A small RNA controls bacterial sensitivity to gentamicin during iron starvation

Small regulatory RNAs as key modulators of antibiotic resistance in pathogenic bacteria

The escalating antibiotic resistance crisis poses a significant challenge to global public health, threatening the efficacy of current treatments and driving the emergence of multidrug-resistant pathogens. Among the various factors associated with bacterial antibiotic resistance, small regulatory RNAs (sRNAs) have emerged as pivotal post-transcriptional regulators which orchestrate bacterial adaptation to antibiotic pressure via diverse mechanisms. This review consolidates the current knowledge on sRNA-mediated mechanisms, focusing on drug uptake, drug efflux systems, lipopolysaccharides, cell wall modification, biofilm formation, and mutagenesis. Recent advances in transcriptomics and functional analyses have revealed novel sRNAs and their regulatory networks, expanding our understanding of resistance mechanisms. These findings highlight the potential of targeting sRNA-mediated pathways as an innovative therapeutic strategy to combat antibiotic resistance, and offer promising avenues for managing challenging bacterial infections.

Control of iron acquisition by multiple small RNAs unravels a new role for transcriptional terminator loops in gene regulation

Small RNAs (sRNAs) controlling gene expression by imperfect base-pairing with mRNA(s) are widespread in bacteria. They regulate multiple genes, including genes involved in iron homeostasis, through a wide variety of mechanisms. We previously showed that OmrA and OmrB sRNAs repress the synthesis of the Escherichia coli FepA receptor for iron-enterobactin complexes. We now report that five additional sRNAs, namely RprA, RybB, ArrS, RseX and SdsR, responding to different environmental cues, also repress fepA, independently of one another. While RprA follows the canonical mechanism of pairing with the translation initiation region, repression by ArrS or RseX requires a secondary structure far upstream within the long fepA 5' untranslated region. We also demonstrate a dual action of SdsR, whose 5'-part pairs with the fepA translation initiation region while its 3'-end behaves like ArrS or RseX. Strikingly, mutation analysis shows a key role for the loops of these sRNAs' intrinsic terminators in the regulation. Furthermore, regulation depends on both the Hfq chaperone and the RNase E endonuclease. Overall, our data strongly suggest that FepA levels must be tightly controlled under a variety of conditions and highlight the diversity of mechanisms that underly the regulation of gene expression by sRNAs in bacteria.

Small RNA OxyS induces resistance to aminoglycosides during oxidative stress by controlling Fe-S cluster biogenesis in Escherichia coli

Fe-S clusters are essential cofactors involved in many reactions across all domains of life. Their biogenesis in Escherichia coli and other enterobacteria involves two machineries: Isc and Suf. Under conditions where cells operate with the Suf system, such as during oxidative stress or iron limitation, the entry of aminoglycosides is reduced, leading to resistance to these antibiotics. The transition between Isc and Suf machineries is controlled by the transcriptional regulator IscR. Here, we found that two small regulatory RNAs (sRNAs), FnrS and OxyS, control iscR expression by base pairing to the 5'-UTR of the iscR mRNA. These sRNAs act in opposite ways and in opposite conditions: FnrS, expressed in anaerobiosis, represses the expression of iscR while OxyS, expressed during oxidative stress, activates it. Using an E. coli strain experiencing protracted oxidative stress, we further demonstrate that iscR expression is rapidly and significantly enhanced in the presence of OxyS. Consequently, we further show that OxyS induces resistance to aminoglycosides during oxidative stress through regulation of Fe-S cluster biogenesis, revealing a major role for this sRNA.

Genome-wide sRNA and mRNA transcriptomic profiling insights into carbapenem-resistant Acinetobacter baumannii

Introduction

Acinetobacter baumannii (AB) is rising as a human pathogen of critical priority worldwide as it is the leading cause of opportunistic infections in healthcare settings and carbapenem-resistant AB is listed as a "super bacterium" or "priority pathogen for drug resistance" by the World Health Organization.

