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Bacterial growth and division rely on intricate regulation of morphogenetic complexes to remodel the cell envelope without compromising envelope integrity. Significant progress has been made in recent years towards understanding the regulation of cell wall metabolic enzymes. However, other cell envelope components play a role in morphogenesis as well. A primary factor required to protect envelope integrity in low osmolarity environments is OpgH, the synthase of osmoregulated periplasmic glucans (OPGs). Here, we demonstrate that OpgH is essential in the α-proteobacterium Caulobacter crescentus. Unexpectedly, depletion of OpgH or attempted complementation with a catalytically dead OpgH variant results in striking asymmetric bulging and cell lysis. These shape defects are accompanied by reduced cell wall synthesis and mislocalization of morphogenetic complexes. Interestingly, overactivation of the CenKR two-component system that has been implicated in cell envelope stress homeostasis in α-proteobacteria phenocopies the morphogenetic defects associated with OpgH depletion. Each of these perturbations leads to an increase in the levels of the elongasome protein, MreB, and decreases in the levels of divisome proteins FtsZ and MipZ as well as OpgH, itself. Constitutive production of OpgH during CenKR overactivation prevents cell bulging, but cells still exhibit morphogenetic defects. We propose that OPG depletion activates CenKR, leading to changes in the expression of cell envelope-related genes, but that OPGs also exert CenKR-independent effects on morphogenesis. Our data establish a surprising function for an OpgH homolog in morphogenesis and reveal an essential role of OpgH in maintaining cell morphology in Caulobacter.IMPORTANCEBacteria must synthesize and fortify the cell envelope in a tightly regulated manner to orchestrate growth and adaptation. Osmoregulated periplasmic glucans (OPGs) are important, but poorly understood, constituents of Gram-negative cell envelopes that contribute to envelope integrity and protect against osmotic stress. Here, we determined that the OPG synthase OpgH plays a surprising, essential role in morphogenesis in Caulobacter crescentus. Loss of OpgH causes asymmetric cell bulging and lysis via misregulation of the localization and activity of morphogenetic complexes. Overactivation of the CenKR two-component system involved in envelope homeostasis phenocopies OpgH depletion, suggesting that depletion of OpgH activates CenKR. Because cell envelope integrity is critical for bacterial survival, understanding how OpgH activity contributes to morphogenesis and maintenance of envelope integrity could aid in the development of antibiotic therapies.

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Hybrid two-component systems (HTCSs) comprise a major class of transcription regulators of polysaccharide utilization genes in Bacteroides. Distinct from classical two-component systems in which signal transduction is carried out by intermolecular phosphotransfer between a histidine kinase (HK) and a cognate response regulator (RR), HTCSs contain the membrane sensor HK and the RR transcriptional regulator within a single polypeptide chain. Tethering the DNA-binding domain (DBD) of the RR with the dimeric HK domain in an HTCS could potentially promote dimerization of the DBDs and would thus require a mechanism to suppress DNA-binding activity in the absence of stimulus. Analysis of phosphorylation and DNA-binding activities of several HTCSs from Bacteroides thetaiotaomicron revealed a DBD suppression mechanism in which an inhibitory interaction between the DBD and the phosphoryl group-accepting receiver domain (REC) decreases autophosphorylation rates of HTCS-RECs and represses DNA-binding activities in the absence of phosphorylation. Sequence analyses and structure predictions identified a highly conserved sequence motif correlated with a conserved inhibitory domain arrangement of REC and DBD. The presence of the motif, as in most HTCSs, or its absence, in a small subset of HTCSs, is likely predictive of two distinct regulatory mechanisms evolved for different glycans. Substitutions within the conserved motif relieve the inhibitory interaction and result in elevated DNA-binding activities in the absence of phosphorylation. Our data suggest a fundamental regulatory mechanism shared by most HTCSs to suppress DBD activities using a conserved inhibitory interdomain arrangement to overcome the challenge of the fused HK and RR components. IMPORTANCE: Different dietary and host-derived complex carbohydrates shape the gut microbial community and impact human health. In Bacteroides, the prevalent gut bacteria genus, utilization of these diverse carbohydrates relies on different gene clusters that are under sophisticated control by various signaling systems, including the hybrid two-component systems (HTCSs). We have uncovered a highly conserved regulatory mechanism in which the output DNA-binding activity of HTCSs is suppressed by interdomain interactions in the absence of stimulating phosphorylation. A consensus amino acid motif is found to correlate with the inhibitory interaction surface while deviations from the consensus can lead to constitutive activation. Understanding of such conserved HTCS features will be important to make regulatory predictions for individual systems as well as to engineer novel systems with substitutions in the consensus to explore the glycan regulation landscape in Bacteroides.

