PerR-regulated manganese ion uptake contributes to oxidative stress defense in an oral streptococcus

Transcriptome analysis reveals manganese tolerance mechanisms in a novel native bacterium of Bacillus altitudinis strain HM-12

Bacillus altitudinis HM-12, isolated from ferromanganese ore tailings, can resist up to 1200 mM Mn(II) when exposed to concentrations from 50 mM to 1400 mM. HM-12 exhibited high Mn(II) removal efficiency (90.6 %). We report the transcriptional profile of HM-12 using RNA-Seq and found 423 upregulated and 536 downregulated differentially expressed genes (DEGs) compared to the control. Gene Ontology analysis showed that DEGs were mainly linked with transporter activity, binding, catalytic activity in molecular function, cellular anatomical entity in cellular component, cellular process, and metabolic process. Kyoto Encyclopedia of Genes and Genomes analysis showed that DEGs were mostly mapped to membrane transport, signal transduction, carbohydrate and amino acid metabolism, energy metabolism, and cellular community pathways. Transport analysis showed that two manganese importer systems, mntH and mntABC, were significantly downregulated. The manganese efflux genes (mneS, yceF and ykoY) exhibited significant upregulation. Manganese homeostasis seems to be subtly regulated by manganese uptake and efflux genes. Moreover, it was found that copA as a Mn(II) oxidase gene and a copper chaperone gene copZ were considerably upregulated by signal transduction analysis. csoR encoding a transcriptional repressor which can regulate the copZA operon was upregulated. The strong Mn(II) oxidizing activity of HM-12 was also confirmed by physicochemical characterization. In metabolism and environmental information processing, yjqC encoding manganese catalase was significantly upregulated, while katE and katX encoding heme catalases were significantly downregulated. The antioxidant gene pcaC was significantly upregulated, but ykuU encoding alkyl hydroperoxide reductase, yojM encoding superoxide dismutase, and perR encoding redox-sensing transcriptional repressor were downregulated. These results highlight the oxidative activity of HM-12 by regulating the transcription of oxidase, catalase, peroxidase, and superoxide dismutase to sense the cellular redox status and prevent Mn(II) intoxication. This study provides relevant information on the biological tolerance and oxidation mechanisms in response to Mn(II) stress.

Metal Homeostasis in Pathogenic Streptococci

Streptococcus spp. are an important genus of Gram-positive bacteria, many of which are opportunistic pathogens that are capable of causing invasive disease in a wide range of populations. Metals, especially transition metal ions, are an essential nutrient for all organisms. Therefore, to survive across dynamic host environments, Streptococci have evolved complex systems to withstand metal stress and maintain metal homeostasis, especially during colonization and infection. There are many different types of transport systems that are used by bacteria to import or export metals that can be highly specific or promiscuous. Focusing on the most well studied transition metals of zinc, manganese, iron, nickel, and copper, this review aims to summarize the current knowledge of metal homeostasis in pathogenic Streptococci, and their role in virulence.

PerR-Regulated Manganese Import Contributes to Oxidative Stress Defense in Streptococcus suis

Streptococcus suis has been increasingly recognized as a porcine zoonotic pathogen that threatens the health of both pigs and humans. Metal homeostasis plays a critical role in the antioxidative capability of bacteria, thus facilitating the escape of pathogenic species from the innate immunity systems of hosts. Here, we revealed that manganese increased the ability of S. suis to resist oxidative stress. RNA sequencing was used to identify potential candidate genes involved in the maintenance of intracellular manganese homeostasis. Four genes, termed troABCD, were identified by NCBI BLASTp analysis. The troA, troB, troC, and troD deletion mutant strains exhibited decreased intracellular manganese content and tolerance to H₂O₂ compared to the wild-type strain. Thus, troABCD were determined to be involved in manganese uptake and played an important role in H₂O₂ tolerance in S. suis. Furthermore, the inactivation of perR increased the survival of H₂O₂-pulsed S. suis 2.18-fold and elevated the intracellular manganese content. H₂O₂-pulsed S. suis and perR deletion mutants upregulated troABCD. This finding suggested that H₂O₂ released the suppression of troABCD by perR. In addition, an electrophoretic mobility shift assay (EMSA) showed that PerR at 500 ng binds to the troABCD promoter, indicating that troABCD were directly regulated by PerR. In conclusion, this study revealed that manganese increases tolerance to H₂O₂ by upregulating the expression of troABCD. Moreover, PerR-regulated Mn import in S. suis and increased the tolerance of S. suis to oxidative stress by regulating troABCD. IMPORTANCE During infection, it is extremely important for bacteria to defend against oxidative stress. While manganese plays an important role in this process, its role is unclear in S. suis. Here, we demonstrated that manganese increased S. suis tolerance to oxidative stress. Four manganese ABC transporter genes, troABCD, were identified. Oxidative stress increased the content of manganese in the cell. Furthermore, PerR increased the tolerance to oxidative stress of S. suis by regulating troABCD. Manganese played an important role in bacterial defense against oxidative stress. These findings provide novel insight into the mechanism by which S. suis resists oxidative stress and approaches to inhibit bacterial infection by limiting manganese intake.

