The MarR Repressor of the Multiple Antibiotic Resistance (mar) Operon in Escherichia coli: Prototypic Member of a Family of Bacterial Regulatory Proteins Involved in Sensing Phenolic Compounds

A library of reporters of the global regulators of gene expression in Escherichia coli

The topology of the transcription factor network (TFN) of Escherichia coli is far from uniform, with 22 global regulator (GR) proteins controlling one-third of all genes. So far, their production rates cannot be tracked by comparable fluorescent proteins. We developed a library of fluorescent reporters for 16 GRs for this purpose. Each consists of a single-copy plasmid coding for green fluorescent protein (GFP) fused to the full-length copy of the native promoter. We tracked their activity in exponential and stationary growth, as well as under weak and strong stresses. We show that the reporters have high sensitivity and specificity to all stresses tested and detect single-cell variability in transcription rates. Given the influence of GRs on the TFN, we expect that the new library will contribute to dissecting global transcriptional stress-response programs of E. coli. Moreover, the library can be invaluable in bioindustrial applications that tune those programs to, instead of cell growth, favor productivity while reducing energy consumption.IMPORTANCECells contain thousands of genes. Many genes are involved in the control of cellular activities. Some activities require a few hundred genes to run largely synchronous transcriptional programs. To achieve this, cells have evolved global regulator (GR) proteins that can influence hundreds of genes simultaneously. We have engineered a library of Escherichia coli strains to track the levels over time of these, phenotypically critical, GRs. Each strain has a single-copy plasmid coding for a fast-maturing green fluorescent protein whose transcription is controlled by a copy of the natural GR promoter. By allowing the tracking of GR levels, with sensitivity and specificity, this library should become of wide use in scientific research on bacterial gene expression (from molecular to synthetic biology) and, later, be used in applications in therapeutics and bioindustries.

The intracellular concentrations of fluoroquinolones determined the antibiotic resistance response of Escherichia coli

The extensive consumption of antibiotics has been reported to significantly promote the generation of antibiotic resistance (ABR), however, a quantitative causal relationship between antibiotic exposure and ABR response is absent. This study aimed to pinpoint the accurate regulatory concentration of fluoroquinolones (FQs) and to understand the biochemical mechanism of the mutual action between FQ exposure and FQ resistance response. Highly sensitive analytical methods were developed by using UPLC-MS/MS to determine the total residual, extracellular residual, total intracellular, intracellular residual and intracellular degraded concentration of three representative FQs, including ciprofloxacin (CIP), ofloxacin (OFL) and norfloxacin (NOR), with detection limits in the range of 0.002-0.057 μg/L, and recoveries in the range of 80-93%. The MICs of Escherichia coli (E. coli) were 7.0-31.4-fold of the respective MIC0 after 40-day FQ exposure, and significant negative associations were discovered between the intracellular (residual, degraded or the sum) FQ concentrations and FQ resistance. Transcriptional expression and whole-genome sequencing results indicated that reduced membrane permeability and enhanced multi-drug efflux pumps contributed to the decreasing intracellular concentration. These results unveiled the pivotal role of intracellular concentration in triggering FQ resistance, providing important information to understand the dose-response relationship between FQ exposure and FQ resistance response, and ascertain the target dose metric of FQs for eliminating FQ resistance crisis.

Prophage maintenance is determined by environment-dependent selective sweeps rather than mutational availability

Prophages, viral sequences integrated into bacterial genomes, can be beneficial and costly. Despite the risk of prophage activation and subsequent bacterial death, active prophages are present in most bacterial genomes. However, our understanding of the selective forces that maintain prophages in bacterial populations is limited. Combining experimental evolution with stochastic modeling, we show that prophage maintenance and loss are primarily determined by environmental conditions that alter the net fitness effect of a prophage on its bacterial host. When prophages are too costly, they are rapidly lost through environment-specific sequences of selective sweeps. Conflicting selection pressures that select against the prophage but for a prophage-encoded accessory gene can maintain prophages. The dynamics of prophage maintenance additionally depend on the sociality of this accessory gene. Prophage-encoded genes that exclusively benefit the lysogen maintain prophages at higher frequencies compared with genes that benefit the entire population. That is because the latter can protect phage-free "cheaters," reducing the benefit of maintaining the prophage. Our simulations suggest that environmental variation plays a larger role than mutation rates in determining prophage maintenance. These findings highlight the complexity of selection pressures that act on mobile genetic elements and challenge our understanding of the role of environmental factors relative to random chance events in shaping the evolutionary trajectory of bacterial populations. By shedding light on the key factors that shape microbial populations in the face of environmental changes, our study significantly advances our understanding of the complex dynamics of microbial evolution and diversification.

Salicylic acids and pathogenic bacteria: new perspectives on an old compound

Salicylic acids have been used in human and veterinary medicine for their anti-pyretic, anti-inflammatory, and analgesic properties for centuries. A key role of salicylic acid-immune modulation in response to microbial infection-was first recognized during studies of their botanical origin. The effects of salicylic acid on bacterial physiology are diverse. In many cases, they impose selective pressures leading to development of cross-resistance to antimicrobial compounds. Initial characterization of these interactions was in Escherichia coli, where salicylic acid activates the multiple antibiotic resistance (mar) operon, resulting in decreased antibiotic susceptibility. Studies suggest that stimulation of the mar phenotype presents similarly in closely related Enterobacteriaceae. Salicylic acids also affect virulence in many opportunistic pathogens by decreasing their ability to form biofilms and increasing persister cell populations. It is imperative to understand the effects of salicylic acid on bacteria of various origins to illuminate potential links between environmental microbes and their clinically relevant antimicrobial-resistant counterparts. This review provides an update on known effects of salicylic acid and key derivatives on a variety of bacterial pathogens, offers insights to possible potentiation of current treatment options, and highlights cellular regulatory networks that have been established during the study of this important class of medicines.

Acori Tatarinowii Rhizoma prevents the fluoxetine-induced multiple-drug resistance of Escherichia coli against antibiotics

BACKGROUND

In treating depression, the residual anti-depressant in gut interacts with the microbiome, leading to the appearance of multiple drug resistant (MDR) mutants, which poses a challenge for the treatment of infectious complications. Strategy is needed to combat this issue. Acori Tatarinowii Rhizoma (ATR, rhizome of Acorus tatarinowii Schott, Araceae), a traditional Chinese medicine, has been widely used for treatment of neurological disorders and gastrointestinal digestive disease in China. Here, ATR was demonstrated an excellent MDR-preventing effect in fluoxetine-induced Escherichia coli (E. coli).

AIM OF THE STUDY

This study aimed to reveal the effective role of ATR and its signaling cascades involved in preventing fluoxetine-induced MDR.

MATERIALS AND METHODS

The water extract of ATR was co-applied with sub-minimum inhibitory concentration (100 mg/l) of fluoxetine in E. coli to evaluate its anti-MDR potential. Formation of reactive oxygen species (ROS) and expression of MDR-related genes in bacteria were measured by dichloro-dihydro-fluorescein diacetate assay and real-time PCR, respectively. Two fluorescent dyes, 1-N-phenylnapthylamine and 3,3'-dipropylthiadicarbocyanine were used to analyze the outer membrane permeability and inner membrane depolarization of E. coli. The accumulation of fluoxetine in the treated E. coli was determined via HPLC. The active fraction of ATR was identified.

RESULTS

The water extract of ATR significantly decreased the number of MDR mutants induced by fluoxetine and had half effective concentrations (EC50) of 55.5 μg/ml and 16.8 μg/ml for chloramphenicol and tetracycline, respectively. ATR robustly reversed the fluoxetine-induced superoxide response and membrane damage in E. coli. In addition, the inclusion of ATR significantly reduced the accumulation of fluoxetine in E. coli. After further fractionation, the polysaccharide of ATR was demonstrated as the fraction with the most significant anti-MDR activity.

CONCLUSIONS

This is the first report to investigate the MDR-preventing effect of ATR. The results of this study proposed ATR as an excellent herbal product to prevent MDR issues, as induced by fluoxetine, with the potential to reduce the side effects during the drug therapy of depression.

