), which may in turn be oxidized, reduced or disproportionated. Although the potential processes affecting these intermediates are well-known from microbial culture and geochemical studies, their significance and rates in the environment are not well constrained. In the study presented here, time-course concentration measurements of intermediate sulfur species were made in amended freshwater water column and sediment incubation experiments in order to constrain consumption rates and processes. In sediment incubations, consumption rates were S colloidal 0 > S x 2 - > SO 3 2 - ≈ S4O 6 2 - > S2O 3 2 - , which is consistent with previous measurements of SO 3 2 - , S4O 6 2 - , and S2O 3 2 - consumption rates in marine sediments. In water column incubations, however, the relative reactivity was S colloidal 0 > SO 3 2 - > S x 2 - > S2O 3 2 - > S4O 6 2 - . Consumption of thiosulfate, tetrathionate and sulfite was primarily biological, whereas it was not possible to distinguish between abiotic and biological polysulfide consumption in either aqueous or sediment incubations. S colloidal 0 consumption in water column experiments was biologically mediated, however, rapid sedimentary consumption was likely due to reactions with iron minerals. These experiments provide important constraints on the biogeochemical reactivity of intermediate sulfur species and give further insight into the diversity of biological and geochemical processes that comprise (cryptic) environmental sulfur cycling.

Sulfur Cycling and the Intestinal Microbiome

In this review, we focus on the activities transpiring in the anaerobic segment of the sulfur cycle occurring in the gut environment where hydrogen sulfide is produced. While sulfate-reducing bacteria are considered as the principal agents for hydrogen sulfide production, the enzymatic desulfhydration of cysteine by heterotrophic bacteria also contributes to production of hydrogen sulfide. For sulfate-reducing bacteria respiration, molecular hydrogen and lactate are suitable as electron donors while sulfate functions as the terminal electron acceptor. Dietary components provide fiber and macromolecules that are degraded by bacterial enzymes to monomers, and these are fermented by intestinal bacteria with the production to molecular hydrogen which promotes the metabolic dominance by sulfate-reducing bacteria. Sulfate is also required by the sulfate-reducing bacteria, and this can be supplied by sulfate- and sulfonate-containing compounds that are hydrolyzed by intestinal bacterial with the release of sulfate. While hydrogen sulfide in the intestinal biosystem may be beneficial to bacteria by increasing resistance to antibiotics, and protecting them from reactive oxygen species, hydrogen sulfide at elevated concentrations may become toxic to the host.

Sulfide Generation by Dominant Halanaerobium Microorganisms in Hydraulically Fractured Shales

Hydraulic fracturing of black shale formations has greatly increased United States oil and natural gas recovery. However, the accumulation of biomass in subsurface reservoirs and pipelines is detrimental because of possible well souring, microbially induced corrosion, and pore clogging. Temporal sampling of produced fluids from a well in the Utica Shale revealed the dominance of Halanaerobium strains within the in situ microbial community and the potential for these microorganisms to catalyze thiosulfate-dependent sulfidogenesis. From these field data, we investigated biogenic sulfide production catalyzed by a Halanaerobium strain isolated from the produced fluids using proteogenomics and laboratory growth experiments. Analysis of Halanaerobium isolate genomes and reconstructed genomes from metagenomic data sets revealed the conserved presence of rhodanese-like proteins and anaerobic sulfite reductase complexes capable of converting thiosulfate to sulfide. Shotgun proteomics measurements using a Halanaerobium isolate verified that these proteins were more abundant when thiosulfate was present in the growth medium, and culture-based assays identified thiosulfate-dependent sulfide production by the same isolate. Increased production of sulfide and organic acids during the stationary growth phase suggests that fermentative Halanaerobium uses thiosulfate to remove excess reductant. These findings emphasize the potential detrimental effects that could arise from thiosulfate-reducing microorganisms in hydraulically fractured shales, which are undetected by current industry-wide corrosion diagnostics. IMPORTANCE Although thousands of wells in deep shale formations across the United States have been hydraulically fractured for oil and gas recovery, the impact of microbial metabolism within these environments is poorly understood. Our research demonstrates that dominant microbial populations in these subsurface ecosystems contain the conserved capacity for the reduction of thiosulfate to sulfide and that this process is likely occurring in the environment. Sulfide generation (also known as "souring") is considered deleterious in the oil and gas industry because of both toxicity issues and impacts on corrosion of the subsurface infrastructure. Critically, the capacity for sulfide generation via reduction of sulfate was not detected in our data sets. Given that current industry wellhead tests for sulfidogenesis target canonical sulfate-reducing microorganisms, these data suggest that new approaches to the detection of sulfide-producing microorganisms may be necessary.