Elucidating the Role and Structure-Activity Relationships of the Streptococcus oligofermentans Competence-Stimulating Peptide

Streptococcus oligofermentans is an early colonizer of the oral microbiome with documented bactericidal activity against the oral pathogen Streptococcus mutans. S. oligofermentans has been observed to possess the typical comABCDE competence regulon found within most oral streptococci; however, the competence-stimulating peptide (CSP) responsible for QS activation and the regulatory role of the competence regulon is yet to be explored. Herein, we have both confirmed the identity of the S. oligofermentans CSP and utilized a wide range of phenotypic assays to characterize its regulatory role in competence, biofilm formation, and hydrogen peroxide formation. To determine the importance of each amino acid residue in CSP/ComD binding, we performed systematic replacement of amino acid residues within the S. oligofermentans CSP and developed a luciferase-based reporter system to assess the ability of these mutated analogues to modulate the competence regulon. Additionally, we performed CD analysis on mutated CSP analogues to determine the correlation between the peptide secondary structure and QS activation. To further explore S. oligofermentans' potential as a biotherapeutic against S. mutans infection, lead QS activators and inhibitors were used in interspecies competition assays to assess the effect of QS modulation on interactions between these two species. Lastly, we have documented a lack of S. oligofermentans-induced cytotoxicity, highlighting the potential of this native flora as a biotherapeutic with minimal health risks.

RNase Z Oxidative Degradation Impedes tRNA Maturation and is Involved in Streptococcal Translation Regulation in Response to Oxidative Stress

When encountering oxidative stress, organisms selectively upregulate antioxidant genes and simultaneously suppress the translation of most other proteins. Eukaryotes employ multiple strategies to adjust translation at both the initiation and elongation stages; however, how prokaryotes modulate translation under oxidative stress remains unclear. Here, we report that upon hydrogen peroxide (H₂O₂) challenge, Streptococcus oligofermentans reduced translation via RNase Z (So-RNaseZ) oxidative degradation, thus hindering tRNA maturation. S. oligofermentans encodes all CCA-less tRNAs that require So-RNaseZ for 3′ end maturation. A combination of nonreducing SDS-PAGE and liquid chromatography/tandem mass spectrometry (LC/MS-MS) assays demonstrated that H₂O₂ oxidation induced Cys38-Cys149 disulfide linkages in recombinant So-RNaseZ protein, and serine substitution of Cys38 or Cys149 abolished these disulfide linkages. Consistently, redox Western blotting also determined intramolecular disulfide-linked So-RNaseZ in H₂O₂-treated S. oligofermentans cells. The disulfide-linked So-RNaseZ and monomer were both subject to proteolysis, whereas C149S mutation alleviated oxidative degradation of So-RNaseZ, suggesting that H₂O₂-mediated disulfide linkages substantially contributed to So-RNaseZ degradation. Accordingly, Northern blotting determined that tRNA precursor accumulation and mature tRNA species decrease in H₂O₂-treated S. oligofermentans. Moreover, reduced overall protein synthesis, as indicated by puromycin incorporation, and retarded growth of S. oligofermentans occurred in an H₂O₂ concentration-dependent manner. Overexpression of So-RNaseZ not only elevated tRNA precursor processing and protein synthesis but also partly rescued H₂O₂-suppressed S. oligofermentans growth. Moreover, So-RNaseZ oxidative degradation-mediated translation repression elevated S. oligofermentans survival under high H₂O₂ stress. Therefore, this work found that So-RNaseZ oxidative degradation-impeded tRNA maturation contributes to streptococcal translation repression and provides the oxidative stress adaptability for S. oligofermentans.