Fitness Costs of Antibiotic Resistance Impede the Evolution of Resistance to Other Antibiotics

Antibiotic resistance is a major threat to global health, claiming the lives of millions every year. With a nearly dry antibiotic development pipeline, novel strategies are urgently needed to combat resistant pathogens. One emerging strategy is the use of sequential antibiotic therapy, postulated to reduce the rate at which antibiotic resistance evolves. Here, we use the soft agar gradient evolution (SAGE) system to carry out high-throughput in vitro bacterial evolution against antibiotic pressure. We find that evolution of resistance to the antibiotic chloramphenicol (CHL) severely affects bacterial fitness, slowing the rate at which resistance to the antibiotics nitrofurantoin and streptomycin emerges. In vitro acquisition of compensatory mutations in the CHL-resistant cells markedly improves fitness and nitrofurantoin adaptation rates but fails to restore rates to wild-type levels against streptomycin. Genome sequencing reveals distinct evolutionary paths to resistance in fitness-impaired populations, suggesting resistance trade-offs in favor of mitigation of fitness costs. We show that the speed of bacterial fronts in SAGE plates is a reliable indicator of adaptation rates and evolutionary trajectories to resistance. Identification of antibiotics whose mutational resistance mechanisms confer stable impairments may help clinicians prescribe sequential antibiotic therapies that are less prone to resistance evolution.

Simvastatin induces human gut bacterial cell surface genes

Drugs intended to target mammalian cells can have broad off-target effects on the human gut microbiota with potential downstream consequences for drug efficacy and side effect profiles. Yet, despite a rich literature on antibiotic resistance, we still know very little about the mechanisms through which commensal bacteria evade non-antibiotic drugs. Here, we focus on statins, one of the most prescribed drug types in the world and an essential tool in the prevention and treatment of high circulating cholesterol levels. Prior work in humans, mice, and cell culture support an off-target effect of statins on human gut bacteria; however, the genetic determinants of statin sensitivity remain unknown. We confirmed that simvastatin inhibits the growth of diverse human gut bacterial strains grown in communities and in pure cultures. Drug sensitivity varied between phyla and was dose-dependent. We selected two representative simvastatin-sensitive species for more in-depth analysis: Eggerthella lenta (phylum: Actinobacteriota) and Bacteroides thetaiotaomicron (phylum: Bacteroidota). Transcriptomics revealed that both bacterial species upregulate genes in response to simvastatin that alter the cell membrane, including fatty acid biogenesis (E. lenta) and drug efflux systems (B. thetaiotaomicron). Transposon mutagenesis identified a key efflux system in B. thetaiotaomicron that enables growth in the presence of statins. Taken together, these results emphasize the importance of the bacterial cell membrane in countering the off-target effects of host-targeted drugs. Continued mechanistic dissection of the various mechanisms through which the human gut microbiota evades drugs will be essential to understand and predict the effects of drug administration in human cohorts and the potential downstream consequences for health and disease.

Comprehensive Bioinformatics Analysis of the Biodiversity of Lsm Proteins in the Archaea Domain

The Sm protein superfamily includes Sm, like-Sm (Lsm), and Hfq proteins. Sm and Lsm proteins are found in the Eukarya and Archaea domains, respectively, while Hfq proteins exist in the Bacteria domain. Even though Sm and Hfq proteins have been extensively studied, archaeal Lsm proteins still require further exploration. In this work, different bioinformatics tools are used to understand the diversity and distribution of 168 Lsm proteins in 109 archaeal species to increase the global understanding of these proteins. All 109 archaeal species analyzed encode one to three Lsm proteins in their genome. Lsm proteins can be classified into two groups based on molecular weight. Regarding the gene environment of lsm genes, many of these genes are located adjacent to transcriptional regulators of the Lrp/AsnC and MarR families, RNA-binding proteins, and ribosomal protein L37e. Notably, only proteins from species of the class Halobacteria conserved the internal and external residues of the RNA-binding site identified in Pyrococcus abyssi, despite belonging to different taxonomic orders. In most species, the Lsm genes show associations with 11 genes: rpl7ae, rpl37e, fusA, flpA, purF, rrp4, rrp41, hel308, rpoD, rpoH, and rpoN. We propose that most archaeal Lsm proteins are related to the RNA metabolism, and the larger Lsm proteins could perform different functions and/or act through other mechanisms of action.

Optimal transcriptional regulation of dynamic bacterial responses to sudden drug exposures

Cellular responses to the presence of toxic compounds in their environment require prompt expression of the correct levels of the appropriate enzymes, which are typically regulated by transcription factors that control gene expression for the duration of the response. The characteristics of each response dictate the choice of regulatory parameters such as the affinity of the transcription factor to its binding sites and the strength of the promoters it regulates. Although much is known about the dynamics of cellular responses, we still lack a framework to understand how different regulatory strategies evolved in natural systems relate to the selective pressures acting in each particular case. Here, we analyze a dynamical model of a typical antibiotic response in bacteria, where a transcriptionally repressed enzyme is induced by a sudden exposure to the drug that it processes. We identify strategies of gene regulation that optimize this response for different types of selective pressures, which we define as a set of costs associated with the drug, enzyme, and repressor concentrations during the response. We find that regulation happens in a limited region of the regulatory parameter space. While responses to more costly (toxic) drugs favor the usage of strongly self-regulated repressors, responses where expression of enzyme is more costly favor the usage of constitutively expressed repressors. Only a very narrow range of selective pressures favor weakly self-regulated repressors. We use this framework to determine which costs and benefits are most critical for the evolution of a variety of natural cellular responses that satisfy the approximations in our model and to analyze how regulation is optimized in new environments with different demands.

RNA-Seq Analysis Discovers the Critical Role of Rel in ppGpp Synthesis, Pathogenicity, and the VBNC State of Clavibacter michiganensis

The viable but nonculturable (VBNC) state is a unique survival strategy of bacteria in response to stress conditions. It was confirmed that Clavibacter michiganensis, the causal agent of bacterial canker in tomato, could be induced into the VBNC state by exposure to CuSO₄ in an oligotrophic solution. RNA-sequencing analysis was used to monitor the mechanisms of the VBNC state during CuSO₄ induction in C. michiganensis. The results identified that numerous genes involved in stringent response, copper resistance, and stress resistance were upregulated, and some involved in cell division were downregulated significantly. The study investigated the importance of Rel, which is an essential enzyme in the synthesis of the molecular alarmone ppGpp, via the generation of a Δrel mutant and its complementation strain. Biological characterization revealed that deficiency of rel reduced the bacterial growth, production of exopolysaccharides, and pathogenicity as well as ppGpp production. The Δrel mutant increased the sensitivity to environmental stress, exhibiting reduced growth on minimal media and a propensity to enter the VBNC state in response to CuSO₄. These findings have important implications for the understanding of survival mechanism and management of C. michiganensis and other phytopathogenic bacteria.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

As(III) Exposure Induces a Zinc Scarcity Response and Restricts Iron Uptake in High-Level Arsenic-Resistant Paenibacillus taichungensis Strain NC1

The Gram-positive bacterium Paenibacillus taichungensis NC1 was isolated from the Zijin gold-copper mine and shown to display high resistance to arsenic (MICs of 10 mM for arsenite in minimal medium). Genome sequencing indicated the presence of a number of potential arsenic resistance determinants in NC1. Global transcriptomic analysis under arsenic stress showed that NC1 not only directly upregulated genes in an arsenic resistance operon but also responded to arsenic toxicity by increasing the expression of genes encoding antioxidant functions, such as cat, perR, and gpx. In addition, two highly expressed genes, marR and arsV, encoding a putative flavin-dependent monooxygenase and located adjacent to the ars resistance operon, were highly induced by As(III) exposure and conferred resistance to arsenic and antimony compounds. Interestingly, the zinc scarcity response was induced under exposure to high concentrations of arsenite, and genes responsible for iron uptake were downregulated, possibly to cope with oxidative stress associated with As toxicity. IMPORTANCE Microbes have the ability to adapt and respond to a variety of conditions. To better understand these processes, we isolated the arsenic-resistant Gram-positive bacterium Paenibacillus taichungensis NC1 from a gold-copper mine. The transcriptome responding to arsenite exposure showed induction of not only genes encoding arsenic resistance determinants but also genes involved in the zinc scarcity response. In addition, many genes encoding functions involved in iron uptake were downregulated. These results help to understand how bacteria integrate specific responses to arsenite exposure with broader physiological responses.