Reprint of: Contribution of enrichments and resampling for sulfate reducing bacteria diversity assessment by high-throughput cultivation

The development of new high-throughput cultivation methods aims to increase the isolation efficiency as compared to standard techniques that often require enrichment procedures to compensate the low microbial recovery. In the current study, estuarine sulfate-reducing bacteria were isolated using an anaerobic isolation procedure in 384-well microplates. Ninety-nine strains were recovered from initial sediments. Isolates were identified according to their partial 16S rRNA sequences and clustered into 13 phylotypes. Besides, the increase in species richness obtained through enrichments or resampling was investigated. Forty-four enrichment procedures were conducted and shifts in sulfate-reducing bacterial communities were investigated through dsrAB gene fingerprinting. Despite efforts in conducting numerous enrichment conditions only few of them were statistically different from initial sample. The cultural diversity obtained from 3 of the most divergent enrichments, as well as from resampled sediments equally contributed to raise the sulfate-reducing diversity up to 22 phylotypes. Enrichments (selection of metabolism) or resampling (transient populations and micro-heterogeneity) may still be helpful to assess new microbial phylotypes. Nevertheless, all the newly cultivated strains were all representatives of minor Operational Taxonomic Units and could eventually be recovered by maintaining high-throughput isolation effort from the initial sediments.

Engineering bacterial thiosulfate and tetrathionate sensors for detecting gut inflammation

There is a groundswell of interest in using genetically engineered sensor bacteria to study gut microbiota pathways, and diagnose or treat associated diseases. Here, we computationally identify the first biological thiosulfate sensor and an improved tetrathionate sensor, both two‐component systems from marine Shewanella species, and validate them in laboratory Escherichia coli. Then, we port these sensors into a gut‐adapted probiotic E. coli strain, and develop a method based upon oral gavage and flow cytometry of colon and fecal samples to demonstrate that colon inflammation (colitis) activates the thiosulfate sensor in mice harboring native gut microbiota. Our thiosulfate sensor may have applications in bacterial diagnostics or therapeutics. Finally, our approach can be replicated for a wide range of bacterial sensors and should thus enable a new class of minimally invasive studies of gut microbiota pathways.

What's in a Name? Species-Wide Whole-Genome Sequencing Resolves Invasive and Noninvasive Lineages of Salmonella enterica Serotype Paratyphi B

For 100 years, it has been obvious that Salmonella enterica strains sharing the serotype with the formula 1,4,[5],12:b:1,2—now known as Paratyphi B—can cause diseases ranging from serious systemic infections to self-limiting gastroenteritis. Despite considerable predicted diversity between strains carrying the common Paratyphi B serotype, there remain few methods that subdivide the group into groups that are congruent with their disease phenotypes. Paratyphi B therefore represents one of the canonical examples in Salmonella where serotyping combined with classical microbiological tests fails to provide clinically informative information. Here, we use genomics to provide the first high-resolution view of this serotype, placing it into a wider genomic context of the Salmonella enterica species. These analyses reveal why it has been impossible to subdivide this serotype based upon phenotypic and limited molecular approaches. By examining the genomic data in detail, we are able to identify common features that correlate with strains of clinical importance. The results presented here provide new diagnostic targets, as well as posing important new questions about the basis for the invasive disease phenotype observed in a subset of strains.

Salmonella enterica strains carrying the serotype Paratyphi B have long been known to possess Jekyll and Hyde characteristics; some cause gastroenteritis, while others cause serious invasive disease. Understanding what makes up the population of strains carrying this serotype, as well as the source of their invasive disease, is a 100-year-old puzzle that we address here using genomics. Our analysis provides the first high-resolution view of this serotype, placing strains carrying serotype Paratyphi B into the wider genomic context of the Salmonella enterica species. This work reveals a history of disease dating back to the middle ages, caused by a group of distinct lineages with various abilities to cause invasive disease. By quantifying the key genomic differences between the invasive and noninvasive populations, we are able to identify key virulence-related targets that can form the basis of simple, rapid, point-of-care tests.