Methods

Clinical isolates of A. baumannii were collected and tested for antimicrobial susceptibility. Among them, carbapenem-resistant and carbapenem-sensitive A. baumannii were subjected to prokaryotic transcriptome sequencing. The change of sRNA and mRNA expression was analyzed by bioinformatics and validated by quantitative reverse transcription-PCR.

Results

A total of 687 clinical isolates were collected, of which 336 strains of A. baumannii were resistant to carbapenem. Five hundred and six differentially expressed genes and nineteen differentially expressed sRNA candidates were discovered through transcriptomic profile analysis between carbapenem-resistant isolates and carbapenem-sensitive isolates. Possible binding sites were predicted through software for sRNA21 and adeK, sRNA27 and pgaC, sRNA29 and adeB, sRNA36 and katG, indicating a possible targeting relationship. A negative correlation was shown between sRNA21 and adeK (r = -0.581, P = 0.007), sRNA27 and pgaC (r = -0.612, P = 0.004), sRNA29 and adeB (r = -0.516, P = 0.020).

Discussion

This study preliminarily screened differentially expressed mRNA and sRNA in carbapenem-resistant A. baumannii, and explored possible targeting relationships, which will help further reveal the resistance mechanism and provide a theoretical basis for the development of drugs targeting sRNA for the prevention and treatment of carbapenem-resistant A. baumannii infection.

Metal-regulated antibiotic resistance and its implications for antibiotic therapy

Antibiotic resistance, one of the major medical threats worldwide, can be selected and induced by metals through multiple mechanisms such as co-resistance, cross-resistance, and co-regulation. Compared with co-resistance and cross-resistance which are attributed to the physically or functionally linked metal and antibiotic resistance genes, co-regulation of antibiotic resistance genes by metal-responsive regulators and pathways is much more complex and elusive. Here, we discussed the main mechanisms by which antibiotic resistance is regulated in response to metals and showed recent attempts to combat antibiotic resistance by interfering with metal-based signalling pathways. Further efforts to depict the intricate metal-based regulatory network of antibiotic resistance will provide tremendous opportunities for the discovery of novel anti-resistance targets, and blocking or rewiring the metal-based signalling pathways is emerging as a promising stratagem to reverse bacterial resistance to antibiotics and rejuvenate the efficacy of conventional antibiotics.

Genetic dissection of the bacterial Fe-S protein biogenesis machineries

Iron‑sulfur (Fe-S) clusters are one of the most ancient and versatile inorganic cofactors present in the three domains of life. Fe-S clusters are essential cofactors for the activity of a large variety of metalloproteins that play crucial physiological roles. Fe-S protein biogenesis is a complex process that starts with the acquisition of the elements (iron and sulfur atoms) and their assembly into an Fe-S cluster that is subsequently inserted into the target proteins. The Fe-S protein biogenesis is ensured by multiproteic systems conserved across all domains of life. Here, we provide an overview on how bacterial genetics approaches have permitted to reveal and dissect the Fe-S protein biogenesis process in vivo.

Molecular mechanism of Hfq-dependent sRNA1039 and sRNA1600 regulating antibiotic resistance and virulence in Shigella sonnei