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Bacteria are capable of withstanding large changes in osmolality and cytoplasmic pH, unlike eukaryotes that tightly regulate their pH and cellular composition. Previous studies on the bacterial acid stress response described a rapid, brief acidification, followed by immediate recovery. More recent experiments with better pH probes have imaged single living cells, and we now appreciate that following acid stress, bacteria maintain an acidic cytoplasm for as long as the stress remains. This acidification enables pathogens to sense a host environment and turn on their virulence programs, for example, enabling survival and replication within acidic vacuoles. Single-cell analysis identified an intracellular pH threshold of ~6.5. Acid stress reduces the internal pH below this threshold, triggering the assembly of a type III secretion system in Salmonella and the secretion of virulence factors in the host. These pathways are significant because preventing intracellular acidification of Salmonella renders it avirulent, suggesting that acid stress pathways represent a potential therapeutic target. Although we refer to the acid stress response as singular, it is actually a complex response that involves numerous two-component signaling systems, several amino acid decarboxylation systems, as well as cellular buffering systems and electron transport chain components, among others. In a recent paper in the Journal of Bacteriology, M. G. Gorelik, H. Yakhnin, A. Pannuri, A. C. Walker, C. Pourciau, D. Czyz, T. Romeo, and P. Babitzke (J Bacteriol 206:e00354-23, 2024, https://doi.org/10.1128/jb.00354-23) describe a new connection linking the carbon storage regulator CsrA to the acid stress response, highlighting new additional layers of complexity.

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Streptococcus mutans is a cariogenic bacterium that produces a variety of bacteriocins and retains resistance to these bacteriocins. In this study, we investigated the susceptibility of 127 S. mutans strains to nukacins produced by Staphylococcus spp., which are commensal bacteria in humans. We detected diverse susceptibilities among strains. Nineteen strains had a disrupted LctF (type I), which is responsible for nukacin susceptibility, whereas the remaining 108 strains had an intact LctF (type II) and displayed resistance to nukacins. However, the type I strains still showed resistance to nukacins to some extent. Interestingly, 18/19 (94.7%) type I strains carried a mukA-T locus, which is related to the synthesis of mutacin K8, and mukFEG, an ABC transporter. In contrast, among type II strains, only 6/108 strains (5.6%) had both the mukA-T locus and mukFEG, 19/108 strains (17.6%) carried only mukFEG, and 83/108 strains (76.9%) harbored neither mukA-T nor mukFEG. We also found that MukF had two variants: 305 amino acids (type α) and 302 amino acids (type ß). All type I strains showed a type α (MukFα), whereas most type II strains with mukFEG (22/25 strains) had a type ß (MukFß). Then, we constructed a mukFEG-deletion mutant complemented with MukFαEG or MukFßEG and found that only MukFαEG was involved in nukacin resistance. The nukacin resistance capability of type II-LctFEG was stronger than that of MukFαEG. In conclusion, we identified a novel nukacin resistance factor, MukFEG, and either LctFEG or MukFEG was active in most strains via genetic polymorphisms depending on mukA-T genes. IMPORTANCE: Streptococcus mutans is an important pathogenic bacterium not only for dental caries but also for systemic diseases. S. mutans is known to produce a variety of bacteriocins and to retain resistance these bacteriocins. In this study, two ABC transporters, LctFEG and MukFEG, were implicated in nukacin resistance and each ABC transporter has two subtypes, active and inactive. Of the two ABC transporters, only one ABC transporter was always resistant, while the other ABC transporter was inactivated by genetic mutation. Interestingly, this phenomenon was defined by the presence or absence of the mutacin K8 synthesis gene region, one of the bacteriocins of S. mutans. This suggests that the resistance acquisition is tightly controlled in each strain. This study provides important evidence that the insertion of bacteriocin synthesis genes is involved in the induction of genetic polymorphisms and suggests that bacteriocin synthesis genes may play an important role in bacterial evolution.