IMPORTANCE Translation regulation is a common strategy used by organisms to reduce oxidative damage. Catalase-negative streptococci produce as well as tolerate high levels of H₂O₂. This work reports a novel translation regulation mechanism employed by Streptococcus oligofermentans in response to H₂O₂ challenge, in which the key tRNA endonuclease So-RNaseZ is oxidized to form Cys38-Cys149 disulfide linkages and both the disulfide-linked So-RNaseZ and monomers are subject to proteolysis; thus, tRNA maturation, protein translation, and growth are all suppressed. Notably, So-RNaseZ oxidative degradation-mediated translation repression offers oxidative adaptability to S. oligofermentans and enhances its survival against high H₂O₂ challenge. So-RNaseZ orthologs and H₂O₂-sensitive cysteines (Cys38 and Cys149) are widely distributed in Streptococcus and Lactococcus species genomes, which also encode all CCA-less tRNAs and lack catalase. Therefore, RNase Z oxidative degradation-based translation regulation could be widely employed by these lactic acid bacteria, including pathogenic streptococci, to cope with H₂O₂.

Regulation and distinct physiological roles of manganese in bacteria

Manganese (Mn²⁺) is an essential trace element within organisms spanning the entire tree of life. In this review, we provide an overview of Mn²⁺ transport and the regulation of its homeostasis in bacteria, with a focus on its functions beyond being a cofactor for enzymes. Crucial differences in Mn²⁺ homeostasis exist between bacterial species that can be characterized to have an iron- or manganese-centric metabolism. Highly iron-centric species require minimal Mn²⁺ and mostly use it as a mechanism to cope with oxidative stress. As a consequence, tight regulation of Mn²⁺ uptake is required, while organisms that use both Fe²⁺ and Mn²⁺ need other layers of regulation for maintaining homeostasis. We will focus in detail on manganese-centric bacterial species, in particular lactobacilli, that require little to no Fe²⁺ and use Mn²⁺ for a wider variety of functions. These organisms can accumulate extraordinarily high amounts of Mn²⁺ intracellularly, enabling the nonenzymatic use of Mn²⁺ for decomposition of reactive oxygen species while simultaneously functioning as a mechanism of competitive exclusion. We further discuss how Mn²⁺ accumulation can provide both beneficial and pathogenic bacteria with advantages in thriving in their niches.

Increased Oxidative Stress Tolerance of a Spontaneously Occurring perR Gene Mutation in Streptococcus mutans UA159

The ability of bacteria, such as the dental pathogen Streptococcus mutans, to coordinate a response against damage-inducing oxidants is a critical aspect of their pathogenicity. The oxidative stress regulator SpxA1 has been demonstrated to be a major player in the ability of S. mutans to withstand both disulfide and peroxide stresses. While studying spontaneously occurring variants of an S. mutans ΔspxA1 strain, we serendipitously discovered that our S. mutans UA159 host strain bore a single-nucleotide deletion within the coding region of perR, resulting in a premature truncation of the encoded protein. PerR is a metal-dependent transcriptional repressor that senses and responds to peroxide stress such that loss of PerR activity results in activation of oxidative stress responses. To determine the impact of loss of PerR regulation, we obtained a UA159 isolate bearing an intact perR copy and created a clean perR deletion mutant. Our findings indicate that loss of PerR activity results in a strain that is primed to tolerate oxidative stresses in the laboratory setting. Interestingly, RNA deep sequencing (RNA-Seq) and targeted transcriptional expression analyses reveal that PerR offers a minor contribution to the ability of S. mutans to orchestrate a transcriptional response to peroxide stress. Furthermore, we detected loss-of-function perR mutations in two other commonly used laboratory strains of S. mutans, suggesting that this may be not be an uncommon occurrence. This report serves as a cautionary tale regarding the so-called domestication of laboratory strains and advocates for the implementation of more stringent strain authentication practices.IMPORTANCE A resident of the human oral biofilm, Streptococcus mutans is one of the major bacterial pathogens associated with dental caries. This report highlights a spontaneously occurring mutation within the laboratory strain S. mutans UA159 found in the coding region of perR, a gene encoding a transcriptional repressor associated with peroxide tolerance. Though perR mutant strains of S. mutans showed a distinct growth advantage and enhanced tolerance toward H₂O₂, a ΔperR deletion strain showed a small number of differentially expressed genes compared to the parent strain, suggesting few direct regulatory targets. In addition to characterizing the role of PerR in S. mutans, our findings serve as a warning to laboratory researchers regarding bacterial adaptation to in vitro growth conditions.