Bacillary dysentery caused by Shigella spp. is a significant concern for human health. Small non-coding RNA (sRNA) plays a crucial role in regulating antibiotic resistance and virulence in Shigella spp. However, the specific mechanisms behind this phenomenon are still not fully understood. This study discovered two sRNAs (sRNA1039 and sRNA1600) that may be involved in bacterial resistance and virulence. By constructing deletion mutants (WT/ΔSR1039 and WT/ΔSR1600), this study found that the WT/ΔSR1039 mutants caused a two-fold increase in sensitivity to ampicillin, gentamicin and cefuroxime, and the WT/ΔSR1600 mutants caused a two-fold increase in sensitivity to cefuroxime. Furthermore, the WT/ΔSR1600 mutants caused a decrease in the adhesion and invasion of bacteria to HeLa cells (P<0.01), and changed the oxidative stress level of bacteria to reduce their survival rate (P<0.001). Subsequently, this study explored the molecular mechanisms by which sRNA1039 and sRNA1600 regulate antibiotic resistance and virulence. The deletion of sRNA1039 accelerated the degradation of target gene cfa mRNA and reduced its expression, thereby regulating the expression of pore protein gene ompD indirectly and negatively to increase bacterial sensitivity to ampicillin, gentamicin and cefuroxime. The inactivation of sRNA1600 reduced the formation of persister cells to reduce resistance to cefuroxime, and reduced the expression of type-III-secretion-system-related genes to reduce bacterial virulence by reducing the expression of target gene tomB. These results provide new insights into Hfq-sRNA-mRNA regulation of the resistance and virulence network of Shigella sonnei, which could potentially promote the development of more effective treatment strategies.

Aminoglycoside uptake, stress, and potentiation in Gram-negative bacteria: new therapies with old molecules

SUMMARYAminoglycosides (AGs) are long-known molecules successfully used against Gram-negative pathogens. While their use declined with the discovery of new antibiotics, they are now classified as critically important molecules because of their effectiveness against multidrug-resistant bacteria. While they can efficiently cross the Gram-negative envelope, the mechanism of AG entry is still incompletely understood, although this comprehension is essential for the development of new therapies in the face of the alarming increase in antibiotic resistance. Increasing antibiotic uptake in bacteria is one strategy to enhance effective treatments. This review aims, first, to consolidate old and recent knowledge about AG uptake; second, to explore the connection between AG-dependent bacterial stress and drug uptake; and finally, to present new strategies of potentiation of AG uptake for more efficient antibiotic therapies. In particular, we emphasize on the connection between sugar transport and AG potentiation.

Conserved metabolic regulator ArcA responds to oxygen availability, iron limitation, and cell envelope perturbations during bacteremia

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.

Identification of the active mechanism of aminoglycoside entry in V. cholerae through characterization of sRNA ctrR, regulating carbohydrate utilization and transport

The possible active entry of aminoglycosides in bacterial cells has been debated since the development of this antibiotic family. Here we report the identification of their active transport mechanism in Vibrio species. We combined genome-wide transcriptional analysis and fitness screens to identify alterations driven by treatment of V. cholerae with sub-minimum inhibitory concentrations (sub-MIC) of the aminoglycoside tobramycin. RNA-seq data showed downregulation of the small non-coding RNA ncRNA586 during such treatment, while Tn-seq revealed that inactivation of this sRNA was associated with improved fitness in the presence of tobramycin. This sRNA is located near sugar transport genes and previous work on a homologous region in Vibrio tasmaniensis suggested that this sRNA stabilizes gene transcripts for carbohydrate transport and utilization, as well as phage receptors. The role for ncRNA586, hereafter named ctrR, in the transport of both carbohydrates and aminoglycosides, was further investigated. Flow cytometry on cells treated with a fluorescent aminoglycoside confirmed the role of ctrR and of carbohydrate transporters in differential aminoglycoside entry. Despite sequence diversity, ctrR showed functional conservation across the Vibrionales. This system in directly modulated by carbon sources, suggesting regulation by carbon catabolite repression, a widely conserved mechanism in Gram-negative bacteria, priming future research on aminoglycoside uptake by sugar transporters in other bacterial species.