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Drug-resistant bacteria are a serious threat to human health as antibiotics are gradually losing their clinical efficacy. Comprehending the mechanism of action of antimicrobials and their resistance mechanisms plays a key role in developing new agents to fight antimicrobial resistance. The lipopeptide daptomycin is an antibiotic that selectively disrupts Gram-positive bacterial membranes, thereby showing slower resistance development than many classical drugs. Consequently, it is often used as a last resort antibiotic to preserve its use as one of the least potent antibiotics at our disposal. The mode of action of daptomycin has been debated but was recently found to involve the formation of a tripartite complex between undecaprenyl precursors of cell wall biosynthesis and the anionic phospholipid phosphatidylglycerol. BceAB-type ABC transporters are known to confer resistance to antimicrobial peptides that sequester some precursors of the peptidoglycan, such as the undecaprenyl pyrophosphate or lipid II. The expression of these transporters is upregulated by dedicated two-component regulatory systems in the presence of antimicrobial peptides that are recognized by the system. Here, we investigated whether daptomycin evades resistance mediated by the BceAB transporter from the bacterial pathogen Streptococcus pneumoniae. Although daptomycin can bind to the transporter, our data showed that the BceAB transporter does not mediate resistance to the drug and its expression is not induced in its presence. These findings show that the pioneering membrane-active daptomycin has the potential to escape the resistance mechanism mediated by BceAB-type transporters and confirm that the development of this class of compounds has promising clinical applications.IMPORTANCEAntibiotic resistance is rising in all parts of the world. New resistance mechanisms are emerging and dangerously spreading, threatening our ability to treat common infectious diseases. Daptomycin is an antimicrobial peptide that is one of the last antibiotics approved for clinical use. Understanding the resistance mechanisms toward last-resort antibiotics such as daptomycin is critical for the success of future antimicrobial therapies. BceAB-type ABC transporters confer resistance to antimicrobial peptides that target precursors of cell-wall synthesis. In this study, we showed that the BceAB transporter from the human pathogen Streptococcus pneumoniae does not confer resistance to daptomycin, suggesting that this drug and other calcium-dependent lipopeptide antibiotics have the potential to evade the action of this type of ABC transporters in other bacterial pathogens.

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IMPORTANCE: We explore when and why large classes of proteins expand into new sequence space. We used an unsupervised machine learning approach to observe the sequence landscape of REC domains of bacterial response regulator proteins. We find that within-gene recombination can switch effector domains and, consequently, change the regulatory context of the duplicated protein.

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Among the 16 two-component systems in the opportunistic human pathogen Staphylococcus aureus, only WalKR is essential. Like the orthologous systems in other Bacillota, S. aureus WalKR controls autolysins involved in peptidoglycan remodeling and is therefore intimately involved in cell division. However, despite the importance of WalKR in S. aureus, the basis for its essentiality is not understood and the regulon is poorly defined. Here, we defined a consensus WalR DNA-binding motif and the direct WalKR regulon by using functional genomics, including chromatin immunoprecipitation sequencing, with a panel of isogenic walKR mutants that had a spectrum of altered activities. Consistent with prior findings, the direct regulon includes multiple autolysin genes. However, this work also revealed that WalR directly regulates at least five essential genes involved in lipoteichoic acid synthesis (ltaS): translation (rplK), DNA compaction (hup), initiation of DNA replication (dnaA, hup) and purine nucleotide metabolism (prs). Thus, WalKR in S. aureus serves as a polyfunctional regulator that contributes to fundamental control over critical cell processes by coordinately linking cell wall homeostasis with purine biosynthesis, protein biosynthesis, and DNA replication. Our findings further address the essentiality of this locus and highlight the importance of WalKR as a bona fide target for novel anti-staphylococcal therapeutics. IMPORTANCE The opportunistic human pathogen Staphylococcus aureus uses an array of protein sensing systems called two-component systems (TCS) to sense environmental signals and adapt its physiology in response by regulating different genes. This sensory network is key to S. aureus versatility and success as a pathogen. Here, we reveal for the first time the full extent of the regulatory network of WalKR, the only staphylococcal TCS that is indispensable for survival under laboratory conditions. We found that WalKR is a master regulator of cell growth, coordinating the expression of genes from multiple, fundamental S. aureus cellular processes, including those involved in maintaining cell wall metabolism, protein biosynthesis, nucleotide metabolism, and the initiation of DNA replication.