Pyruvate secretion by oral streptococci modulates hydrogen peroxide dependent antagonism

Many commensal oral streptococci generate H₂O₂ via pyruvate oxidase (SpxB) to inhibit the growth of competing bacteria like Streptococcus mutans, a major cariogenic species. In Streptococcus sanguinis SK36 (SK36) and Streptococcus gordonii DL1 (DL1), spxB expression and H₂O₂ release are subject to carbon catabolite repression by the catabolite control protein A (CcpA). Surprisingly, ccpA deletion mutants of SK36 and DL1 fail to inhibit S. mutans despite their production of otherwise inhibitory levels of H₂O₂. Using H₂O₂-deficient spxB deletion mutants of SK36 and DL1, it was subsequently discovered that both strains confer protection in trans to other bacteria when H₂O₂ is added exogenously. This protective effect depends on the direct detoxification of H₂O₂ by the release of pyruvate. The pyruvate dependent protective effect is also present in other spxB-encoding streptococci, such as the pneumococcus, but is missing from spxB-negative species like S. mutans. Targeted and transposon-based mutagenesis revealed Nox (putative H₂O-forming NADH dehydrogenase) as an essential component required for pyruvate release and oxidative protection, while other genes such as sodA and dps play minor roles. Furthermore, pyruvate secretion is only detectable in aerobic growth conditions at biofilm-like cell densities and is responsive to CcpA-dependent catabolite control. This ability of spxB-encoding streptococci reveals a new facet of the competitive interactions between oral commensals and pathobionts and provides a mechanistic basis for the variable levels of inhibitory potential observed among H₂O₂-producing strains of commensal oral streptococci.

Redox-Regulated Adaptation of Streptococcus oligofermentans to Hydrogen Peroxide Stress

The catalase-negative streptococci produce as well as tolerate high levels of H₂O₂. This work reports the molecular mechanisms of low-H₂O₂-concentration-induced adaptation to higher H₂O₂ stress in a Streptococcus species, in which the peroxide-responsive repressor PerR and its redox regulons play the major role. Distinct from the Bacillus subtilis PerR, which is inactivated by H₂O₂ through histidine oxidation by the Fe²⁺-triggered Fenton reaction, the streptococcal PerR is inactivated by H₂O₂ oxidation of the structural Zn²⁺ binding cysteine residues and thus derepresses the expression of genes defending against oxidative stress. The reversible cysteine oxidation could provide flexibility for PerR regulation in streptococci, and the mechanism might be widely used by lactic acid bacteria, including pathogenic streptococci, containing high levels of cellular manganese, in coping with oxidative stress. The adaptation mechanism could also be applied in oral hygiene by facilitating the fitness and adaptability of the oral commensal streptococci to suppress the pathogens.

Preexposure to a low concentration of H₂O₂ significantly increases the survivability of catalase-negative streptococci in the presence of a higher concentration of H₂O₂. However, the mechanisms of this adaptation remain unknown. Here, using a redox proteomics assay, we identified 57 and 35 cysteine-oxidized proteins in Streptococcus oligofermentans bacteria that were anaerobically cultured and then pulsed with 40 μM H₂O₂ and that were statically grown in a 40-ml culture, respectively. The oxidized proteins included the peroxide-responsive repressor PerR, the manganese uptake repressor MntR, thioredoxin system proteins Trx and Tpx, and most glycolytic proteins. Cysteine oxidations of these proteins were verified through redox Western blotting, immunoprecipitation, and liquid chromatography-tandem mass spectrometry assays. In particular, Zn²⁺-coordinated Cys139 and Cys142 mutations eliminated the H₂O₂ oxidation of PerR, and inductively coupled plasma mass spectrometry detected significantly decreased amounts of Zn²⁺ in H₂O₂-treated PerR, demonstrating that cysteine oxidation results in Zn²⁺ loss. An electrophoretic mobility shift assay (EMSA) determined that the DNA binding of Mn²⁺-bound PerR protein (PerR:Zn,Mn) was abolished by H₂O₂ treatment but was restored by dithiothreitol reduction, verifying that H₂O₂ inactivates streptococcal PerR:Zn,Mn through cysteine oxidation, analogous to the findings for MntR. Quantitative PCR and EMSA demonstrated that tpx, mntA, mntR, and dpr belonged to the PerR regulons but that only dpr was directly regulated by PerR; mntA was also controlled by MntR. Deletion of mntR significantly reduced the low-H₂O₂-concentration-induced adaptation of S. oligofermentans to a higher H₂O₂ concentration, while the absence of PerR completely abolished the self-protection. Therefore, a low H₂O₂ concentration resulted in the cysteine-reversible oxidations of PerR and MntR to derepress their regulons, which function in cellular metal and redox homeostasis and which endow streptococci with the antioxidative capability. This work reveals a novel Cys redox-based H₂O₂ defense strategy employed by catalase-negative streptococci in Mn²⁺-rich cellular environments.

