Osmotic Stress

Metabolomic Disparities in Intraocular Fluid Across Varied Stages of Cataract Progression: Implications for the Analysis of Cataract Development

Introduction: The lens's metabolic demands are met through a continuous circulation of aqueous humor, encompassing a spectrum of components such as organic and inorganic ions, carbohydrates, glutathione, urea, amino acids, proteins, oxygen, carbon dioxide, and water. Metabolomics is a pivotal tool, offering an initial insight into the complexities of integrated metabolism. In this investigative study, we systematically scrutinize the composition of intraocular fluid in individuals afflicted with cataracts. Methods: The investigation involved a comprehensive analysis of aqueous humor samples from a cohort comprising 192 patients. These individuals were stratified by utilizing the SPONCS classification system, delineating distinct groups characterized by the hardness of cataracts. The analytical approach employed targeted quantitative metabolite analysis using HILIC-based liquid chromatography coupled with high-resolution mass spectrometric detection. The metabolomics data analysis was performed with MetaboAnalyst 5.0. Results: The results of the enrichment analysis have facilitated the inference that the discerned disparities among groups arise from disruptions in taurine and hypotaurine metabolism, variations in tryptophan metabolism, and modifications in mitochondrial beta-oxidation of short-chain saturated fatty acids and pyrimidine metabolism. Conclusion: A decline in taurine concentration precipitates diminished glutathione activity, prompting an elevated requirement for NAD+ and instigating tryptophan metabolism along the kynurenine pathway. Activation of this pathway is additionally prompted by interferon-gamma and UV radiation, leading to the induction of IDO. Concurrently, heightened mitochondrial beta-oxidation signifies a distinctive scenario in translocating fatty acids into the mitochondria, enhancing energy production.

Comparison of Glucose Metabolizing Properties of Enterobacterial Probiotic Strains In Vitro

Before the absorption in the intestine, glucose encounters gut bacteria, which may serve as a barrier against hyperglycemia by metabolizing glucose. In the present study, we compared the capacity of enterobacterial strains to lower glucose levels in an in vitro model of nutrient-induced bacterial growth. Two probiotic strains, Hafnia alvei HA4597 (H. alvei) and Escherichia coli (E. coli) Nissle 1917, as well as E. coli K12, were studied. To mimic bacterial growth in the gut, a planktonic culture was supplemented twice daily by the Luria Bertani milieu with or without 0.5% glucose. Repeated nutrient provision resulted in the incremental growth of bacteria. However, in the presence of glucose, the maximal growth of both strains of E. coli but not of H. alvei was inhibited. When glucose was added to the culture medium, a continuous decrease in its concentration was observed during each feeding phase. At its highest density, H. alvei displayed more efficient glucose consumption accompanied by a more pronounced downregulation of glucose transporters' expression than E. coli K12. Thus, the study reveals that the probiotic strain H. alvei HA4597 is more resilient to maintain its growth than E. coli in the presence of 0.5% glucose accompanied by more efficient glucose consumption. This experimental approach offers a new strategy for the identification of probiotics with increased glucose metabolizing capacities potentially useful for the prevention and co-treatment of type 2 diabetes.

Enhanced catabolism of glycine betaine and derivatives provides improved osmotic stress protection in Methylorubrum extorquens PA1

Integration of metabolites into the overall metabolic network of a cell requires careful coordination dependent upon the ultimate usage of the metabolite. Different stoichiometric needs, and thus pathway fluxes, must exist for compounds destined for diverse uses, such as carbon sources, nitrogen sources, or stress-protective agents. Herein, we expand upon our previous work that highlighted the nature of glycine betaine (GB) metabolism in Methylobacteria to examine the utilization of GB-derivative compounds dimethylglycine (DMG) and sarcosine into Methylorubrum extorquens in different metabolic capacities, including as sole nitrogen and/or carbon sources. We isolated gain-of-function mutations that allowed M. extorquens PA1 to utilize dimethylglycine as a carbon source and dimethylglycine and sarcosine as nitrogen source. Characterization of mutants demonstrated selection for variants of the AraC-like regulator Mext_3735 that confer constitutive expression of the GB metabolic gene cluster, allowing direct utilization of the downstream GB derivatives. Finally, among the distinct isolates examined, we found that catabolism of the osmoprotectant used for selection (GB or dimethylglycine) enhanced osmotic stress resistance provided in the presence of that particular osmolyte. Thus, access to the carbon and nitrogen and osmoprotective effects of GB and DMG are made readily accessible through adaptive mutations. In M. extorquens PA1, the limitations to exploiting this group of compounds appear to exist predominantly at the levels of gene regulation and functional activity, rather than being constrained by transport or toxicity.IMPORTANCEOsmotic stress is a common challenge for bacteria colonizing the phyllosphere, where glycine betaine (GB) can be found as a prevalent osmoprotectant. Though Methylorubrum extorquens PA1 cannot use GB or its demethylation products, dimethylglycine (DMG) and sarcosine, as a sole carbon source, utilization is highly selectable via single nucleotide changes for both GB and DMG growth. The innate inability to use these compounds is due to limited flux through steps in the pathway and regulatory constraints. Herein, the characterization of the transcriptional regulator, Mext_3735 (GbdR), expands our understanding of the various roles in which GB derivatives can be used in M. extorquens PA1. Interestingly, increased catabolism of GB and derivatives does not interfere with, but rather improves, the ability of cells to thrive under increased salt stress conditions, suggesting that metabolic flux improves stress tolerance rather than providing a distinct tension between uses.