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The nitrogen (N) status transduced via the NtrBC two-component system is a major signaling cue in the root nodule endosymbiosis of diazotrophic rhizobia with legumes. NtrBC is upregulated in the N-limiting rhizosphere environment at the onset of nodulation but silenced in nodules to favor the assimilation of the fixed N into plant biomass. We reported that the trans-acting sRNA NfeR1 (Nodule Formation Efficiency RNA) broadly influences the symbiotic performance of the α-rhizobium Sinorhizobium meliloti. Here, we show that NfeR1 is indeed an N-responsive sRNA that fine-tunes NtrBC output during the symbiotic transition. Biochemical and genetic approaches unveiled that NtrC and the LysR-type symbiotic regulator LsrB bind at distinct nearby sites in the NfeR1 promoter, acting antagonistically as repressor and activator of transcription, respectively. This complex transcriptional control specifies peak NfeR1 steady-state levels in N-starved and endosymbiotic bacteria. Furthermore, NfeR1 base pairs the translation initiation region of the histidine kinase coding mRNA ntrB, causing a decrease in both NtrB and NtrC abundance as assessed by double-plasmid genetic assays. In the context of endogenous regulation, NfeR1-mediated ntrBC silencing most likely amends the effective strength of the known operon autorepression exerted by NtrC. Accordingly, a lack of NfeR1 shifts the wild-type NtrBC output, restraining the fitness of free-living rhizobia under N stress and plant growth upon nodulation. The mixed NtrBC-NfeR1 double-negative feedback loop is thus an unprecedented adaptive network motif that helps α-rhizobia adjust N metabolism to the demands of an efficient symbiosis with legume plants. IMPORTANCE Root nodule endosymbioses between diazotrophic rhizobia and legumes provide the largest input of combined N to the biosphere, thus representing an alternative to harmful chemical fertilizers for sustainable crop production. Rhizobia have evolved intricate strategies to coordinate N assimilation for their own benefit with N2 fixation to sustain plant growth. The rhizobial N status is transduced by the NtrBC two-component system, the seemingly ubiquitous form of N signal transduction in Proteobacteria. Here, we show that the regulatory sRNA NfeR1 (nodule formation efficiency RNA) of the alfalfa symbiont Sinorhizobium meliloti is transcribed from a complex promoter repressed by NtrC in a N-dependent manner and feedback silences ntrBC by complementary base-pairing. These findings unveil a more prominent role of NtrC as a transcriptional repressor than hitherto anticipated and a novel RNA-based mechanism for NtrBC regulation. The NtrBC-NfeR1 double-negative feedback loop accurately rewires symbiotic S. meliloti N metabolism and is likely conserved in α-rhizobia.

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IMPORTANCE: Infections of the bloodstream are life-threatening and can result in sepsis. Gram-negative bacteria cause a significant portion of bloodstream infections, which is also referred to as bacteremia. The long-term goal of our work is to understand how such bacteria establish and maintain infection during bacteremia. We have previously identified the transcription factor ArcA, which promotes fermentation in bacteria, as a likely contributor to the growth and survival of bacteria in this environment. Here, we study ArcA in the Gram-negative species Citrobacter freundii, Klebsiella pneumoniae, and Serratia marcescens. Our findings aid in determining how these bacteria sense their environment, utilize nutrients, and generate energy while countering the host immune system. This information is critical for developing better models of infection to inform future therapeutic development.

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Bacterial two-component systems are crucial features of bacterial pathogens such as methicillin-resistant Staphylococcus aureus to overcome environmental and antimicrobial stresses by activating regulons to interfere with the bactericidal mechanisms. GraRS is a unique subset of two-component systems belonging to the intramembrane-sensing histidine kinase family (IM-HK) and is responsible for resistance to cationic host defense peptides. However, the precise manner by which the short 9-residue extracellular loop of the membrane sensor GraS detects the antimicrobial peptides and transduces the signal is not comprehensively understood. Here, we show that a single point mutation (D35A) in the extracellular loop of GraS blocked activation of GraRS, but this effect was also abrogated with graS mutations in the N-terminal transmembrane segments without any accompanying effect on GraS protein expression. Additionally, mutations in H120 and T172 in the dimerization/histidine phosphotransfer (DHp) domain of GraS increased activation without any accompanying enhancement in dimerization, likely due to disruption of the H120-T172 interaction that restricts rotational movements of the DHp helices since swapping H120 and T172 did not alter GraS activation. Notably, the enhancing effects of H120 and T172 mutations were abolished with a D35 mutation, highlighting the pivotal role of D35 in the 9-residue extracellular loop of GraS in GraR phosphorylation. In summary, our study delivers the significance of the D35 in the extracellular loop of GraS and ensuing changes in the N-terminal transmembrane helices as a model to illustrate signaling in the IM-HK subset of two-component regulatory systems. IMPORTANCE Methicillin-resistant Staphylococcus aureus (MRSA) is a human pathogen capable of infecting skin, blood, internal organs, and artificial medical devices. Generally, personal hygiene and a robust immune system can limit the spread of this pathogen; however, MRSA possesses an assortment of phenotypic tools to survive the hostile host environment including host defense peptides. More specifically, S. aureus utilizes two-component systems to sense noxious environmental cues to respond to harmful environmental elements. Our study focused on a two-component system called GraRS that S. aureus deploys against host defense peptides. We showed that one single residue in the extracellular loop of GraS and the adjacent membrane segment controlled the activation of GraRS, indicating the importance of a well-tuned-charged residue in the extracellular loop of GraS for sensing activity.