IMPORTANCE The catalase-negative streptococci produce as well as tolerate high levels of H₂O₂. This work reports the molecular mechanisms of low-H₂O₂-concentration-induced adaptation to higher H₂O₂ stress in a Streptococcus species, in which the peroxide-responsive repressor PerR and its redox regulons play the major role. Distinct from the Bacillus subtilis PerR, which is inactivated by H₂O₂ through histidine oxidation by the Fe²⁺-triggered Fenton reaction, the streptococcal PerR is inactivated by H₂O₂ oxidation of the structural Zn²⁺ binding cysteine residues and thus derepresses the expression of genes defending against oxidative stress. The reversible cysteine oxidation could provide flexibility for PerR regulation in streptococci, and the mechanism might be widely used by lactic acid bacteria, including pathogenic streptococci, containing high levels of cellular manganese, in coping with oxidative stress. The adaptation mechanism could also be applied in oral hygiene by facilitating the fitness and adaptability of the oral commensal streptococci to suppress the pathogens.

The complete genome sequence of Bifidobacterium animalis subsp. lactis 01 and its integral components of antioxidant defense system

The strain Bifidobacterium animalis 01, isolated from centenarians, showed promising antioxidant potential in our previous studies. In this study, the genome information on strain 01 and the important antioxidant components are presented. The complete genome comprises a single circular chromosome (1,931,632 bp; 60.49% G + C content) with 1569 coding DNA sequences, 52 tRNA, and 9 rRNA operons. Based on phylogenomic analyses, strain 01 was designated as B. animalis subsp. lactis 01. The genomic analysis reveals that at least eight protein-coding genes are antioxidant-related genes. The conditions for simulating the oxidative stress have been determined. The results of quantitative reverse transcription PCR further demonstrated that the genes encoding the thioredoxin system (ahpC, ahpF, bcp, trxB, trxA, nrdH, and msrAB) and non-enzyme factors of the divalent cation transporter gene (mntH) were upregulated under the H₂O₂ challenge, indicating that the eight genes were effective components of the antioxidant system. The results of this study could benefit for understanding the antioxidant mechanism of B. animalis 01 and future utilization of it as a potential antioxidant agent.

A Streptococcus aquaporin acts as peroxiporin for efflux of cellular hydrogen peroxide and alleviation of oxidative stress

Aquaporins (AQPs) are transmembrane proteins widely distributed in various organisms, and they facilitate bidirectional diffusion of water and uncharged solutes. The catalase-negative bacterium Streptococcus oligofermentans produces the highest H₂O₂ levels reported to date, which has to be exported to avoid oxidative stress. Here, we report that a S. oligofermentans aquaporin functions as a peroxiporin facilitating bidirectional transmembrane H₂O₂ transport. Knockout of this aquaporin homolog, So-AqpA, reduced H₂O₂ export by ∼50% and increased endogenous H₂O₂ retention, as indicated by the cellular H₂O₂ reporter HyPer. Heterologous expression of So-aqpA accelerated exogenous H₂O₂ influx into Saccharomyces cerevisiae and Escherichia coli cells, indicating that So-AqpA acts as an H₂O₂-transferring aquaporin. Alanine substitution revealed Phe-40 as a key residue for So-AqpA-mediated H₂O₂ transport. Northern blotting, qPCR, and luciferase reporter assays disclosed that H₂O₂ induces a >10-fold expression of So-aqpA Super-resolution imaging showed that H₂O₂ treatment increases So-AqpA protein molecules per cell by 1.6- to 3-fold. Inactivation of two redox-regulatory transcriptional repressors, PerR and MntR, reduced H₂O₂-induced So-aqpA expression to 1.8- and 4-fold, respectively. Electrophoretic mobility shift assays determined that MntR, but not PerR, binds to the So-aqpA promoter, indicating that MntR directly regulates H₂O₂-induced So-aqpA expression. Importantly, So-aqpA deletion decreased oxic growth and intraspecies competition and diminished the competitive advantages of S. oligofermentans over the caries pathogen Streptococcus mutans Of note, So-aqpA orthologs with the functionally important Phe-40 are present in all streptococci. Our work has uncovered an intrinsic, H₂O₂-inducible bacterial peroxiporin that has a key physiological role in H₂O₂ detoxification in S. oligofermentans.