Dynamic Boolean modelling reveals the influence of energy supply on bacterial efflux pump expression

Antimicrobial resistance (AMR) is a global health issue. One key factor contributing to AMR is the ability of bacteria to export drugs through efflux pumps, which relies on the ATP-dependent expression and interaction of several controlling genes. Recent studies have shown that significant cell-to-cell ATP variability exists within clonal bacterial populations, but the contribution of intrinsic cell-to-cell ATP heterogeneity is generally overlooked in understanding efflux pumps. Here, we consider how ATP variability influences gene regulatory networks controlling expression of efflux pump genes in two bacterial species. We develop and apply a generalizable Boolean modelling framework, developed to incorporate the dependence of gene expression dynamics on available cellular energy supply. Theoretical results show that differences in energy availability can cause pronounced downstream heterogeneity in efflux gene expression. Cells with higher energy availability have a superior response to stressors. Furthermore, in the absence of stress, model bacteria develop heterogeneous pulses of efflux pump gene expression which contribute to a sustained sub-population of cells with increased efflux expression activity, potentially conferring a continuous pool of intrinsically resistant bacteria. This modelling approach thus reveals an important source of heterogeneity in cell responses to antimicrobials and sheds light on potentially targetable aspects of efflux pump-related antimicrobial resistance.

Plant Bioactive Compounds as an Intrinsic and Sustainable Tool to Enhance the Microbial Safety of Crops

Outbreaks of produce-associated foodborne illness continue to pose a threat to human health worldwide. New approaches are necessary to improve produce safety. Plant innate immunity has potential as a host-based strategy for the deactivation of enteric pathogens. In response to various biotic and abiotic threats, plants mount defense responses that are governed by signaling pathways. Once activated, these result in the release of reactive oxygen and nitrogen species in addition to secondary metabolites that aim at tempering microbial infection and pest attack. These phytochemicals have been investigated as alternatives to chemical sanitization, as many are effective antimicrobial compounds in vitro. Their antagonistic activity toward enteric pathogens may also provide an intrinsic hurdle to their viability and multiplication in planta. Plants can detect and mount basal defenses against enteric pathogens. Evidence supports the role of plant bioactive compounds in the physiology of Salmonella enterica, Escherichia coli, and Listeria monocytogenes as well as their fitness on plants. Here, we review the current state of knowledge of the effect of phytochemicals on enteric pathogens and their colonization of plants. Further understanding of the interplay between foodborne pathogens and the chemical environment on/in host plants may have lasting impacts on crop management for enhanced microbial safety through translational applications in plant breeding, editing technologies, and defense priming.

Accurate annotation of protein coding sequences with IDTAXA

The observed diversity of protein coding sequences continues to increase far more rapidly than knowledge of their functions, making classification algorithms essential for assigning a function to proteins using only their sequence. Most pipelines for annotating proteins rely on searches for homologous sequences in databases of previously annotated proteins using BLAST or HMMER. Here, we develop a new approach for classifying proteins into a taxonomy of functions and demonstrate its utility for genome annotation. Our algorithm, IDTAXA, was more accurate than BLAST or HMMER at assigning sequences to KEGG ortholog groups. Moreover, IDTAXA correctly avoided classifying sequences with novel functions to existing groups, which is a common error mode for classification approaches that rely on E-values as a proxy for confidence. We demonstrate IDTAXA's utility for annotating eukaryotic and prokaryotic genomes by assigning functions to proteins within a multi-level ontology and applied IDTAXA to detect genome contamination in eukaryotic genomes. Finally, we re-annotated 8604 microbial genomes with known antibiotic resistance phenotypes to discover two novel associations between proteins and antibiotic resistance. IDTAXA is available as a web tool (http://DECIPHER.codes/Classification.html) or as part of the open source DECIPHER R package from Bioconductor.

High-throughput functional variant screens via in vivo production of single-stranded DNA

Creating and characterizing individual genetic variants remains limited in scale, compared to the tremendous variation both existing in nature and envisioned by genome engineers. Here we introduce retron library recombineering (RLR), a methodology for high-throughput functional screens that surpasses the scale and specificity of CRISPR-Cas methods. We use the targeted reverse-transcription activity of retrons to produce single-stranded DNA (ssDNA) in vivo, incorporating edits at >90% efficiency and enabling multiplexed applications. RLR simultaneously introduces many genomic variants, producing pooled and barcoded variant libraries addressable by targeted deep sequencing. We use RLR for pooled phenotyping of synthesized antibiotic resistance alleles, demonstrating quantitative measurement of relative growth rates. We also perform RLR using the sheared genomic DNA of an evolved bacterium, experimentally querying millions of sequences for causal variants, demonstrating that RLR is uniquely suited to utilize large pools of natural variation. Using ssDNA produced in vivo for pooled experiments presents avenues for exploring variation across the genome.

Sub-lethal Concentrations of Phytochemicals (Carvacrol and Oregano) Select for Reduced Susceptibility Mutants of Escherichia coli O23:H52

In vitro studies aimed at studying the mechanism of action of carvacrol and oregano as natural anti-bacterial agents to control multiple antibiotic-resistant avian pathogenic Escherichia coli (APEC) strain O23:H52 isolated from chicken were performed. Derivatives with increased minimum inhibitory concentrations (MIC) to the phytochemicals were selected after growing Escherichia coli (E. coli) strain O23:H52 at sub-lethal concentrations of carvacrol and oregano for a period of 60 days. Whole-genome sequencing (WGS) of two derivatives revealed a missense mutation in cadC and marR: the genes responsible for survival mechanisms and antibiotic resistance by efflux, respectively.

In vitro studies aimed at studying the mechanism of action of carvacrol and oregano as natural anti-bacterial agents to control multiple antibiotic-resistant avian pathogenic Escherichia coli (APEC) strain O23:H52 isolated from chicken were performed. Derivatives with increased minimum inhibitory concentrations (MIC) to the phytochemicals were selected after growing Escherichia coli (E. coli) strain O23:H52 at sub-lethal concentrations of carvacrol and oregano for a period of 60 days. Whole-genome sequencing (WGS) of two derivatives revealed a missense mutation in cadC and marR: the genes responsible for survival mechanisms and antibiotic resistance by efflux, respectively.

1-benzyl-1,4-diazepane reduces the efflux of resistance-nodulation-cell division pumps in Escherichia coli

Aim: To investigate the action mechanism of 1-benzyl-1,4-diazepane (1-BD) as efflux pump inhibitor (EPI) in Escherichia coli mutants: ΔacrAB or overexpressing AcrAB and AcrEF efflux pumps. Materials & methods: Effect of 1-BD on: antibiotic potentiation, by microdilution method; membrane functionality, by fluorimetric assays; ethidium bromide accumulation, by fluorometric real-time efflux assay; AcrB expression, by quantitative photoactivated localization microscopy. Results: 1-BD decreases the minimal inhibitory concentration of levofloxacin and other antibiotics and increase ethidium bromide accumulation in E. coli overexpressing efflux pumps but not in the ΔacrAB strain. 1-BD increases membranes permeability, without sensibly affecting inner membrane polarity and decreases acrAB transcription. Conclusion: 1-BD acts as an EPI in E. coli with a mixed mechanism, different from that of major reference EPIs.

CarR, a MarR-family regulator from Corynebacterium glutamicum, modulated antibiotic and aromatic compound resistance

MarR (multiple antibiotic resistance regulator) proteins are a family of transcriptional regulators that is prevalent in Corynebacterium glutamicum. Understanding the physiological and biochemical function of MarR homologs in C. glutamicum has focused on cysteine oxidation-based redox-sensing and substrate metabolism-involving regulators. In this study, we characterized the stress-related ligand-binding functions of the C. glutamicum MarR-type regulator CarR (C. glutamicum antibiotic-responding regulator). We demonstrate that CarR negatively regulates the expression of the carR (ncgl2886)-uspA (ncgl2887) operon and the adjacent, oppositely oriented gene ncgl2885, encoding the hypothetical deacylase DecE. We also show that CarR directly activates transcription of the ncgl2882-ncgl2884 operon, encoding the peptidoglycan synthesis operon (PSO) located upstream of carR in the opposite orientation. The addition of stress-associated ligands such as penicillin and streptomycin induced carR, uspA, decE, and PSO expression in vivo, as well as attenuated binding of CarR to operator DNA in vitro. Importantly, stress response-induced up-regulation of carR, uspA, and PSO gene expression correlated with cell resistance to β-lactam antibiotics and aromatic compounds. Six highly conserved residues in CarR were found to strongly influence its ligand binding and transcriptional regulatory properties. Collectively, the results indicate that the ligand binding of CarR induces its dissociation from the carR-uspA promoter to derepress carR and uspA transcription. Ligand-free CarR also activates PSO expression, which in turn contributes to C. glutamicum stress resistance. The outcomes indicate that the stress response mechanism of CarR in C. glutamicum occurs via ligand-induced conformational changes to the protein, not via cysteine oxidation-based thiol modifications.