Epigenetic-Based Regulation of Transcriptome in Escherichia coli Adaptive Antibiotic Resistance

Adaptive antibiotic resistance is a transient metabolic adaptation of bacteria limiting their sensitivity to low, progressively increased, concentrations of antibiotics. Unlike innate and acquired resistance, adaptive resistance is dependent on the presence of antibiotics, and it disappears when the triggering factor is removed. Low concentrations of antibiotics are largely diffused in natural environments, in the food industry or in certain body compartments of humans when used therapeutically, or in animals when used for growth promotion. However, molecular mechanisms underlying this phenomenon are still poorly characterized. Here, we present experiments suggesting that epigenetic modifications, triggered by low concentrations of ampicillin, gentamicin, and ciprofloxacin, may modulate the sensitivity of bacteria to antibiotics. The epigenetic modifications we observed were paralleled by modifications of the expression pattern of many genes, including some of those that have been found mutated in strains with permanent antibiotic resistance. As the use of low concentrations of antibiotics is spreading in different contexts, our findings may suggest new targets and strategies to avoid adaptive antibiotic resistance. This might be very important as, in the long run, this transient adaptation may increase the chance, allowing the survival and the flourishing of bacteria populations, of the onset of mutations leading to stable resistance. IMPORTANCE In this study, we characterized the modifications of epigenetic marks and of the whole transcriptome in the adaptive response of Escherichia coli cells to low concentrations of ampicillin, gentamicin, and ciprofloxacin. As the transient adaptation does increase the chance of permanent resistance, possibly allowing the survival and flourishing of bacteria populations where casual mutations providing resistance may give an immediate advantage, the importance of this study is not only in the identification of possible molecular mechanisms underlying adaptive resistance to antibiotics, but also in suggesting new strategies to avoid adaptation.

Bioenergetic State of Escherichia coli Controls Aminoglycoside Susceptibility

Aminoglycosides (AG) have been used against Gram-negative bacteria for decades. Yet, how bacterial metabolism and environmental conditions modify AG toxicity is poorly understood. Here, we show that the level of AG susceptibility varies depending on the nature of the respiratory chain that Escherichia coli uses for growth, i.e., oxygen, nitrate, or fumarate. We show that all components of the fumarate respiratory chain, namely, hydrogenases 2 and 3, the formate hydrogenlyase complex, menaquinone, and fumarate reductase are required for AG-mediated killing under fumarate respiratory conditions. In addition, we show that the AAA+ ATPase RavA and its Von Wildebrand domain-containing partner, ViaA, are essential for AG to act under fumarate respiratory conditions. This effect was true for all AG that were tested but not for antibiotics from other classes. In addition, we show that the sensitizing effect of RavA-ViaA is due to increased gentamicin uptake in a proton motive force-dependent manner. Interestingly, the sensitizing effect of RavA-ViaA was prominent in poor energy conservation conditions, i.e., with fumarate, but dispensable under high energy conservation conditions, i.e., in the presence of nitrate or oxygen. We propose that RavA-ViaA can facilitate uptake of AG across the membrane in low-energy cellular states. IMPORTANCE Antibiotic resistance is a major public health, social, and economic problem. Aminoglycosides (AG) are known to be highly effective against Gram-negative bacteria, but their use is limited to life-threatening infections because of their nephrotoxicity and ototoxicity at therapeutic dose. Elucidation of AG-sensitization mechanisms in bacteria would allow reduced effective doses of AG. Here, we have identified the molecular components involved in anaerobic fumarate respiration that are required for AG to kill. In addition to oxidoreductases and menaquinone, this includes new molecular players, RavA, an AAA+ ATPase, and ViaA, its partner that has the VWA motif. Remarkably, the influence of RavA-ViaA on AG susceptibility varies according to the type of bioenergetic metabolism used by E. coli. This is a significant advance because anaerobiosis is well known to reduce the antibacterial activity of AG. This study highlights the critical importance of the relationship between culture conditions, metabolism, and antibiotic susceptibility.