A Model for Manganese interaction with Deinococcus radiodurans proteome network involved in ROS response and defense

A complex network of regulatory proteins takes part in the mechanism underlying the radioresistance of Deinoccocus radiodurans bacterium (DR). The interaction of Mn(II) ions with DR-proteins and peptides seems to be responsible for proteins protection from oxidative damage induced by Reactive Oxygen Species during irradiation. In the present work we describe a combined approach of bioinformatic strategies based on structural data and annotation to predict the Mn(II)-binding proteins encoded by the genome of DR and, in parallel, the same predictions for other bacteria were performed; the comparison revealed that, in most of the cases, the content of Mn(II)-binding proteins is significantly higher in radioresistant than in radiosensitive bacteria. Moreover, we report the in silico protein-protein interaction network of the putative Mn(II)-proteins, remodeled in order to enhance the knowledge about the impact of Mn-binding proteins in DR ability to protect also DNA from various damaging agents such as ionizing radiation, UV radiation and oxidative stress.

Comparative genome analysis reveals key genetic factors associated with probiotic property in Enterococcus faecium strains

BACKGROUND

Enterococcus faecium though commensal in the human gut, few strains provide a beneficial effect to humans as probiotics while few are responsible for the nosocomial infection. Comparative genomics of E. faecium can decipher the genomic differences responsible for probiotic, pathogenic and non-pathogenic properties. In this study, we compared E. faecium strain 17OM39 with a marketed probiotic, non-pathogenic non-probiotic (NPNP) and pathogenic strains.

RESULTS

E. faecium 17OM39 was found to be closely related with marketed probiotic strain T110 based on core genome analysis. Strain 17OM39 was devoid of known vancomycin, tetracycline resistance and functional virulence genes. Moreover, E. faecium 17OM39 genome was found to be more stable due to the absence of frequently found transposable elements. Genes imparting beneficial functional properties were observed to be present in marketed probiotic T110 and 17OM39 strains. Genes associated with colonization and survival within gastrointestinal tract was also detected across all the strains.

CONCLUSIONS

Beyond shared genetic features; this study particularly identified genes that are responsible for imparting probiotic, non-pathogenic and pathogenic features to the strains of E. faecium. Higher genomic stability, absence of known virulence factors and antibiotic resistance genes and close genomic relatedness with marketed probiotics makes E. faecium 17OM39 a potential probiotic candidate. The work presented here demonstrates that comparative genome analyses can be applied to large numbers of genomes, to find potential probiotic candidates.

bifA Regulates Biofilm Development of Pseudomonas putida MnB1 as a Primary Response to H2O2 and Mn2

Pseudomonas putida (P. putida) MnB1 is a widely used model strain in environment science and technology for determining microbial manganese oxidation. Numerous studies have demonstrated that the growth and metabolism of P. putida MnB1 are influenced by various environmental factors. In this study, we investigated the effects of hydrogen peroxide (H₂O₂) and manganese (Mn²⁺) on proliferation, Mn²⁺ acquisition, anti-oxidative system, and biofilm formation of P. putida MnB1. The related orthologs of 4 genes, mco, mntABC, sod, and bifA, were amplified from P. putida GB1 and their involvement were assayed, respectively. We found that P. putida MnB1 degraded H₂O₂, and quickly recovered for proliferation, but its intracellular oxidative stress state was maintained, with rapid biofilm formation after H₂O₂ depletion. The data from mco, mntABC, sod and bifA expression levels by qRT-PCR, elucidated a sensitivity toward bifA-mediated biofilm formation, in contrary to intracellular anti-oxidative system under H₂O₂ exposure. Meanwhile, Mn²⁺ ion supply inhibited biofilm formation of P. putida MnB1. The expression pattern of these genes showed that Mn²⁺ ion supply likely functioned to modulate biofilm formation rather than only acting as nutrient substrate for P. putida MnB1. Furthermore, blockade of BifA activity by GTP increased the formation and development of biofilms during H₂O₂ exposure, while converse response to Mn²⁺ ion supply was evident. These distinct cellular responses to H₂O₂ and Mn²⁺ provide insights on the common mechanism by which environmental microorganisms may be protected from exogenous factors. We postulate that BifA-mediated biofilm formation but not intracellular anti-oxidative system may be a primary protective strategy adopted by P. putida MnB1. These findings will highlight the understanding of microbial adaptation mechanisms to distinct environmental stresses.

Preliminary study on the tick population of Benin wildlife at the moment of its invasion by the Rhipicephalus microplus tick (Canestrini, 1888)

BACKGROUND AND AIM

Rhipicephalus microplus (Rm) is one of the most problematic livestock tick species in the world. Its rapid propagation and resistance to acaricides make it control difficult in the sub-region and Benin particularly. The aim of this work was to check its presence in wildlife and to confirm the possible role of reservoir wildlife may play in the propagation of the parasite. This will help to design more efficient control strategy.