CosR Is a Global Regulator of the Osmotic Stress Response with Widespread Distribution among Bacteria

Bacteria accumulate small, organic compounds called compatible solutes via uptake from the environment or biosynthesis from available precursors to maintain the turgor pressure of the cell in response to osmotic stress. The halophile Vibrio parahaemolyticus has biosynthesis pathways for the compatible solutes ectoine (encoded by ectABC-asp_ect) and glycine betaine (encoded by betIBA-proXWV), four betaine-carnitine-choline transporters (encoded by bccT1 to bccT4), and a second ProU transporter (encoded by proVWX). All of these systems are osmotically inducible with the exception of bccT2 Previously, it was shown that CosR, a MarR-type regulator, was a direct repressor of ectABC-asp_ect in Vibrio species. In this study, we investigated whether CosR has a broader role in the osmotic stress response. Expression analyses demonstrated that betIBA-proXWV, bccT1, bccT3, bccT4, and proVWX are repressed in low salinity. Examination of an in-frame cosR deletion mutant showed that expression of these systems is derepressed in the mutant at low salinity compared with the wild type. DNA binding assays demonstrated that purified CosR binds directly to the regulatory region of both biosynthesis systems and four transporters. In Escherichia coli green fluorescent protein (GFP) reporter assays, we demonstrated that CosR directly represses transcription of betIBA-proXWV, bccT3, and proVWX Similar to Vibrio harveyi, we showed betIBA-proXWV was directly activated by the quorum-sensing LuxR homolog OpaR, suggesting a conserved mechanism of regulation among Vibrio species. Phylogenetic analysis demonstrated that CosR is ancestral to the Vibrionaceae family, and bioinformatics analysis showed widespread distribution among Gammaproteobacteria in general. Incidentally, in Aliivibrio fischeri, Aliivibrio finisterrensis, Aliivibrio sifiae, and Aliivibrio wodanis, an unrelated MarR-type regulator gene named ectR was clustered with ectABC-asp, which suggests the presence of another novel ectoine biosynthesis regulator. Overall, these data show that CosR is a global regulator of osmotic stress response that is widespread among bacteria.IMPORTANCE Vibrio parahaemolyticus can accumulate compatible solutes via biosynthesis and transport, which allow the cell to survive in high salinity conditions. There is little need for compatible solutes under low salinity conditions, and biosynthesis and transporter systems need to be repressed. However, the mechanism(s) of this repression is not known. In this study, we showed that CosR played a major role in the regulation of multiple compatible solute systems. Phylogenetic analysis showed that CosR is present in all members of the Vibrionaceae family as well as numerous Gammaproteobacteria Collectively, these data establish CosR as a global regulator of the osmotic stress response that is widespread in bacteria, controlling many more systems than previously demonstrated.

MarR family proteins are important regulators of clinically relevant antibiotic resistance

There has been a rapid spread of multidrug-resistant (MDR) bacteria across the world. MDR efflux transporters are an important mechanism of antibiotic resistance in many pathogens among both Gram positive and Gram negative bacteria. These pumps can recognize a variety of chemically and structurally different compounds, including innate and clinically administered antibiotics. Intriguingly, these efflux pumps are often regulated by transcription factors that themselves bind a diverse set of substrates thereby allowing them to regulate the expression of their cognate MDR efflux pumps. One significant family of such transcription factors is the Multiple antibiotic resistance Repressor (MarR) family. Members of this family are well conserved across different bacterial species and in some cases are known to regulate vital bacterial functions. This review focusses on the role of MarR family transcriptional factors in antibiotic resistance within a select group of clinically relevant pathogens.

The mutational landscape of quinolone resistance in Escherichia coli

The evolution of antibiotic resistance is influenced by a variety of factors, including the availability of resistance mutations, and the pleiotropic effects of such mutations. Here, we isolate and characterize chromosomal quinolone resistance mutations in E. coli, in order to gain a systematic understanding of the rate and consequences of resistance to this important class of drugs. We isolated over fifty spontaneous resistance mutants on nalidixic acid, ciprofloxacin, and levofloxacin. This set of mutants includes known resistance mutations in gyrA, gyrB, and marR, as well as two novel gyrB mutations. We find that, for most mutations, resistance tends to be higher to nalidixic acid than relative to the other two drugs. Resistance mutations had deleterious impacts on one or more growth parameters, suggesting that quinolone resistance mutations are generally costly. Our findings suggest that the prevalence of specific gyrA alleles amongst clinical isolates are driven by high levels of resistance, at no more cost than other resistance alleles.

Biology of MarR family transcription factors and implications for targets of antibiotics against tuberculosis

The emergence of multidrug resistant (MDR) Mycobacterium tuberculosis strains and increased incidence of HIV coinfection fueled the difficulty in controlling tuberculosis (TB). MarR (multiple antibiotic resistance regulator) family transcription factors can regulate marRAB operon and are involved in resistance to multiple environmental stresses. We have summarized the structure, function, distribution, and regulation of the MarR family proteins, as well as their implications for novel targets for antibiotics, especially for tuberculosis.

Transcriptomic Changes in Medicago truncatula and Lotus japonicus Root Nodules during Drought Stress

Drought is one of the major environmental factors limiting biomass and seed yield production in agriculture. In this research, we focused on plants from the Fabaceae family, which has a unique ability for the establishment of symbiosis with nitrogen-fixing bacteria, and are relatively susceptible to water limitation. We have presented the changes in nitrogenase activity and global gene expression occurring in Medicago truncatula and Lotus japonicus root nodules during water deficit. Our results proved a decrease in the efficiency of nitrogen fixation, as well as extensive changes in plant and bacterial transcriptomes, shortly after watering cessation. We showed for the first time that not only symbiotic plant components but also Sinorhizobium meliloti and Mesorhizobium loti bacteria residing in the root nodules of M. truncatula and L. japonicus, respectively, adjust their gene expression in response to water shortage. Although our results demonstrated that both M. truncatula and L. japonicus root nodules were susceptible to water deprivation, they indicated significant differences in plant and bacterial response to drought between the tested species, which might be related to the various types of root nodules formed by these species.

Cross Talk between MarR-Like Transcription Factors Coordinates the Regulation of Motility in Uropathogenic Escherichia coli

The MarR-like protein PapX represses the transcription of the flagellar master regulator genes flhDC in uropathogenic Escherichia coli (UPEC), the primary cause of uncomplicated urinary tract infections (UTIs). PapX is encoded by the pap operon, which also encodes the adherence factors termed P fimbriae.

The MarR-like protein PapX represses the transcription of the flagellar master regulator genes flhDC in uropathogenic Escherichia coli (UPEC), the primary cause of uncomplicated urinary tract infections (UTIs). PapX is encoded by the pap operon, which also encodes the adherence factors termed P fimbriae. Both adherence and motility are critical for productive colonization of the urinary tract. However, the mechanisms involved in coordinating the transition between adherence and motility are not well characterized. UPEC strain CFT073 carries both papX and a homolog, focX, located in the foc operon encoding F1C fimbriae. In this study, we characterized the dose effects of “X” genes on flagellar gene expression and cross talk between focX and papX. We found that both FocX and PapX repress flhD transcription. However, we determined that the ΔpapX mutant was hypermotile, while the loss of focX did not affect motility. We further investigated this phenotype and found that FocX functions as a repressor of papX. Additionally, we identified a proximal independent promoter upstream of both focX and papX and assessed the expression of focX and papX during culture in human urine and on LB agar plates compared to LB medium. Finally, we characterized the contributions of PapX and FocX to fitness in the ascending murine model of UTI and observed a subtle, but not statistically significant, fitness defect in colonization of the kidneys. Altogether, these results expand our understanding of the impact of carrying multiple X genes on the coordinated regulation of motility and adherence in UPEC.