Iron restriction induces the small-colony variant phenotype in Staphylococcus aureus

Pathogens such as Staphylococcus aureus must overcome host-induced selective pressures, including limited iron availability. To cope with the harsh conditions of the host environment, S. aureus can adapt its physiology in multiple ways. One of these adaptations is the fermenting small-colony variant (SCV) phenotype, which is known to be inherently tolerant to certain classes of antibiotics and heme toxicity. We hypothesized that SCVs might also behave uniquely in response to iron starvation since one of the major cellular uses of iron is the respiration machinery. In this study, a respiring strain of S. aureus and fermenting SCV strains were treated with different concentrations of the iron chelator, 2,2' dipyridyl (DIP). Our data demonstrate that a major impact of iron starvation in S. aureus is the repression of respiration and the induction of the SCV phenotype. We demonstrate that the SCV phenotype transiently induced by iron starvation mimics the aminoglycoside recalcitrance exhibited by genetic SCVs. Furthermore, prolonged growth in iron starvation promotes increased emergence of stable aminoglycoside-resistant SCVs relative to the naturally occurring subpopulation of SCVs within an S. aureus community. These findings may have relevance to physiological and evolutionary processes occurring within bacterial populations infecting iron-limited host environments.

Key players in regulatory RNA realm of bacteria

Precise regulation of gene expression is crucial for living cells to adapt for survival in diverse environmental conditions. Among the common cellular regulatory mechanisms, RNA-based regulators play a key role in all domains of life. Discovery of regulatory RNAs have made a paradigm shift in molecular biology as many regulatory functions of RNA have been identified beyond its canonical roles as messenger, ribosomal and transfer RNA. In the complex regulatory RNA network, riboswitches, small RNAs, and RNA thermometers can be identified as some of the key players. Herein, we review the discovery, mechanism, and potential therapeutic use of these classes of regulatory RNAs mainly found in bacteria. Being highly adaptive organisms that inhabit a broad range of ecological niches, bacteria have adopted tight and rapid-responding gene regulation mechanisms. This review aims to highlight how bacteria utilize versatile RNA structures and sequences to build a sophisticated gene regulation network.

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The three major classes of prokaryotic ncRNAs and their characterized mechanisms of operation in gene regulation.

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sRNAs emerging as major players in global gene regulatory networks.

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Riboswitch mediated gene control mechanisms through on/off switches in response to ligand binding.

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RNA thermo sensors for temperature-dependent gene expression.

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Therapeutic importance of ncRNAs and computational approaches involved in the discovery of ncRNAs.

The ArcAB Two-Component System: Function in Metabolism, Redox Control, and Infection

ArcAB, also known as the Arc system, is a member of the two-component system family of bacterial transcriptional regulators and is composed of sensor kinase ArcB and response regulator ArcA. In this review, we describe the structure and function of these proteins and assess the state of the literature regarding ArcAB as a sensor of oxygen consumption. The bacterial quinone pool is the primary modulator of ArcAB activity, but questions remain for how this regulation occurs. This review highlights the role of quinones and their oxidation state in activating and deactivating ArcB and compares competing models of the regulatory mechanism. The cellular processes linked to ArcAB regulation of central metabolic pathways and potential interactions of the Arc system with other regulatory systems are also reviewed. Recent evidence for the function of ArcAB under aerobic conditions is challenging the long-standing characterization of this system as strictly an anaerobic global regulator, and the support for additional ArcAB functionality in this context is explored. Lastly, ArcAB-controlled cellular processes with relevance to infection are assessed.

Mutations in respiratory complex I promote antibiotic persistence through alterations in intracellular acidity and protein synthesis

Antibiotic persistence describes the presence of phenotypic variants within an isogenic bacterial population that are transiently tolerant to antibiotic treatment. Perturbations of metabolic homeostasis can promote antibiotic persistence, but the precise mechanisms are not well understood. Here, we use laboratory evolution, population-wide sequencing and biochemical characterizations to identify mutations in respiratory complex I and discover how they promote persistence in Escherichia coli. We show that persistence-inducing perturbations of metabolic homeostasis are associated with cytoplasmic acidification. Such cytoplasmic acidification is further strengthened by compromised proton pumping in the complex I mutants. While RpoS regulon activation induces persistence in the wild type, the aggravated cytoplasmic acidification in the complex I mutants leads to increased persistence via global shutdown of protein synthesis. Thus, we propose that cytoplasmic acidification, amplified by a compromised complex I, can act as a signaling hub for perturbed metabolic homeostasis in antibiotic persisters.