MATERIALS AND METHODS

This study was conducted from February to March 2017 in the National Parks of Benin (Pendjari and W Park) and wildfowl's assembly and selling point in Benin. Ticks were manually picked with forceps from each animal after slaughtering by hunters then stored in 70° ethanol. Collected ticks were counted and identified in the laboratory using the identification key as described by Walker.

RESULTS

Overall, seven species of ticks (Amblyomma variegatum, Boophilus decoloratus, Rm, Boophilus spp., Hyalomma spp., Rhipicephalus sanguineus, Rhipicephalus spp.) were identified on nine wild animal species sampled (Cane rat, wildcat, Hare, Doe, Cricetoma, Buffalo, Buffon Cobe, and Bushbuck and Warthog). The average number of ticks varies from 3 to 6 between animal species, 3 to 7 between localities visited, and 2 to 5 between tick species. However, these differences are statistically significant only for localities. Considering tick species and animal species, the parasite load of Rm and Rhipicephalus spp. is higher; the buffalo being more infested. The analysis of deviance reveals that the abundance of ticks observed depends only on the observed localities (p>0.05). However, the interactions between animal species and localities on the one hand and between animal and tick species on the other hand, although not significant, have influenced the abundance of ticks as they reduce the residual deviance after their inclusion in the model.

CONCLUSIONS

This study reported the presence of Rm in wildlife of Benin and confirmed its role in the maintenance and spread of the parasites. It is, therefore, an important risk factor that we must not neglect in the epidemiological surveillance and ticks control strategies in the West African sub-region and particularly in Benin.

Stress response of a clinical Enterococcus faecalis isolate subjected to a novel antimicrobial surface coating

Emerging antibiotic resistance among pathogenic bacteria, paired with their ability to form biofilms on medical and technical devices, represents a serious problem for effective and long-term decontamination in health-care environments and gives rise to an urgent need for new antimicrobial materials. Here we present the impact of AGXX^®, a novel broad-spectrum antimicrobial surface coating consisting of micro-galvanic elements formed by silver and ruthenium, on the transcriptome of Enterococcus faecalis. A clinical E. faecalis isolate was subjected to metal stress by growing it for different periods in presence of the antimicrobial coating or silver-coated steel meshes. Subsequently, total RNA was isolated and next-generation RNA sequencing was performed to analyze variations in gene expression in presence of the antimicrobial materials with focus on known stress genes. Exposure to the antimicrobial coating had a large impact on the transcriptome of E. faecalis. After 24min almost 1/5 of the E. faecalis genome displayed differential expression. At each time-point the cop operon was strongly up-regulated, providing indirect evidence for the presence of free Ag⁺-ions. Moreover, exposure to the antimicrobial coating induced a broad general stress response in E. faecalis. Genes coding for the chaperones GroEL and GroES and the Clp proteases, ClpE and ClpB, were among the top up-regulated heat shock genes. Differential expression of thioredoxin, superoxide dismutase and glutathione synthetase genes indicates a high level of oxidative stress. We postulate a mechanism of action where the combination of Ag⁺-ions and reactive oxygen species generated by AGXX^® results in a synergistic antimicrobial effect, superior to that of conventional silver coatings.

The essential genomic landscape of the commensal Bifidobacterium breve UCC2003

Bifidobacteria are common gut commensals with purported health-promoting effects. This has encouraged scientific research into bifidobacteria, though recalcitrance to genetic manipulation and scarcity of molecular tools has hampered our knowledge on the precise molecular determinants of their health-promoting attributes and gut adaptation. To overcome this problem and facilitate functional genomic analyses in bifidobacteria, we created a large Tn5 transposon mutant library of the commensal Bifidobacterium breve UCC2003 that was further characterized by means of a Transposon Directed Insertion Sequencing (TraDIS) approach. Statistical analysis of transposon insertion distribution revealed a set of 453 genes that are essential for or markedly contribute to growth of this strain under laboratory conditions. These essential genes encode functions involved in the so-called bifid-shunt, most enzymes related to nucleotide biosynthesis and a range of housekeeping functions. Comparison to the Bifidobacterium and B. breve core genomes highlights a high degree of conservation of essential genes at the species and genus level, while comparison to essential gene datasets from other gut bacteria identified essential genes that appear specific to bifidobacteria. This work establishes a useful molecular tool for scientific discovery of bifidobacteria and identifies targets for further studies aimed at characterizing essential functions not previously examined in bifidobacteria.