Enhanced Biocide Mitigation of Field Biofilm Consortia by a Mixture of D-Amino Acids

Microbiologically influenced corrosion (MIC) is a major problem in the oil and gas industry as well as in many other industries. Current treatment methods rely mostly on pigging and biocide dosing. Biocide resistance is a growing concern. Thus, it is desirable to use biocide enhancers to improve the efficacy of existing biocides. D-Amino acids are naturally occurring. Our previous work demonstrated that some D-amino acids are biocide enhancers. Under a biocide stress of 50 ppm (w/w) hydroxymethyl phosphonium sulfate (THPS) biocide, 1 ppm D-tyrosine and 100 ppm D-methionine used separately successfully mitigated the Desulfovibrio vulgaris biofilm on carbon steel coupons. The data reported in this work revealed that 50 ppm of an equimolar mixture of D-methionine, D-tyrosine, D-leucine, and D-tryptophan greatly enhanced 50 ppm THPS biocide treatment of two recalcitrant biofilm consortia containing sulfate reducing bacteria (SRB), nitrate reducing bacteria (NRB), and fermentative bacteria, etc., from oil-field operations. The data also indicated that individual D-amino acids were inadequate for the biofilm consortia.

Visual detection technique for efficient screening and isolation of Salmonella based on a novel enrichment assay using chromatography membrane

To detect Salmonella more efficiently and isolate strains more easily, a novel and simple detection method that uses an enrichment assay and two chromogenic reactions on a chromatography membrane was developed. Grade 3 chromatography paper is used as functionalized solid phase support (SPS), which contains specially optimized medium. One reaction for screening is based on the sulfate-reducing capacity of Salmonella. Hydrogen sulfide (H2S) generated by Salmonella reacts with ammonium ferric citrate to produce black colored ferrous sulfide. Another reaction is based on Salmonella C8 esterase that is unique for Enterobacteriaceae except Serratia and interacts with 4-methylumbelliferyl caprylate (MUCAP) to produce fluorescent umbelliferone, which is visible under ultraviolet light. A very low detection limit (10(1) CFU ml(-1)) for Salmonella was achieved on the background of 10(5) CFU ml(-1) Escherichia coli. More importantly, testing with more than 1,000 anal samples indicated that our method has a high positive detection rate and is relatively low cost, compared with the traditional culture-based method. It took only 1 day for the preliminary screening and 2 days to efficiently isolate the Salmonella cells, indicating that the new assay is specific, rapid, and simple for Salmonella detection. In contrast to the traditional culture-based method, this method can be easily used to screen and isolate targeted strains with the naked eye. The results of quantitative and comparative experiments showed that the visual detection technique is an efficient alternative method for the screening of Salmonella spp. in many applications of large-sized samples related to public health surveillance.

Hydrogen Sulfide in Physiology and Diseases of the Digestive Tract

Hydrogen sulfide (H₂S) is a Janus-faced molecule. On one hand, several toxic functions have been attributed to H₂S and exposure to high levels of this gas is extremely hazardous to health. On the other hand, H₂S delivery based clinical therapies are being developed to combat inflammation, visceral pain, oxidative stress related tissue injury, thrombosis and cancer. Since its discovery, H₂S has been found to have pleiotropic effects on physiology and health. H₂S is a gasotransmitter that exerts its effect on different systems, such as gastrointestinal, neuronal, cardiovascular, respiratory, renal, and hepatic systems. In the gastrointestinal tract, in addition to H₂S production by mammalian cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE), H₂S is also generated by the metabolic activity of resident gut microbes, mainly by colonic Sulfate-Reducing Bacteria (SRB) via a dissimilatory sulfate reduction (DSR) pathway. In the gut, H₂S regulates functions such as inflammation, ischemia/ reperfusion injury and motility. H₂S derived from gut microbes has been found to be associated with gastrointestinal disorders such as ulcerative colitis, Crohn’s disease and irritable bowel syndrome. This underscores the importance of gut microbes and their production of H₂S on host physiology and pathophysiology.