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Clostridioides difficile is a Gram-positive opportunistic pathogen that results in 220,000 infections, 12,000 deaths, and upwards of $1 billion in medical costs in the United States each year. C. difficile is highly resistant to a variety of antibiotics, but we have a poor understanding of how C. difficile senses and responds to antibiotic stress and how such sensory systems affect clinical outcomes. We have identified a spontaneous C. difficile mutant that displays increased daptomycin resistance. We performed whole-genome sequencing and found a nonsense mutation, S605*, in draS, which encodes a putative sensor histidine kinase of a two-component system (TCS). The draSS605* mutant has an ~4- to 8-fold increase in the daptomycin MIC compared to the wild type (WT). We found that the expression of constitutively active DraRD54E in the WT increases daptomycin resistance 8- to 16-fold and increases bacitracin resistance ~4-fold. We found that a selection of lipid II-inhibiting compounds leads to the increased activity of the luciferase-based reporter PdraR-slucopt, including vancomycin, bacitracin, ramoplanin, and daptomycin. Using RNA sequencing (RNA-seq), we identified the DraRS regulon. Interestingly, we found that DraRS can induce the expression of the previously identified hex locus required for the synthesis of a novel glycolipid produced in C. difficile. Our data suggest that the induction of the hex locus by DraR explains some, but not all, of the DraR-induced daptomycin and bacitracin resistance. IMPORTANCE Clostridioides difficile is a major cause of hospital-acquired diarrhea and represents an urgent concern due to the prevalence of antibiotic resistance and the rate of recurrent infections. C. difficile encodes ~50 annotated two-component systems (TCSs); however, only a few have been studied. The function of these unstudied TCSs is not known. Here, we show that the TCS DraRS plays a role in responding to a subset of lipid II-inhibiting antibiotics and mediates resistance to daptomycin and bacitracin in part by inducing the expression of the recently identified hex locus, which encodes enzymes required for the production of a novel glycolipid in C. difficile.

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The control of virulence two-component gene regulatory system (CovRS) is critical to the pathogenesis of many medically important streptococci. In emm1 group A streptococci (GAS), CovR directly binds the promoters of numerous GAS virulence factor-encoding genes. Elimination of CovS phosphatase activity increases CovR phosphorylation (CovR~P) levels and abrogates GAS virulence. Given the emm type-specific diversity of CovRS function, in this study we used chromatin immunoprecipitation sequencing (ChIP-seq) to define global CovR DNA occupancy in the wild-type emm3 strain MGAS10870 (medium CovR~P) and its CovS phosphatase-negative derivative 10870-CovS-T284A (high CovR~P). In the wild-type emm3 strain, 89% of the previously identified emm1 CovR binding sites present in the emm3 genome were also enriched; additionally, we ascertained unique CovR binding, primarily to genes in mobile genetic elements and other sites of interstrain chromosomal differences. Elimination of CovS phosphatase activity specifically increased CovR occupancy at the promoters of a broad array of CovR repressed virulence factor-encoding genes, including those encoding the key GAS regulator Mga and M protein. However, a limited number of promoters had augmented enrichment at low CovR~P levels. Differential motif searches using sequences enriched at high versus low CovR~P levels revealed two distinct binding patterns. At high CovR~P, a pseudopalindromic AT-rich consensus sequence (WTWTTATAAWAAAAWNATDA) consistent with CovR binding as a dimer was determined. Conversely, sequences specifically enriched at low CovR~P contained isolated ATTARA motifs suggesting an interaction with a monomer. These data extend understanding of global CovR DNA occupancy beyond emm1 GAS and provide a mechanism for previous observations regarding hypovirulence induced by CovS phosphatase abrogation. IMPORTANCE Given its key role in pathogenesis of Gram-positive bacteria, CovR is one of the most important members of the OmpR/PhoB family of transcriptional regulators. Herein we extend recent GAS CovR global binding analyses done in emm1 to a non-emm1 strain, which is important considering the known inter-emm-type heterogeneity in GAS CovRS function. Our data provide mechanistic understanding for variation in CovRS function between emm types and the profound hypovirulence of CovS phosphatase-negative strains in addition to indicating differential targeting by phosphorylated and nonphosphorylated CovR isoforms at specific CovR binding sites. These findings advance knowledge regarding how a key bacterial virulence regulator impacts pathogenesis and add to the growing appreciation of the function of nonphosphorylated OmpR/PhoB family members.