Over expression of the constitutive expression refractory Mycobacterium tuberculosis Rv3488 protein with an acid-inducible expression system

As inducible protein expression plasmids available for mycobacterial species are limited, here we demonstrate the utility of an acid-inducible promoter driving gene and subsequent protein expression of a difficult to express protein. We wanted to assess the use of an acid inducible promoter expression system to produce proteins in a mycobacterial system, specifically proteins that when overexpressed interfere with mycobacterial cell growth. Overexpression of those types of proteins would require a tightly regulated promoter system. We employed the Mycobacterium tuberculosis lipF minimal acid-inducible promoter (mpr) which had previously been shown to be upregulated by acidic stress >100 X and to have a low basal level of expression in the absence of acidic stress. It is active from pH 4.3 up to pH 6.4 making this an acid range that is compatible with mycobacterial growth or survival and active at acidic ranges encountered in vivo within acidified phagosomes of macrophages. We therefore cloned the M. tuberculosis gene Rv3488, whose product had been unable to be expressed constitutively, into a plasmid downstream of the lipF mpr promoter and overexpressed this gene in the presence of acidic pH in Mycobacterium smegmatis. Sustained overexpression of the gene leads to inhibition of replication of mycobacterial cells as well as inhibition of carotenoid pigment synthesis, while short-term overexpression leads to appropriate protein expression.

Non-antibiotic antimicrobial triclosan induces multiple antibiotic resistance through genetic mutation

Antibiotic resistance poses a major threat to public health. Overuse and misuse of antibiotics are generally recognized as the key factors contributing to antibiotic resistance. However, whether non-antibiotic, anti-microbial (NAAM) chemicals can directly induce antibiotic resistance is unclear. We aim to investigate whether the exposure to a NAAM chemical triclosan (TCS) has an impact on inducing antibiotic resistance on Escherichia coli. Here, we report that at a concentration of 0.2 mg/L TCS induces multi-drug resistance in wild-type Escherichia coli after 30-day TCS exposure. The oxidative stress induced by TCS caused genetic mutations in genes such as fabI, frdD, marR, acrR and soxR, and subsequent up-regulation of the transcription of genes encoding beta-lactamases and multi-drug efflux pumps, together with down-regulation of genes related to membrane permeability. The findings advance our understanding of the potential role of NAAM chemicals in the dissemination of antibiotic resistance in microbes, and highlight the need for controlling biocide applications.

Whole-Genome Sequencing and Genetic Analysis Reveal Novel Stress Responses to Individual Constituents of Essential Oils in Escherichia coli

Food preservation by the use of essential oils (EOs) is being extensively studied because of the antimicrobial properties of their individual constituents (ICs). Three resistant mutants (termed CAR, CIT, and LIM) of Escherichia coli MG1655 were selected by subculturing with the ICs carvacrol, citral, and (+)-limonene oxide, respectively. These derivative strains showed increased MIC values of ICs and concomitantly enhanced resistance to various antibiotics (ampicillin, trimethoprim, chloramphenicol, tetracycline, kanamycin, novobiocin, norfloxacin, cephalexin, and nalidixic acid) compared to those for the parental strain (wild type [WT]). Whole-genome sequencing (WGS) of these hyperresistant strains permitted the identification of single nucleotide polymorphisms (SNPs) and deletions in comparison to the WT. In order to analyze the contribution of these mutations to the increased antimicrobial resistance detected in hyperresistant strains, derivative strains were constructed by allelic reversion. A role of the SoxR D137Y missense mutation in CAR was confirmed by growth in the presence of some ICs and antibiotics and by its tolerance to ICs but not to lethal heat treatments. In CIT, increased resistance relied on contributions by several detected SNPs, resulting in a frameshift in MarR and an in-frame GyrB ΔG157 mutation. Finally, both the insertion resulting in an AcrR frameshift and large chromosomal deletions found in LIM were correlated with the hyperresistant phenotype of this strain. The nature of the obtained mutants suggests intriguing links to cellular defense mechanisms previously implicated in antibiotic resistance.IMPORTANCE The antimicrobial efficacy of ICs has been proven over the years, together with their potential to improve traditional heat treatments by reducing treatment intensity and, consequently, adverse effects on food quality. However, the mechanisms of bacterial inactivation by ICs are still not well understood, in contrast to antibiotics. We performed WGS of three E. coli strains that are hyperresistant to ICs. The information provided detailed insight into the mechanisms of bacterial resistance arising from exposure to carvacrol, citral, and (+)-limonene oxide. Future experiments will undoubtedly yield additional insights into genes and pathways contributing to the acquisition of endogenous resistance to ICs.

Regulation of the AcrAB-TolC efflux pump in Enterobacteriaceae

Bacterial multidrug efflux systems are a major mechanism of antimicrobial resistance and are fundamental to the physiology of Gram-negative bacteria. The resistance-nodulation-division (RND) family of efflux pumps is the most clinically significant, as it is associated with multidrug resistance. Expression of efflux systems is subject to multiple levels of regulation, involving local and global transcriptional regulation as well as post-transcriptional and post-translational regulation. The best-characterised RND system is AcrAB-TolC, which is present in Enterobacteriaceae. This review describes the current knowledge and new data about the regulation of the acrAB and tolC genes in Escherichia coli and Salmonella enterica.

Inactivation of MarR gene homologs increases susceptibility to antimicrobials in Bacteroides fragilis

Bacteroides fragilis is the strict anaerobic bacteria most commonly found in human infections, and has a high mortality rate. Among other virulence factors, the remarkable ability to acquire resistance to a variety of antimicrobial agents and to tolerate nanomolar concentrations of oxygen explains in part their success in causing infection and colonizing the mucosa. Much attention has been given to genes related to multiple drug resistance derived from plasmids, integrons or transposon, but such genes are also detected in chromosomal systems, like the mar (multiple antibiotic resistance) locus, that confer resistance to a range of drugs. Regulators like MarR, that control expression of the locus mar, also regulate resistance to organic solvents, disinfectants and oxygen reactive species are important players in these events. Strains derived from the parental strain 638R, with mutations in the genes hereby known as marRI (BF638R_3159) and marRII (BF638R_3706) were constructed by gene disruption using a suicide plasmid. Phenotypic response of the mutant strains to hydrogen peroxide, cell survival assay against exposure to oxygen, biofilm formation, resistance to bile salts and resistance to antibiotics was evaluated. The results showed that the mutant strains exhibit statistically significant differences in their response to oxygen stress, but no changes were observed in survival when exposed to bile salts. Biofilm formation was not affected by either gene disruption. Both mutant strains however, became more sensitive to multiple antimicrobial drugs tested. This indicates that as observed in other bacterial species, MarR are an important resistance mechanism in B. fragilis.

Structural analysis of the regulatory mechanism of MarR protein Rv2887 in M. tuberculosis

MarR family proteins are transcriptional regulators that control expression of bacterial proteins involved in metabolism, virulence, stress responses and multi-drug resistance, mainly via ligand-mediated attenuation of DNA binding. Greater understanding of their underlying regulatory mechanism may open up new avenues for the effective treatment of bacterial infections. To gain molecular insight into the mechanism of Rv2887, a MarR family protein in M. tuberculosis, we first showed that it binds salicylate (SA) and para-aminosalicylic acid (PAS), its structural analogue and an antitubercular drug, in a 1:1 stoichiometry with high affinity. Subsequent determination and analysis of Rv2887 crystal structures in apo form, and in complex with SA, PAS and DNA showed that SA and PAS bind to Rv2887 at similar sites, and that Rv2887 interacts with DNA mainly by insertion of helix α4 into the major groove. Ligand binding triggers rotation of the wHTH domain of Rv2887 toward the dimerization domain, causing changes in protein conformation such that it can no longer bind to a 27 bp recognition sequence in the upstream region of gene Rv0560c. The structures provided here lay a foundation for the design of small molecules that target Rv2887, a potential new approach for the development of anti-mycobacterials.