The EutQ and EutP proteins are novel acetate kinases involved in ethanolamine catabolism: physiological implications for the function of the ethanolamine metabolosome in Salmonella enterica

Salmonella enterica catabolizes ethanolamine inside a compartment known as the metabolosome. The ethanolamine utilization (eut) operon of this bacterium encodes all functions needed for the assembly and function of this structure. To date, the roles of EutQ and EutP were not known. Herein we show that both proteins have acetate kinase activity and that EutQ is required during anoxic growth of S. enterica on ethanolamine and tetrathionate. EutP and EutQ-dependent ATP synthesis occurred when enzymes were incubated with ADP, Mg(II) ions and acetyl-phosphate. EutQ and EutP also synthesized acetyl-phosphate from ATP and acetate. Although EutP had acetate kinase activity, ΔeutP strains lacked discernible phenotypes under the conditions where ΔeutQ strains displayed clear phenotypes. The kinetic parameters indicate that EutP is a faster enzyme than EutQ. Our evidence supports the conclusion that EutQ and EutP represent novel classes of acetate kinases. We propose that EutQ is necessary to drive flux through the pathway under physiological conditions, preventing a buildup of acetaldehyde. We also suggest that ATP generated by these enzymes may be used as a substrate for EutT, the ATP-dependent corrinoid adenosyltransferase and for the EutA ethanolamine ammonia-lyase reactivase.

An allosteric model for control of pore opening by substrate binding in the EutL microcompartment shell protein

The ethanolamine utilization (Eut) microcompartment is a protein-based metabolic organelle that is strongly associated with pathogenesis in bacteria that inhabit the human gut. The exterior shell of this elaborate protein complex is composed from a few thousand copies of BMC-domain shell proteins, which form a semi-permeable diffusion barrier that provides the interior enzymes with substrates and cofactors while simultaneously retaining metabolic intermediates. The ability of this protein shell to regulate passage of substrate and cofactor molecules is critical for microcompartment function, but the details of how this diffusion barrier can allow the passage of large cofactors while still retaining small intermediates remain unclear. Previous work has revealed two conformations of the EutL shell protein, providing substantial evidence for a gated pore that might allow the passage of large cofactors. Here we report structural and biophysical evidence to show that ethanolamine, the substrate of the Eut microcompartment, acts as a negative allosteric regulator of EutL pore opening. Specifically, a series of X-ray crystal structures of EutL from Clostridium perfringens, along with equilibrium binding studies, reveal that ethanolamine binds to EutL at a site that exists in the closed-pore conformation and which is incompatible with opening of the large pore for cofactor transport. The allosteric mechanism we propose is consistent with the cofactor requirements of the Eut microcompartment, leading to a new model for EutL function. Furthermore, our results suggest the possibility of redox modulation of the allosteric mechanism, opening potentially new lines of investigation.

Microalgal Metabolic Network Model Refinement through High-Throughput Functional Metabolic Profiling

Metabolic modeling provides the means to define metabolic processes at a systems level; however, genome-scale metabolic models often remain incomplete in their description of metabolic networks and may include reactions that are experimentally unverified. This shortcoming is exacerbated in reconstructed models of newly isolated algal species, as there may be little to no biochemical evidence available for the metabolism of such isolates. The phenotype microarray (PM) technology (Biolog, Hayward, CA, USA) provides an efficient, high-throughput method to functionally define cellular metabolic activities in response to a large array of entry metabolites. The platform can experimentally verify many of the unverified reactions in a network model as well as identify missing or new reactions in the reconstructed metabolic model. The PM technology has been used for metabolic phenotyping of non-photosynthetic bacteria and fungi, but it has not been reported for the phenotyping of microalgae. Here, we introduce the use of PM assays in a systematic way to the study of microalgae, applying it specifically to the green microalgal model species Chlamydomonas reinhardtii. The results obtained in this study validate a number of existing annotated metabolic reactions and identify a number of novel and unexpected metabolites. The obtained information was used to expand and refine the existing COBRA-based C. reinhardtii metabolic network model iRC1080. Over 254 reactions were added to the network, and the effects of these additions on flux distribution within the network are described. The novel reactions include the support of metabolism by a number of d-amino acids, l-dipeptides, and l-tripeptides as nitrogen sources, as well as support of cellular respiration by cysteamine-S-phosphate as a phosphorus source. The protocol developed here can be used as a foundation to functionally profile other microalgae such as known microalgae mutants and novel isolates.