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The type VI secretion system (T6SS) is used by bacteria for virulence, resistance to grazing, and competition with other bacteria. We previously demonstrated that the role of the T6SS in interbacterial competition and in resistance to grazing is enhanced in Vibrio cholerae in the presence of subinhibitory concentrations of polymyxin B. Here, we performed a global quantitative proteomic analysis and a targeted transcriptomic analysis of the T6SS-known regulators in V. cholerae grown with and without polymyxin B. The proteome of V. cholerae is greatly modified by polymyxin B with more than 39% of the identified cellular proteins displaying a difference in their abundance, including T6SS-related proteins. We identified a regulator whose abundance and expression are increased in the presence of polymyxin B, vxrB, the response regulator of the two-component system VxrAB (VCA0565-66). In vxrAB, vxrA and vxrB deficient mutants, the expression of both hcp copies (VC1415 and VCA0017), although globally reduced, was not modified by polymyxin B. These hcp genes encode an identical protein Hcp, which is the major component of the T6SS syringe. Thus, the upregulation of the T6SS in the presence of polymyxin B appears to be, at least in part, due to the two-component system VxrAB.

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Metal resistance of Cupriavidus metallidurans is based on determinants that were acquired in the past by horizontal gene transfer during evolution. Some of these determinants encode transmembrane metal efflux systems. Expression of most of the respective genes is controlled by two-component regulatory systems composed of a membrane-bound sensor/sensory histidine kinase (HK) and a cytoplasmic, DNA-binding response regulator (RR). Here, we investigated the interplay between the three closely related two-component regulatory systems CzcRS, CzcR2S2, and AgrRS. All three systems regulate the response regulator CzcR, while the RRs AgrR and CzcR2 were not involved in czc regulation. Target promoters were czcNp and czcPp for genes upstream and downstream of the central czc gene region. The two systems together repressed CzcRS-dependent upregulation of czcP-lacZ at low zinc concentrations in the presence of CzcS but activated this signal transmission at higher zinc concentrations. AgrRS and CzcR2S2 interacted to quench CzcRS-mediated expression of czcNp-lacZ and czcPp-lacZ. Together, cross talk between the three two-component regulatory systems enhanced the capabilities of the Czc systems by controlling expression of the additional genes czcN and czcP. IMPORTANCE Bacteria are able to acquire genes encoding resistance to metals and antibiotics by horizontal gene transfer. To bestow an evolutionary advantage on their host cell, new genes must be expressed, and their expression should be regulated so that resistance-mediating proteins are produced only when needed. Newly acquired regulators may interfere with those already present in a host cell. Such an event was studied here in the metal-resistant bacterium Cupriavidus metallidurans. The results demonstrate how regulation by the acquired genes interacts with the host's extant regulatory network. This leads to emergence of a new system level of complexity that optimizes the response of the cell to periplasmic signals.