LdtR is a master regulator of gene expression in Liberibacter asiaticus

Huanglongbing or citrus greening disease is causing devastation to the citrus industry. Liberibacter asiaticus, an obligate intracellular pathogen of citrus, is one the causative agents of the disease. Most of the knowledge about this bacterium has been deduced from the in silico exploration of its genomic sequence. L. asiaticus differentially expresses genes during its transmission from the psyllid vector, Diaphorina citri, to the plant. However, the regulatory mechanisms for the adaptation of the bacterium into either hosts remain unknown. Here we show that LdtR, a MarR family transcriptional regulator, activates or represses transcription genome-wide. We performed a double approach to identify the components of the LdtR regulon: a transcriptome analysis in both the related bacterium Liberibacter crescens and citrus-infected leaves, strengthened with an in silico prediction of LdtR regulatory sites. Our results demonstrated that LdtR controls the expression of nearly 180 genes in L. asiaticus, distributed in processes such as cell motility, cell wall biogenesis, energy production, and transcription. These results provide new evidence about the regulatory network of L. asiaticus, where the differential expression of genes from these functional categories could be of great importance during the adaptation of the bacterium to either hosts.

Indole-Induced Activities of β-Lactamase and Efflux Pump Confer Ampicillin Resistance in Pseudomonas putida KT2440

Indole, which is widespread in microbial communities, has received attention because of its effects on bacterial physiology. Pseudomonas putida and Pseudomonas aeruginosa can acquire ampicillin (Amp) resistance during growth on indole-Amp agar. Transcriptome, mutant, and inhibitor studies have suggested that Amp resistance induced by indole can be attributed to increased gene expression of ttgAB encoding two genes of RND-type multidrug efflux operons and an ampC encoding β-lactamase. Expression, enzyme activities, and mutational analyses indicated that AmpC β-lactamase is important for acquiring Amp resistance of P. putida in the presence of indole. Here, we show, for the first time, that volatile indole increased Amp-resistant cells. Consistent with results of the volatile indole assay, a low concentration of indole in liquid culture promoted growth initially, but led to mutagenesis after indole was depleted, which could not be observed at high indole concentrations. Interestingly, ttgAB and ampC gene expression levels correlate with the concentration of indole, which might explain the low number of Amp-mutated cells in high indole concentrations. The expression levels of genes involved in mutagenesis, namely rpoS, recA, and mutS, were also modulated by indole. Our data indicates that indole reduces Amp-induced heterogeneity by promoting expression of TtgABC or MexAB-OprM efflux pumps and the indole-induced β-lactamase in P. putida and P. aeruginosa.

Agrobacterium tumefaciens estC, Encoding an Enzyme Containing Esterase Activity, Is Regulated by EstR, a Regulator in the MarR Family

Analysis of the A. tumefaciens genome revealed estC, which encodes an esterase located next to its transcriptional regulator estR, a regulator of esterase in the MarR family. Inactivation of estC results in a small increase in the resistance to organic hydroperoxides, whereas a high level of expression of estC from an expression vector leads to a reduction in the resistance to organic hydroperoxides and menadione. The estC gene is transcribed divergently from its regulator, estR. Expression analysis showed that only high concentrations of cumene hydroperoxide (CHP, 1 mM) induced expression of both genes in an EstR-dependent manner. The EstR protein acts as a CHP sensor and a transcriptional repressor of both genes. EstR specifically binds to the operator sites OI and OII overlapping the promoter elements of estC and estR. This binding is responsible for transcription repression of both genes. Exposure to organic hydroperoxide results in oxidation of the sensing cysteine (Cys16) residue of EstR, leading to a release of the oxidized repressor from the operator sites, thereby allowing transcription and high levels of expression of both genes. The estC is the first organic hydroperoxide-inducible esterase-encoding gene in alphaproteobacteria.

Control of MarRAB Operon in Escherichia coli via Autoactivation and Autorepression

Choice of network topology for gene regulation has been a question of interest for a long time. How do simple and more complex topologies arise? In this work, we analyze the topology of the marRAB operon in Escherichia coli, which is associated with control of expression of genes associated with conferring resistance to low-level antibiotics to the bacterium. Among the 2102 promoters in E. coli, the marRAB promoter is the only one that encodes for an autoactivator and an autorepressor. What advantages does this topology confer to the bacterium? In this work, we demonstrate that, compared to control by a single regulator, the marRAB regulatory arrangement has the least control cost associated with modulating gene expression in response to environmental stimuli. In addition, the presence of dual regulators allows the regulon to exhibit a diverse range of dynamics, a feature that is not observed in genes controlled by a single regulator.

The yjdF riboswitch candidate regulates gene expression by binding diverse azaaromatic compounds

The yjdF motif RNA is an orphan riboswitch candidate that almost exclusively associates with the yjdF protein-coding gene in many bacteria. The function of the YjdF protein is unknown, which has made speculation regarding the natural ligand for this putative riboswitch unusually challenging. By using a structure-probing assay for ligand binding, we found that a surprisingly broad diversity of nitrogen-containing aromatic heterocycles, or "azaaromatics," trigger near-identical changes in the structures adopted by representative yjdF motif RNAs. Regions of the RNA that undergo ligand-induced structural modulation reside primarily in portions of the putative aptamer region that are highly conserved in nucleotide sequence, as is typical for riboswitches. Some azaaromatic molecules are bound by the RNA with nanomolar dissociation constants, and a subset of these ligands activate riboswitch-mediated gene expression in cells. Furthermore, genetic elements most commonly adjacent to the yjdF motif RNA or to the yjdF protein-coding region are homologous to protein regulators implicated in mitigating the toxic effects of diverse phenolic acids or polycyclic compounds. Although the precise type of natural ligand sensed by yjdF motif RNAs remains unknown, our findings suggest that this riboswitch class might serve as part of a genetic response system to toxic or signaling compounds with chemical structures similar to azaaromatics.

Bacterial Multidrug Efflux Pumps: Much More Than Antibiotic Resistance Determinants

Bacterial multidrug efflux pumps are antibiotic resistance determinants present in all microorganisms. With few exceptions, they are chromosomally encoded and present a conserved organization both at the genetic and at the protein levels. In addition, most, if not all, strains of a given bacterial species present the same chromosomally-encoded efflux pumps. Altogether this indicates that multidrug efflux pumps are ancient elements encoded in bacterial genomes long before the recent use of antibiotics for human and animal therapy. In this regard, it is worth mentioning that efflux pumps can extrude a wide range of substrates that include, besides antibiotics, heavy metals, organic pollutants, plant-produced compounds, quorum sensing signals or bacterial metabolites, among others. In the current review, we present information on the different functions that multidrug efflux pumps may have for the bacterial behaviour in different habitats as well as on their regulation by specific signals. Since, in addition to their function in non-clinical ecosystems, multidrug efflux pumps contribute to intrinsic, acquired, and phenotypic resistance of bacterial pathogens, the review also presents information on the search for inhibitors of multidrug efflux pumps, which are currently under development, in the aim of increasing the susceptibility of bacterial pathogens to antibiotics.

Sister Dehalobacter Genomes Reveal Specialization in Organohalide Respiration and Recent Strain Differentiation Likely Driven by Chlorinated Substrates

The genomes of two closely related Dehalobacter strains (strain CF and strain DCA) were assembled from the metagenome of an anaerobic enrichment culture that reductively dechlorinates chloroform (CF), 1,1,1-trichloroethane (1,1,1-TCA) and 1,1-dichloroethane (1,1-DCA). The 3.1 Mbp genomes of strain CF (that dechlorinates CF and 1,1,1-TCA) and strain DCA (that dechlorinates 1,1-DCA) each contain 17 putative reductive dehalogenase homologous (rdh) genes. These two genomes were systematically compared to three other available organohalide-respiring Dehalobacter genomes (Dehalobacter restrictus strain PER-K23, Dehalobacter sp. strain E1 and Dehalobacter sp. strain UNSWDHB), and to the genomes of Dehalococcoides mccartyi strain 195 and Desulfitobacterium hafniense strain Y51. This analysis compared 42 different metabolic and physiological categories. The genomes of strains CF and DCA share 90% overall average nucleotide identity and >99.8% identity over a 2.9 Mbp alignment that excludes large insertions, indicating that these genomes differentiated from a close common ancestor. This differentiation was likely driven by selection pressures around two orthologous reductive dehalogenase genes, cfrA and dcrA, that code for the enzymes that reduce CF or 1,1,1-TCA and 1,1-DCA. The many reductive dehalogenase genes found in the five Dehalobacter genomes cluster into two small conserved regions and were often associated with Crp/Fnr transcriptional regulators. Specialization is on-going on a strain-specific basis, as some strains but not others have lost essential genes in the Wood-Ljungdahl (strain E1) and corrinoid biosynthesis pathways (strains E1 and PER-K23). The gene encoding phosphoserine phosphatase, which catalyzes the last step of serine biosynthesis, is missing from all five Dehalobacter genomes, yet D. restrictus can grow without serine, suggesting an alternative or unrecognized biosynthesis route exists. In contrast to D. mccartyi, a complete heme biosynthesis pathway is present in the five Dehalobacter genomes. This pathway corresponds to a newly described alternative heme biosynthesis route first identified in Archaea. This analysis of organohalide-respiring Firmicutes and Chloroflexi reveals profound evolutionary differences despite very similar niche-specific metabolism and function.