Kinetics and mechanism of oxidation of super-reduced cobalamin and cobinamide species by thiosulfate, sulfite and dithionite

We studied the kinetics of reactions of cob(I)alamin and cob(I)inamide with thiosulfate, sulfite, and dithionite by UV-Visible (UV-Vis) and stopped-flow spectroscopy. We found that the two Co(I) species were oxidized by these sulfur-containing compounds to Co(II) forms: oxidation by excess thiosulfate leads to penta-coordinate complexes and oxidation by excess sulfite or dithionite leads to hexa-coordinate Co(II)-SO2(-) complexes. The net scheme involves transfer of three electrons in the case of oxidation by thiosulfate and one electron for oxidation by sulfite and dithionite. On the basis of kinetic data, the nature of the reactive oxidants was suggested, i.e., HS2O3(-) (for oxidation by thiosulfate), S2O5(2-), HSO3(-), and aquated SO2 (for oxidation by sulfite), and S2O4(2-) and SO2(-) (for oxidation by dithionite). No difference was observed in kinetics with cob(i)alamin or cob(i)inamide as reductants.

Parallel independent evolution of pathogenicity within the genus Yersinia

The genus Yersinia has been used as a model system to study pathogen evolution. Using whole-genome sequencing of all Yersinia species, we delineate the gene complement of the whole genus and define patterns of virulence evolution. Multiple distinct ecological specializations appear to have split pathogenic strains from environmental, nonpathogenic lineages. This split demonstrates that contrary to hypotheses that all pathogenic Yersinia species share a recent common pathogenic ancestor, they have evolved independently but followed parallel evolutionary paths in acquiring the same virulence determinants as well as becoming progressively more limited metabolically. Shared virulence determinants are limited to the virulence plasmid pYV and the attachment invasion locus ail. These acquisitions, together with genomic variations in metabolic pathways, have resulted in the parallel emergence of related pathogens displaying an increasingly specialized lifestyle with a spectrum of virulence potential, an emerging theme in the evolution of other important human pathogens.

Vitamin D regulates the gut microbiome and protects mice from dextran sodium sulfate-induced colitis

The active form of vitamin D [1,25-dihydroxycholecalciferol, 1,25(OH)2D3] and the vitamin D receptor (VDR) regulate susceptibility to experimental colitis. The effect of the bacterial microflora on the susceptibility of C57BL/6 mice to dextran sodium sulfate-induced colitis was determined. Mice that cannot produce 1,25(OH)2D3 [Cyp27b1 (Cyp) knockout (KO)], VDR KO as well as their wild-type littermates were used. Cyp KO and VDR KO mice had more bacteria from the Bacteroidetes and Proteobacteria phyla and fewer bacteria from the Firmicutes and Deferribacteres phyla in the feces compared with wild-type. In particular, there were more beneficial bacteria, including the Lactobacillaceae and Lachnospiraceae families, in feces from Cyp KO and VDR KO mice than in feces from wild-type. Helicobacteraceae family member numbers were elevated in Cyp KO compared with wild-type mice. Depletion of the gut bacterial flora using antibiotics protected mice from colitis. 1,25(OH)2D3 treatment (1.25 μg/100 g diet) of Cyp KO mice decreased colitis severity and reduced the numbers of Helicobacteraceae in the feces compared with the numbers in the feces of untreated Cyp KO mice. The mechanisms by which the dysbiosis occurs in VDR KO and Cyp KO mice included lower expression of E-cadherin on gut epithelial and immune cells and fewer tolerogenic dendritic cells that resulted in more gut inflammation in VDR and Cyp KO mice compared with wild-type mice. Increased host inflammation has been shown to provide pathogens with substrates to out-compete more beneficial bacterial species. Our data demonstrate that vitamin D regulates the gut microbiome and that 1,25(OH)2D3 or VDR deficiency results in dysbiosis, leading to greater susceptibility to injury in the gut.