Bacterial Approaches for Assembling Iron-Sulfur Proteins

Building iron-sulfur (Fe-S) clusters and assembling Fe-S proteins are essential actions for life on Earth. The three processes that sustain life, photosynthesis, nitrogen fixation, and respiration, require Fe-S proteins. Genes coding for Fe-S proteins can be found in nearly every sequenced genome. Fe-S proteins have a wide variety of functions, and therefore, defective assembly of Fe-S proteins results in cell death or global metabolic defects. Compared to alternative essential cellular processes, there is less known about Fe-S cluster synthesis and Fe-S protein maturation. Moreover, new factors involved in Fe-S protein assembly continue to be discovered. These facts highlight the growing need to develop a deeper biological understanding of Fe-S cluster synthesis, holo-protein maturation, and Fe-S cluster repair. Here, we outline bacterial strategies used to assemble Fe-S proteins and the genetic regulation of these processes. We focus on recent and relevant findings and discuss future directions, including the proposal of using Fe-S protein assembly as an antipathogen target.

Diversity and Versatility in Small RNA-Mediated Regulation in Bacterial Pathogens

Bacterial gene expression is under the control of a large set of molecules acting at multiple levels. In addition to the transcription factors (TFs) already known to be involved in global regulation of gene expression, small regulatory RNAs (sRNAs) are emerging as major players in gene regulatory networks, where they allow environmental adaptation and fitness. Developments in high-throughput screening have enabled their detection in the entire bacterial kingdom. These sRNAs influence a plethora of biological processes, including but not limited to outer membrane synthesis, metabolism, TF regulation, transcription termination, virulence, and antibiotic resistance and persistence. Almost always noncoding, they regulate target genes at the post-transcriptional level, usually through base-pair interactions with mRNAs, alone or with the help of dedicated chaperones. There is growing evidence that sRNA-mediated mechanisms of actions are far more diverse than initially thought, and that they go beyond the so-called cis- and trans-encoded classifications. These molecules can be derived and processed from 5' untranslated regions (UTRs), coding or non-coding sequences, and even from 3' UTRs. They usually act within the bacterial cytoplasm, but recent studies showed sRNAs in extracellular vesicles, where they influence host cell interactions. In this review, we highlight the various functions of sRNAs in bacterial pathogens, and focus on the increasing examples of widely diverse regulatory mechanisms that might compel us to reconsider what constitute the sRNA.

Oxidative stress antagonizes fluoroquinolone drug sensitivity via the SoxR-SUF Fe-S cluster homeostatic axis

The level of antibiotic resistance exhibited by bacteria can vary as a function of environmental conditions. Here, we report that phenazine-methosulfate (PMS), a redox-cycling compound (RCC) enhances resistance to fluoroquinolone (FQ) norfloxacin. Genetic analysis showed that E. coli adapts to PMS stress by making Fe-S clusters with the SUF machinery instead of the ISC one. Based upon phenotypic analysis of soxR, acrA, and micF mutants, we showed that PMS antagonizes fluoroquinolone toxicity by SoxR-mediated up-regulation of the AcrAB drug efflux pump. Subsequently, we showed that despite the fact that SoxR could receive its cluster from either ISC or SUF, only SUF is able to sustain efficient SoxR maturation under exposure to prolonged PMS period or high PMS concentrations. This study furthers the idea that Fe-S cluster homeostasis acts as a sensor of environmental conditions, and because its broad influence on cell metabolism, modifies the antibiotic resistance profile of E. coli.