HosA, a MarR Family Transcriptional Regulator, Represses Nonoxidative Hydroxyarylic Acid Decarboxylase Operon and Is Modulated by 4-Hydroxybenzoic Acid

Members of the Multiple antibiotic resistance Regulator (MarR) family of DNA binding proteins regulate transcription of a wide array of genes required for virulence and pathogenicity of bacteria. The present study reports the molecular characterization of HosA (Homologue of SlyA), a MarR protein, with respect to its target gene, DNA recognition motif, and nature of its ligand. Through a comparative genomics approach, we demonstrate that hosA is in synteny with nonoxidative hydroxyarylic acid decarboxylase (HAD) operon and is present exclusively within the mutS-rpoS polymorphic region in nine different genera of Enterobacteriaceae family. Using molecular biology and biochemical approach, we demonstrate that HosA binds to a palindromic sequence downstream to the transcription start site of divergently transcribed nonoxidative HAD operon and represses its expression. Furthermore, in silico analysis showed that the recognition motif for HosA is highly conserved in the upstream region of divergently transcribed operon in different genera of Enterobacteriaceae family. A systematic chemical search for the physiological ligand revealed that 4-hydroxybenzoic acid (4-HBA) interacts with HosA and derepresses HosA mediated repression of the nonoxidative HAD operon. Based on our study, we propose a model for molecular mechanism underlying the regulation of nonoxidative HAD operon by HosA in Enterobacteriaceae family.

The Asp20-to-Asn Substitution in the Response Regulator AdeR Leads to Enhanced Efflux Activity of AdeB in Acinetobacter baumannii

Overexpression of the resistance-nodulation-cell division-type efflux pump AdeABC is often associated with multidrug resistance in Acinetobacter baumannii and has been linked to mutations in the genes encoding the AdeRS two-component system. In a previous study, we reported that the Asp20→Asn amino acid substitution in the response regulator AdeR is associated with adeB overexpression and reduced susceptibility to the antimicrobials levofloxacin, tigecycline, and trimethoprim-sulfamethoxazole. To further characterize the effect of the Asp20→Asn substitution on antimicrobial susceptibility, the expression of the efflux genes adeB, adeJ, and adeG, and substrate accumulation, four plasmid constructs [containing adeR(Asp20)S, adeR(Asn20)S, adeR(Asp20)SABC, and adeR(Asn20)SABC] were introduced into the adeRSABC-deficient A. baumannii isolate NIPH 60. Neither adeRS construct induced changes in antimicrobial susceptibility or substrate accumulation from that for the vector-only control. The adeR(Asp20)SABC transformant showed reduced susceptibility to 6 antimicrobials and accumulated 12% less ethidium than the control, whereas the Asn20 variant showed reduced susceptibility to 6 of 8 antimicrobial classes tested, and its ethidium accumulation was only 72% of that observed for the vector-only construct. adeB expression was 7-fold higher in the adeR(Asn20)SABC transformant than in its Asp20 variant. No changes in adeG or adeJ expression or in acriflavine or rhodamine 6G accumulation were detected. The antimicrobial susceptibility data suggest that AdeRS does not regulate any resistance determinants other than AdeABC. Furthermore, the characterization of the Asp20→Asn20 substitution proves that the reduced antimicrobial susceptibility previously associated with this substitution was indeed caused by enhanced efflux activity of AdeB.

Identification of an archaeal mercury regulon by chromatin immunoprecipitation

Mercury is a heavy metal and toxic to all forms of life. Metal exposure can invoke a response to improve survival. In archaea, several components of a mercury response system have been identified, but it is not known whether metal transport is a member of this system. To identify such missing components, a peptide-tagged MerR transcription factor was used to localize enriched chromosome regions by chromosome immunoprecipitation combined with DNA sequence analysis. Such regions could serve as secondary regulatory binding sites to control the expression of additional genes associated with mercury detoxification. Among the 31 highly enriched loci, a subset of five was pursued as potential candidates based on their current annotations. Quantitative reverse transcription-PCR analysis of these regions with and without mercury treatment in WT and mutant strains lacking merR indicated significant regulatory responses under these conditions. Of these, a Family 5 extracellular solute-binding protein and the MarR transcription factor shown previously to control responses to oxidation were most strongly affected. Inactivation of the solute-binding protein by gene disruption increased the resistance of mutant cells to mercury challenge. Inductively coupled plasma-MS analysis of the mutant cell line following metal challenge indicated there was less intracellular mercury compared with the isogenic WT strain. Together, these regulated genes comprise new members of the archaeal MerR regulon and reveal a cascade of transcriptional control not previously demonstrated in this model organism.

Indole: a signaling molecule or a mere metabolic byproduct that alters bacterial physiology at a high concentration?

Indole is an organic compound that is widespread in microbial communities inhabiting diverse habitats, like the soil environment and human intestines. Measurement of indole production is a traditional method for the identification of microbial species. Escherichia coli can produce millimolar concentrations of indole in the stationary growth phase under nutrient-rich conditions. Indole has received considerable attention because of its remarkable effects on various biological functions of the microbial communities, for example, biofilm formation, motility, virulence, plasmid stability, and antibiotic resistance. Indole may function as an intercellular signaling molecule, like a quorum-sensing signal. Nevertheless, a receptor system for indole and the function of this compound in coordinated behavior of a microbial population (which are requirements for a true signaling molecule) have not yet been confirmed. Recent findings suggest that a long-known quorum-sensing regulator, E. coli's SdiA, cannot recognize indole and that this compound may simply cause membrane disruption and energy reduction, which can lead to various changes in bacterial physiology including unstable folding of a quorum-sensing regulator. Indole appears to be responsible for acquisition of antibiotic resistance via the formation of persister cells and activation of an exporter. This review highlights and summarizes the current knowledge about indole as a multitrophic molecule among bacteria, together with recently identified new avenues of research.

Adaptive resistance in bacteria requires epigenetic inheritance, genetic noise, and cost of efflux pumps

Adaptive resistance emerges when populations of bacteria are subjected to gradual increases of antibiotics. It is characterized by a rapid emergence of resistance and fast reversibility to the non-resistant phenotype when the antibiotic is removed from the medium. Recent work shows that adaptive resistance requires epigenetic inheritance and heterogeneity of gene expression patterns that are, in particular, associated with the production of porins and efflux pumps. However, the precise mechanisms by which inheritance and variability govern adaptive resistance, and what processes cause its reversibility remain unclear. Here, using an efflux pump regulatory network (EPRN) model, we show that the following three mechanisms are essential to obtain adaptive resistance in a bacterial population: 1) intrinsic variability in the expression of the EPRN transcription factors; 2) epigenetic inheritance of the transcription rate of EPRN associated genes; and 3) energetic cost of the efflux pumps activity that slows down cell growth. While the first two mechanisms acting together are responsible for the emergence and gradual increase of the resistance, the third one accounts for its reversibility. In contrast with the standard assumption, our model predicts that adaptive resistance cannot be explained by increased mutation rates. Our results identify the molecular mechanism of epigenetic inheritance as the main target for therapeutic treatments against the emergence of adaptive resistance. Finally, our theoretical framework unifies known and newly identified determinants such as the burden of efflux pumps that underlie bacterial adaptive resistance to antibiotics.