Microbial regulation of host hydrogen sulfide bioavailability and metabolism

Hydrogen sulfide (H2S), generated through various endogenous enzymatic and nonenzymatic pathways, is emerging as a regulator of physiological and pathological events throughout the body. Bacteria in the gastrointestinal tract also produce significant amounts of H2S that regulates microflora growth and virulence responses. However, the impact of the microbiota on host global H2S bioavailability and metabolism remains unknown. To address this question, we examined H2S bioavailability in its various forms (free, acid labile, or bound sulfane sulfur), cystathionine γ-lyase (CSE) activity, and cysteine levels in tissues from germ-free versus conventionally housed mice. Free H2S levels were significantly reduced in plasma and gastrointestinal tissues of germ-free mice. Bound sulfane sulfur levels were decreased by 50-80% in germ-free mouse plasma and adipose and lung tissues. Tissue CSE activity was significantly reduced in many organs from germ-free mice, whereas tissue cysteine levels were significantly elevated compared to conventional mice. These data reveal that the microbiota profoundly regulates systemic bioavailability and metabolism of H2S.

The prokaryotic Mo/W-bisPGD enzymes family: a catalytic workhorse in bioenergetic

Over the past two decades, prominent importance of molybdenum-containing enzymes in prokaryotes has been put forward by studies originating from different fields. Proteomic or bioinformatic studies underpinned that the list of molybdenum-containing enzymes is far from being complete with to date, more than fifty different enzymes involved in the biogeochemical nitrogen, carbon and sulfur cycles. In particular, the vast majority of prokaryotic molybdenum-containing enzymes belong to the so-called dimethylsulfoxide reductase family. Despite its extraordinary diversity, this family is characterized by the presence of a Mo/W-bis(pyranopterin guanosine dinucleotide) cofactor at the active site. This review highlights what has been learned about the properties of the catalytic site, the modular variation of the structural organization of these enzymes, and their interplay with the isoprenoid quinones. In the last part, this review provides an integrated view of how these enzymes contribute to the bioenergetics of prokaryotes. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Genomic analysis of Melioribacter roseus, facultatively anaerobic organotrophic bacterium representing a novel deep lineage within Bacteriodetes/Chlorobi group

Melioribacter roseus is a moderately thermophilic facultatively anaerobic organotrophic bacterium representing a novel deep branch within Bacteriodetes/Chlorobi group. To better understand the metabolic capabilities and possible ecological functions of M. roseus and get insights into the evolutionary history of this bacterial lineage, we sequenced the genome of the type strain P3M-2^T. A total of 2838 open reading frames was predicted from its 3.30 Mb genome. The whole proteome analysis supported phylum-level classification of M. roseus since most of the predicted proteins had closest matches in Bacteriodetes, Proteobacteria, Chlorobi, Firmicutes and deeply-branching bacterium Caldithrix abyssi, rather than in one particular phylum. Consistent with the ability of the bacterium to grow on complex carbohydrates, the genome analysis revealed more than one hundred glycoside hydrolases, glycoside transferases, polysaccharide lyases and carbohydrate esterases. The reconstructed central metabolism revealed pathways enabling the fermentation of complex organic substrates, as well as their complete oxidation through aerobic and anaerobic respiration. Genes encoding the photosynthetic and nitrogen-fixation machinery of green sulfur bacteria, as well as key enzymes of autotrophic carbon fixation pathways, were not identified. The M. roseus genome supports its affiliation to a novel phylum Ignavibateriae, representing the first step on the evolutionary pathway from heterotrophic ancestors of Bacteriodetes/Chlorobi group towards anaerobic photoautotrophic Chlorobi.