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Two-component regulatory systems composed of a membrane-bound sensor/sensory histidine kinase (HK) and a cytoplasmic, DNA-binding response regulator (RR) are often associated with transenvelope efflux systems, which export transition metal cations from the periplasm directly out of the cell. Although much work has been done in this field, more evidence is needed for the hypothesis that the respective two-component regulatory systems are indeed sensing periplasmic ions. If so, a regulatory circuit between the concentration of periplasmic metal cations, sensing of these metals, and control of expression of the genes for transenvelope efflux systems that remove periplasmic cations can be assumed. Escherichia coli possesses only one transenvelope efflux system for metal cations, the Cus system for export of Cu(I) and Ag(I). It is composed of the transenvelope efflux system CusCBA, the periplasmic copper chaperone CusF, and the two-component regulatory system CusS (HK) and CusR (RR). Using phoA- and lacZ-reporter gene fusions, it was verified that an assumed periplasmic part of CusS is located in the periplasm. CusS was more important for copper resistance in E. coli under anaerobic conditions than under aerobic conditions and in complex medium more than in mineral salts medium. Predicted copper-binding sites in the periplasmic part of CusS were identified that, individually, were not essential for copper resistance but were in combination. In summary, evidence was obtained that the two-component regulatory system CusSR that controls expression of cusF and cusCBA does indeed sense periplasmic copper ions. IMPORTANCE Homeostasis of essential-but-toxic transition metal cations such as Zn(II) and Cu(II)/Cu(I) is an important contributor to the fitness of environmental bacteria and pathogenic bacteria during their confrontation with an infected host. Highly efficient removal of threatening concentrations of these metals can be achieved by the combined actions of an inner membrane with a transenvelope efflux system, which removes periplasmic ions after their export from the cytoplasm to this compartment. To understand the resulting metal cation homeostasis in the periplasm, it is important to know if a regulatory circuit exists between periplasmic metal cations, their sensing, and the subsequent control of the expression of the transenvelope efflux system. This publication adds evidence to the hypothesis that two-component regulatory systems in control of the expression of genes for transenvelope efflux systems do indeed sense metal cations in the periplasm.

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The regulation of membrane protein activity for cellular functions is critically dependent on the composition of phospholipid membranes. Cardiolipin, a unique phospholipid found in bacterial membranes and mitochondrial membranes of eukaryotes, plays a crucial role in stabilizing membrane proteins and maintaining their function. In the human pathogen Staphylococcus aureus, the SaeRS two-component system (TCS) controls the expression of key virulence factors essential for the bacterium's virulence. The SaeS sensor kinase activates the SaeR response regulator via phosphoryl transfer to bind its gene target promoters. In this study, we report that cardiolipin is critical for sustaining the full activity of SaeRS and other TCSs in S. aureus. The sensor kinase protein SaeS binds directly to cardiolipin and phosphatidylglycerol, enabling SaeS activity. Elimination of cardiolipin from the membrane reduces SaeS kinase activity, indicating that bacterial cardiolipin is necessary for modulating the kinase activities of SaeS and other sensor kinases during infection. Moreover, the deletion of cardiolipin synthase genes cls1 and cls2 leads to reduced cytotoxicity to human neutrophils and lower virulence in a mouse model of infection. These findings suggest a model where cardiolipin modulates the kinase activity of SaeS and other sensor kinases after infection to adapt to the hostile environment of the host and expand our knowledge of how phospholipids contribute to membrane protein function.

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Coxiella burnetii is an obligate intracellular bacterium and the etiological agent of Q fever in humans. C. burnetii transitions between a replicative, metabolically active large-cell variant (LCV) and a spore-like, quiescent small-cell variant (SCV) as a likely mechanism to ensure survival between host cells and mammalian hosts. C. burnetii encodes three canonical two-component systems, four orphan hybrid histidine kinases, five orphan response regulators, and a histidine phosphotransfer protein, which have been speculated to play roles in the signaling required for C. burnetii morphogenesis and virulence. However, very few of these systems have been characterized. By employing a CRISPR interference system for genetic manipulation of C. burnetii, we created single- and multigene transcriptional knockdown strains targeting most of these signaling genes. Through this, we revealed a role for the C. burnetii PhoBR canonical two-component system in virulence, regulation of [Pi] maintenance, and Pi transport. We also outline a novel mechanism by which PhoBR function may be regulated by an atypical PhoU-like protein. We also determined that the GacA.2/GacA.3/GacA.4/GacS orphan response regulators coordinately and disparately regulate expression of SCV-associated genes in C. burnetii LCVs. These foundational results will inform future studies on the role of C. burnetii two-component systems in virulence and morphogenesis. IMPORTANCE C. burnetii is an obligate intracellular bacterium with a spore-like stability allowing it to survive long periods of time in the environment. This stability is likely due to its biphasic developmental cycle, whereby it can transition from an environmentally stable small-cell variant (SCV) to a metabolically active large-cell variant (LCV). Here, we define the role of two-component phosphorelay systems (TCS) in C. burnetii's ability to survive within the harsh environment contained in the phagolysosome of host cells. We show that the canonical PhoBR TCS has an important role in C. burnetii virulence and phosphate sensing. Further examination of the regulons controlled by orphan regulators indicated a role in modulating gene expression of SCV-associated genes, including genes essential for cell wall remodeling.