The Bacillus subtilis tyrZ gene encodes a highly selective tyrosyl-tRNA synthetase and is regulated by a MarR regulator and T box riboswitch

Misincorporation of D-tyrosine (D-Tyr) into cellular proteins due to mischarging of tRNA(Tyr) with D-Tyr by tyrosyl-tRNA synthetase inhibits growth and biofilm formation of Bacillus subtilis. Furthermore, many B. subtilis strains lack a functional gene encoding D-aminoacyl-tRNA deacylase, which prevents misincorporation of D-Tyr in most organisms. B. subtilis has two genes that encode tyrosyl-tRNA synthetase: tyrS is expressed under normal growth conditions, and tyrZ is known to be expressed only when tyrS is inactivated by mutation. We hypothesized that tyrZ encodes an alternate tyrosyl-tRNA synthetase, expression of which allows the cell to grow when D-Tyr is present. We show that TyrZ is more selective for L-Tyr over D-Tyr than is TyrS; however, TyrZ is less efficient overall. We also show that expression of tyrZ is required for growth and biofilm formation in the presence of D-Tyr. Both tyrS and tyrZ are preceded by a T box riboswitch, but tyrZ is found in an operon with ywaE, which is predicted to encode a MarR family transcriptional regulator. Expression of tyrZ is repressed by YwaE and also is regulated at the level of transcription attenuation by the T box riboswitch. We conclude that expression of tyrZ may allow growth when excess D-Tyr is present.

IMPORTANCE

Accurate protein synthesis requires correct aminoacylation of each tRNA with the cognate amino acid and discrimination against related compounds. Bacillus subtilis produces D-Tyr, an analog of L-Tyr that is toxic when incorporated into protein, during stationary phase. Most organisms utilize a D-aminoacyl-tRNA deacylase to prevent misincorporation of D-Tyr. This work demonstrates that the increased selectivity of the TyrZ form of tyrosyl-tRNA synthetase may provide a mechanism by which B. subtilis prevents misincorporation of D-Tyr in the absence of a functional D-aminoacyl-tRNA deacylase gene.

Regulation of fatty acid biosynthesis by the global regulator CcpA and the local regulator FabT in Streptococcus mutans

SMU.1745c, encoding a putative transcriptional regulator of the MarR family, maps to a location proximal to the fab gene cluster in Streptococcus mutans. Deletion of the SMU.1745c (fabTS m ) coding region resulted in a membrane fatty acid composition comprised of longer-chained, unsaturated fatty acids (UFA), compared with the parent strain. Previous reports have indicated a role for FabT in regulation of genes in the fab gene cluster in other organisms, through binding to a palindromic DNA sequence. Consensus FabT motif sequences were identified in S. mutans in the intergenic regions preceding fabM, fabTSm and fabK in the fab gene cluster. Chloramphenicol acetyltransferase (cat) reporter fusions, using the fabM promoter, revealed elevated transcription in a ∆fabTS m background. Transcription of fabTS m was dramatically elevated in cells grown at pH values of 5 and 7 in the ∆ fabTS m background. Transcription of fabTS m was also elevated in a strain carrying a deletion for the carbon catabolite repressor CcpA. Purified FabTS m and CcpA bound to the promoter regions of fabTS m and fabM. Hence, the data indicate that FabTS m acts as a repressor of fabM and fabTS m itself and the global regulator CcpA acts as a repressor for fabTS m .

The CpxR/CpxA two-component regulatory system up-regulates the multidrug resistance cascade to facilitate Escherichia coli resistance to a model antimicrobial peptide

A genome-wide susceptibility assay was used to identify specific CpxR-dependent genes that facilitate Escherichia coli resistance to a model cationic antimicrobial peptide, protamine. A total of 115 strains from the Keio Collection, each of which contained a deletion at a demonstrated or predicted CpxR/CpxA-dependent locus, were tested for protamine susceptibility. One strain that exhibited high susceptibility carried a deletion of tolC, a gene that encodes the outer membrane component of multiple tripartite multidrug transporters. Concomitantly, two of these efflux systems, AcrAB/TolC and EmrAB/TolC, play major roles in protamine resistance. Activation of the CpxR/CpxA system stimulates mar transcription, suggesting a new regulatory circuit that enhances the multidrug resistance cascade. Tripartite multidrug efflux systems contribute to bacterial resistance to protamine differently from the Tat system. DNase I footprinting analysis demonstrated that the CpxR protein binds to a sequence located in the -35 and -10 regions of mar promoter. This sequence resembles the consensus CpxR binding site, however, on the opposite strand. aroK, a CpxR-dependent gene that encodes a shikimate kinase in the tryptophan biosynthesis pathway, was also found to facilitate protamine resistance. Specific aromatic metabolites from this pathway, such as indole, can stimulate expression of well studied CpxR-dependent genes degP and cpxP, which are not components of the tripartite multidrug transporters. Thus, we propose a novel mechanism for E. coli to modulate resistance to protamine and likely other cationic antimicrobial peptides in which the CpxR/CpxA system up-regulates mar transcription in response to specific aromatic metabolites, subsequently stimulating the multidrug resistance cascade.

Aromatic inhibitors derived from ammonia-pretreated lignocellulose hinder bacterial ethanologenesis by activating regulatory circuits controlling inhibitor efflux and detoxification

Efficient microbial conversion of lignocellulosic hydrolysates to biofuels is a key barrier to the economically viable deployment of lignocellulosic biofuels. A chief contributor to this barrier is the impact on microbial processes and energy metabolism of lignocellulose-derived inhibitors, including phenolic carboxylates, phenolic amides (for ammonia-pretreated biomass), phenolic aldehydes, and furfurals. To understand the bacterial pathways induced by inhibitors present in ammonia-pretreated biomass hydrolysates, which are less well studied than acid-pretreated biomass hydrolysates, we developed and exploited synthetic mimics of ammonia-pretreated corn stover hydrolysate (ACSH). To determine regulatory responses to the inhibitors normally present in ACSH, we measured transcript and protein levels in an Escherichia coli ethanologen using RNA-seq and quantitative proteomics during fermentation to ethanol of synthetic hydrolysates containing or lacking the inhibitors. Our study identified four major regulators mediating these responses, the MarA/SoxS/Rob network, AaeR, FrmR, and YqhC. Induction of these regulons was correlated with a reduced rate of ethanol production, buildup of pyruvate, depletion of ATP and NAD(P)H, and an inhibition of xylose conversion. The aromatic aldehyde inhibitor 5-hydroxymethylfurfural appeared to be reduced to its alcohol form by the ethanologen during fermentation, whereas phenolic acid and amide inhibitors were not metabolized. Together, our findings establish that the major regulatory responses to lignocellulose-derived inhibitors are mediated by transcriptional rather than translational regulators, suggest that energy consumed for inhibitor efflux and detoxification may limit biofuel production, and identify a network of regulators for future synthetic biology efforts.

Bacterial multidrug efflux pumps: mechanisms, physiology and pharmacological exploitations

Multidrug resistance (MDR) refers to the capability of bacterial pathogens to withstand lethal doses of structurally diverse drugs which are capable of eradicating non-resistant strains. MDR has been identified as a major threat to the public health of human being by the World Health Organization (WHO). Among the four general mechanisms that cause antibiotic resistance including target alteration, drug inactivation, decreased permeability and increased efflux, drug extrusion by the multidrug efflux pumps serves as an important mechanism of MDR. Efflux pumps not only can expel a broad range of antibiotics owing to their poly-substrate specificity, but also drive the acquisition of additional resistance mechanisms by lowering intracellular antibiotic concentration and promoting mutation accumulation. Over-expression of multidrug efflux pumps have been increasingly found to be associated with clinically relevant drug resistance. On the other hand, accumulating evidence has suggested that efflux pumps also have physiological functions in bacteria and their expression is subject tight regulation in response to various of environmental and physiological signals. A comprehensive understanding of the mechanisms of drug extrusion, and regulation and physiological functions of efflux pumps is essential for the development of anti-resistance interventions. In this review, we summarize the development of these research areas in the recent decades and present the pharmacological exploitation of efflux pump inhibitors as a promising anti-drug resistance intervention.