Molecular Insights into Hydrogen Peroxide-sensing Mechanism of the Metalloregulator MntR in Controlling Bacterial Resistance to Oxidative Stresses

Manganese contributes to anti-oxidative stress particularly in catalase-devoid bacteria, and DtxR family metalloregulators, through sensing cellular Mn²⁺ content, regulate its homeostasis. Here, we show that metalloregulator MntR (So-MntR) functions dually as Mn²⁺ and H₂O₂ sensors in mediating H₂O₂ resistance by an oral streptococcus. H₂O₂ disrupted So-MntR binding to Mn²⁺ transporter mntABC promoter and induced disulfide-linked dimerization of the protein. Mass spectrometry identified Cys-11/Cys-156 and Cys-11/Cys-11 disulfide-linked peptides in H₂O₂-treated So-MntR. Site mutagenesis of Cys-11 and Cys-156 and particularly Cys-11 abolished H₂O₂-induced disulfide-linked dimers and weakened H₂O₂ damage on So-MntR binding, indicating that H₂O₂ inactivates So-MntR via disulfide-linked dimerization. So-MntR C123S mutant was extremely sensitive to H₂O₂ oxidization in dimerization/oligomerization, probably because the mutagenesis caused a conformational change that facilitates Cys-11/Cys-156 disulfide linkage. Intermolecular Cys-11/Cys-11 disulfide was detected in C123S/C156S double mutant. Redox Western blot detected So-MntR oligomers in air-exposed cells but remarkably decreased upon H₂O₂ pulsing, suggesting a proteolysis of the disulfide-linked So-MntR oligomers. Remarkably, elevated C11S and C156S but much lower C123S proteins were detected in H₂O₂-pulsed cells, confirming Cys-11 and Cys-156 contributed to H₂O₂-induced oligomerization and degradation. Accordingly, in the C11S and C156S mutants, expression of mntABC and cellular Mn²⁺ decreased, but H₂O₂ susceptibility increased. In the C123S mutant, increased mntABC expression, cellular Mn²⁺ content, and manganese-mediated H₂O₂ survival were determined. Given the wide distribution of Cys-11 in streptococcal DtxR-like metalloregulators, the disclosed redox regulatory function and mechanism of So-MntR can be employed by the DtxR family proteins in bacterial resistance to oxidative stress.

Transition Metal Homeostasis in Streptococcus pyogenes and Streptococcus pneumoniae

Trace metals such as Fe, Mn, Zn and Cu are essential for various biological functions including proper innate immune function. The host immune system has complicated and coordinated mechanisms in place to either starve and/or overload invading pathogens with various metals to combat the infection. Here, we discuss the roles of Fe, Mn and Zn in terms of nutritional immunity, and also the roles of Cu and Zn in metal overload in relation to the physiology and pathogenesis of two human streptococcal species, Streptococcus pneumoniae and Streptococcus pyogenes. S. pneumoniae is a major human pathogen that is carried asymptomatically in the nasopharynx by up to 70% of the population; however, transition to internal sites can cause a range of diseases such as pneumonia, otitis media, meningitis and bacteraemia. S. pyogenes is a human pathogen responsible for diseases ranging from pharyngitis and impetigo, to severe invasive infections. Both species have overlapping capacity with respect to metal acquisition, export and regulation and how metal homeostasis relates to their virulence and ability to invade and survive within the host. It is becoming more apparent that metals have an important role to play in the control of infection, and with further investigations, it could lead to the potential use of metals in novel antimicrobial therapies.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Comparison of genes required for H2O2 resistance in Streptococcus gordonii and Streptococcus sanguinis

Hydrogen peroxide (H2O2) is produced by several members of the genus Streptococcus mainly through the pyruvate oxidase SpxB under aerobic growth conditions. The acute toxic nature of H2O2 raises the interesting question of how streptococci cope with intrinsically produced H2O2, which subsequently accumulates in the microenvironment and threatens the closely surrounding population. Here, we investigate the H2O2 susceptibility of oral Streptococcus gordonii and Streptococcus sanguinis and elucidate potential mechanisms of how they protect themselves from the deleterious effect of H2O2. Both organisms are considered primary colonizers and occupy the same intraoral niche making them potential targets for H2O2 produced by other species. We demonstrate that S. gordonii produces relatively more H2O2 and has a greater ability for resistance to H2O2 stress. Functional studies show that, unlike in Streptococcus pneumoniae, H2O2 resistance is not dependent on a functional SpxB and confirms the important role of the ferritin-like DNA-binding protein Dps. However, the observed increased H2O2 resistance of S. gordonii over S. sanguinis is likely to be caused by an oxidative stress protection machinery present even under anaerobic conditions, while S. sanguinis requires a longer period of time for adaptation. The ability to produce more H2O2 and be more resistant to H2O2 might aid S. gordonii in the competitive oral biofilm environment, since it is lower in abundance yet manages to survive quite efficiently in the oral biofilm.