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The renowned antimicrobial activity of copper stems in part from its ability to undergo redox cycling between Cu1+/2+ oxidation states. Bacteria counter copper toxicity with a network of sensors that often include two-component signaling systems to direct transcriptional responses. As in typical two-component systems, ligand binding by the extracellular domain of the membrane bound copper sensor component leads to phosphorylation and activation of the cognate response regulator transcription factor. In Listeria monocytogenes, the plasmid-borne CopRS two-component system upregulates both copper resistance and lipoprotein remodeling genes upon copper challenge, but the oxidation state of copper bound by CopS is unknown. Herein, we show CopS utilizes a triad of key residues (His-His-Phe) that are predicted to be at the dimerization interface and that are analogous with the Escherichia coli CusS copper sensor to specifically bind Cu1+/Ag1+ and activate CopR transcription. We demonstrate Cu2+ only induces CopRS if first reduced by electron transport systems, as strains lacking menaquinone carriers were unable to respond to Cu2+. The flavin-dependent extracellular electron transport system (EET) was the main mechanism for metal reduction, capable of either generating inducing ligand (Cu2+ to Cu1+) or removing it by precipitation (Ag1+ to Ag0). We show that EET flux is directly proportional to the rate of Cu2+ reduction and that since EET activity is low under oxygenated conditions when a competing respiratory chain is operating, CopRS signaling in turn is activated only under anaerobic conditions. EET metal reduction thus sensitizes cells to copper while providing resistance to silver under anaerobic growth. IMPORTANCE Two-component extracellular copper sensing from the periplasm of Gram-negative bacteria has been well studied, but copper detection at the cell surface of the Gram-positive L. monocytogenes is less understood. Collectively, our results show that EET is most active under anaerobic conditions and reduces Cu2+ and Ag1+ to, respectively, generate or remove the monovalent ligands that directly bind to CopS and lead to the induction of lipoprotein remodeling genes. This reducing activity regulates CopRS signaling and links the upregulation of copper resistance genes with increasing EET flux. Our studies provide insight into how a two-component copper sensing system is integrated into a model monoderm Firmicute to take cues from the electron transport chain activity.

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Bacteria that colonize eukaryotic surfaces interact with numerous antimicrobial host-produced molecules, including host defense peptides, complement, and antibodies. Bacteria have evolved numerous strategies to both detect and resist these molecules, and in the Enterobacterales order of bacteria these include alterations of the cell surface lipopolysaccharide structure and/or charge and the production of proteases that can degrade these antimicrobial molecules. Here, we show that omptin family proteases from Escherichia coli and Citrobacter rodentium are regulated by the PhoPQ system. Omptin protease activity is induced by growth in low Mg2+, and deletion of PhoP dramatically reduces omptin protease activity, transcriptional regulation, and protein levels. We identify conserved PhoP-binding sites in the promoters of the E. coli omptin genes ompT, ompP, and arlC as well as in croP of Citrobacter rodentium and show that mutation of the putative PhoP-binding site in the ompT promoter abrogates PhoP-dependent expression. Finally, we show that although regulation by PhoPQ is conserved, each of the omptin proteins has differential activity toward host defense peptides, complement components, and resistance to human serum, suggesting that each omptin confers unique survival advantages against specific host antimicrobial factors.

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The ability to sense and respond rapidly to the dynamic environment of the upper respiratory tract (URT) makes Streptococcus pneumoniae (Spn) a highly successful human pathogen. Two-component systems (TCSs) of Spn sense and respond to multiple signals it encounters allowing Spn to adapt and thrive in various host sites. Spn TCS have been implicated in their ability to promote pneumococcal colonization of the URT and virulence. As the disease state can be a dead-end for a pathogen, we considered whether TCS would contribute to pneumococcal transmission. Herein, we determined the role of YesMN, an understudied TCS of Spn, and observe that YesMN contributes toward pneumococcal shedding and transmission but is not essential for colonization. The YesMN regulon includes genes involved in zinc homeostasis and glycan metabolism, which are upregulated during reduced zinc availability in a YesMN-dependent fashion. Thus, we identified the YesMN regulon and a potential molecular signal it senses that lead to the activation of genes involved in zinc homeostasis and glycan metabolism. Furthermore, in contrast to Spn monoinfection, we demonstrate that YesMN is critical for high pneumococcal density in the URT during influenza A virus (IAV) coinfection. We attribute reduced colonization of the yesMN mutant possibly due to increased association with and clearance by the mucus covering the URT epithelial surface. Thus, our results highlight the dynamic interactions that occur between Spn and IAV in the URT, and the role that TCSs play in modulation of these interactions.

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