Cysteine catabolism and cysteine desulfhydrase (CdsH/STM0458) in Salmonella enterica serovar typhimurium

Cysteine is potentially toxic and can affect diverse functions such as oxidative stress, antibiotic resistance, and swarming motility. The contribution of cysteine catabolism in modulating responses to cysteine has not been examined, in part because the genes have not been identified and mutants lacking these genes have not been isolated or characterized. We identified the gene for a previously described cysteine desulfhydrase, which we designated cdsH (formerly STM0458). We also identified a divergently transcribed gene that regulates cdsH expression, which we designated cutR (formerly ybaO, or STM0459). CdsH appears to be the major cysteine-degrading and sulfide-producing enzyme aerobically but not anaerobically. Mutants with deletions of cdsH and ybaO exhibited increased sensitivity to cysteine toxicity and altered swarming motility but unaltered cysteine-enhanced antibiotic resistance and survival in macrophages.

Thiosulfate reduction in Salmonella enterica is driven by the proton motive force

Thiosulfate respiration in Salmonella enterica serovar Typhimurium is catalyzed by the membrane-bound enzyme thiosulfate reductase. Experiments with quinone biosynthesis mutants show that menaquinol is the sole electron donor to thiosulfate reductase. However, the reduction of thiosulfate by menaquinol is highly endergonic under standard conditions (ΔE°' = -328 mV). Thiosulfate reductase activity was found to depend on the proton motive force (PMF) across the cytoplasmic membrane. A structural model for thiosulfate reductase suggests that the PMF drives endergonic electron flow within the enzyme by a reverse loop mechanism. Thiosulfate reductase was able to catalyze the combined oxidation of sulfide and sulfite to thiosulfate in a reverse of the physiological reaction. In contrast to the forward reaction the exergonic thiosulfate-forming reaction was PMF independent. Electron transfer from formate to thiosulfate in whole cells occurs predominantly by intraspecies hydrogen transfer.

General metabolism of Laribacter hongkongensis: a genome-wide analysis

BACKGROUND

Laribacter hongkongensis is associated with community-acquired gastroenteritis and traveler's diarrhea. In this study, we performed an in-depth annotation of the genes and pathways of the general metabolism of L. hongkongensis and correlated them with its phenotypic characteristics.

RESULTS

The L. hongkongensis genome possesses the pentose phosphate and gluconeogenesis pathways and tricarboxylic acid and glyoxylate cycles, but incomplete Embden-Meyerhof-Parnas and Entner-Doudoroff pathways, in agreement with its asaccharolytic phenotype. It contains enzymes for biosynthesis and β-oxidation of saturated fatty acids, biosynthesis of all 20 universal amino acids and selenocysteine, the latter not observed in Neisseria gonorrhoeae, Neisseria meningitidis and Chromobacterium violaceum. The genome contains a variety of dehydrogenases, enabling it to utilize different substrates as electron donors. It encodes three terminal cytochrome oxidases for respiration using oxygen as the electron acceptor under aerobic and microaerophilic conditions and four reductases for respiration with alternative electron acceptors under anaerobic conditions. The presence of complete tetrathionate reductase operon may confer survival advantage in mammalian host in association with diarrhea. The genome contains CDSs for incorporating sulfur and nitrogen by sulfate assimilation, ammonia assimilation and nitrate reduction. The existence of both glutamate dehydrogenase and glutamine synthetase/glutamate synthase pathways suggests an importance of ammonia metabolism in the living environments that it may encounter.

CONCLUSIONS

The L. hongkongensis genome possesses a variety of genes and pathways for carbohydrate, amino acid and lipid metabolism, respiratory chain and sulfur and nitrogen metabolism. These allow the bacterium to utilize various substrates for energy production and survive in different environmental niches.

A breathtaking feat: to compete with the gut microbiota, Salmonella drives its host to provide a respiratory electron acceptor

Salmonella is a common cause of food poisoning. However, after ingestion the pathogen has to compete with resident microbes that already occupy the intestinal lumen (microbiota), which poses a challenge for Salmonella to successfully colonize this niche. Recent data show that Salmonella elicits help from the host immune response to beat the competition. After arriving in the intestine, Salmonella elicits acute intestinal inflammation. The respiratory burst of neutrophils that transmigrate into the intestinal lumen during inflammation oxidizes endogenous sulfur compounds to generate a respiratory electron acceptor, tetrathionate. As a result, Salmonella can use tetrathionate respiration to outgrow the fermenting microbiota in the anaerobic environment of the gut, which promotes transmission of the pathogen. This principle might be used by other gut microbes and contribute to changes in the microbiota composition observed during inflammation.