Our study investigates how phenazine compounds, which are widely present in the environment, impact antibiotic resistance of the Gram-negative bacteria Escherichia coli. The paucity of new antibacterial molecules fuels concern in the wake of increased antibiotic resistance among pathogens. Equally worrying is the realization that environmental conditions can have a drastic influence on the efficiency of antibacterial compounds. Here we report that phenazine, a member of the redox-cycling molecule family, is antagonistic to norfloxacin, a well-known and routinely used fluoroquinolone antibiotic. We show that the mechanism E. coli is using for synthesizing Fe-S clusters controls the phenazine/fluoroquinolone antagonism. Indeed, upon exposure to phenazine, E. coli switches from making Fe-S clusters with the ISC Fe-S biogenesis system to making them with SUF, a consequence of which is the activation of the SoxR transcriptional activator, up-regulation of the AcrAB efflux pump, and efflux of fluoroquinolone out of the cell. This study illustrates the major influence that environmental conditions play in setting antibiotic level resistance and further highlights the major contribution of Fe-S cluster homeostasis in antibiotic susceptibility.

Networks of Resistance: Small RNA Control of Antibiotic Resistance

The golden age of antibiotics has passed, and the threat of untreatable antimicrobial resistant infections is now a reality for many individuals. Understanding how bacteria resist antimicrobial treatment and regulate gene expression in response to antibiotics is an important step towards combating resistance. In this review we focus on a ubiquitous class of bacterial gene regulators termed regulatory small RNAs (sRNAs) and how they contribute to antimicrobial resistance and tolerance. Small RNAs have notable roles in modulating the composition of the bacterial envelope, and through these functions control intrinsic antimicrobial resistance in many human pathogens. Recent technical advances that allow profiling of the 'sRNA interactome' have revealed a complex post-transcriptional network of sRNA interactions that can be used to identify network hubs and regulatory bottlenecks. Sequence-specific inhibition of these sRNAs with programmable RNA-targeting therapeutics may present avenues for treating antimicrobial resistant pathogens or resensitizing to our current antibiotics.

Low iron-induced small RNA BrrF regulates central metabolism and oxidative stress responses in Burkholderia cenocepacia

Regulatory small RNAs play an essential role in maintaining cell homeostasis in bacteria in response to environmental stresses such as iron starvation. Prokaryotes generally encode a large number of RNA regulators, yet their identification and characterisation is still in its infancy for most bacterial species. Burkholderia cenocepacia is an opportunistic pathogen with high innate antimicrobial resistance, which can cause the often fatal cepacia syndrome in individuals with cystic fibrosis. In this study we characterise a small RNA which is involved in the response to iron starvation, a condition that pathogenic bacteria are likely to encounter in the host. BrrF is a small RNA highly upregulated in Burkholderia cenocepacia under conditions of iron depletion and with a genome context consistent with Fur regulation. Its computationally predicted targets include iron-containing enzymes of the tricarboxylic acid (TCA) cycle such as aconitase and succinate dehydrogenase, as well as iron-containing enzymes responsible for the oxidative stress response, such as superoxide dismutase and catalase. Phenotypic and gene expression analysis of BrrF deletion and overexpression mutants show that the regulation of these genes is BrrF-dependent. Expression of acnA, fumA, sdhA and sdhC was downregulated during iron depletion in the wild type strain, but not in a BrrF deletion mutant. TCA cycle genes not predicted as target for BrrF were not affected in the same manner by iron depletion. Likewise, expression of sodB and katB was dowregulated during iron depletion in the wild type strain, but not in a BrrF deletion mutant. BrrF overexpression reduced aconitase and superoxide dismutase activities and increased sensitivity to hydrogen peroxide. All phenotypes and gene expression changes of the BrrF deletion mutant could be complemented by overexpressing BrrF in trans. Overall, BrrF acts as a regulator of central metabolism and oxidative stress response, possibly as an iron-sparing measure to maintain iron homeostasis under conditions of iron starvation.