RpoS and the bacterial general stress response

SUMMARYThe general stress response (GSR) is a widespread strategy developed by bacteria to adapt and respond to their changing environments. The GSR is induced by one or multiple simultaneous stresses, as well as during entry into stationary phase and leads to a global response that protects cells against multiple stresses. The alternative sigma factor RpoS is the central GSR regulator in E. coli and conserved in most γ-proteobacteria. In E. coli, RpoS is induced under conditions of nutrient deprivation and other stresses, primarily via the activation of RpoS translation and inhibition of RpoS proteolysis. This review includes recent advances in our understanding of how stresses lead to RpoS induction and a summary of the recent studies attempting to define RpoS-dependent genes and pathways.

Abundant urinary amino acids activate glutamine synthetase-encoding glnA by two different mechanisms in Escherichia coli

Growth of uropathogenic Escherichia coli in the bladder induces transcription of glnA which codes for the ammonia-assimilating glutamine synthetase (GS) despite the normally suppressive high ammonia concentration. We previously showed that the major urinary component, urea, induces transcription from the Crp-dependent glnAp1 promoter, but the urea-induced transcript is not translated. Our purpose here was to determine whether the most abundant urinary amino acids, which are known to inhibit GS activity in vitro, also affect glnA transcription in vivo. We found that the abundant amino acids impaired growth, which glutamine and glutamate reversed; this implies inhibition of GS activity. In strains with deletions of crp and glnG that force transcription from the glnAp2 and glnAp1 promoters, respectively, we examined growth and glnA transcription with a glnA-gfp transcriptional fusion and quantitative reverse transcription PCR with primers that can distinguish transcription from the two promoters. The abundant urinary amino acids stimulated transcription from the glnAp2 promoter in the absence of urea but from the glnAp1 promoter in the presence of urea. However, transcription from glnAp1 did not produce a translatable mRNA or GS as assessed by a glnA-gfp translational fusion, enzymatic assay of GS, and Western blot to detect GS antigen in urea-containing media. We discuss these results within the context of the extremely rapid growth of uropathogenic E. coli in urine, the different factors that control the two glnA promoters and possible mechanisms that either overcome or bypass the urea-imposed block of glutamine synthesis during bacterial growth in urine.IMPORTANCEKnowledge of the regulatory mechanisms for genes expressed at the site of infection provides insight into the virulence of pathogenic bacteria. During urinary tract infections-most often caused by Escherichia coli-growth in urine induces the glnA gene which codes for glutamine synthetase. The most abundant urinary amino acids amplified the effect of urea which resulted in hypertranscription from the glnAp1 promoter and, unexpectedly, an untranslated transcript. E. coli must overcome this block in glutamine synthesis during growth in urine, and the mechanism of glutamine acquisition or synthesis may suggest a possible therapy.

The Mar, Sox, and Rob Systems

Environments inhabited by Enterobacteriaceae are diverse and often stressful. This is particularly true for Escherichia coli and Salmonella during host association in the gastrointestinal systems of animals. There, E. coli and Salmonella must survive exposure to various antimicrobial compounds produced or ingested by their host. A myriad of changes to cellular physiology and metabolism are required to achieve this feat. A central regulatory network responsible for sensing and responding to intracellular chemical stressors like antibiotics are the Mar, Sox, and Rob systems found throughout the Enterobacteriaceae. Each of these distinct regulatory networks controls expression of an overlapping set of downstream genes whose collective effects result in increased resistance to a wide array of antimicrobial compounds. This collection of genes is known as the mar-sox-rob regulon. This review will provide an overview of the mar-sox-rob regulon and molecular architecture of the Mar, Sox, and Rob systems.

Hypo-osmotic Stress Increases Permeability of Individual Barriers in Escherichia coli Cell Envelope, Enabling Rapid Drug Transport

Survival of foodborne Gram-negative bacteria during osmotic stress often leads to multidrug resistance development. However, despite the concern, how osmoadaptation alters drug penetration across the Gram-negative bacterial cell envelope has remained inconclusive for years. Here, we have investigated drug permeation and accumulation inside hypo-osmotically shocked Escherichia coli. Three different quaternary ammonium compounds (QACs) are used as cationic amine-containing drug representatives; they also serve as envelope permeability indicators in different assays. Propidium iodide fluorescence reveals cytoplasmic accumulation and overall envelope permeability, while crystal violet sorption and second harmonic generation (SHG) spectroscopy reveal periplasmic accumulation and outer membrane permeability. Malachite green sorption and SHG results reveal transport across both the outer and inner membranes and accumulation in the periplasm as well as cytoplasm. The findings are found to be complementary to one another, collectively revealing enhanced permeabilities of both membranes and the periplasmic space in response to hypo-osmotic stress in E. coli. Enhanced permeability leads to faster QACs transport and higher accumulation in subcellular compartments, whereas transport and accumulation both are negligible under isosmotic conditions. The QACs' transport rates are found to be highly influenced by the osmolytes used, where phosphate ion emerges as a key facilitator of transport across the periplasm into the cytoplasm. E. coli is found viable, with morphology unchanged under extreme hypo-osmotic stress; i.e., it adapts to the situation. The outcome shows that the hypo-osmotic shock to E. coli, specifically using phosphate as an osmolyte, can be beneficial for drug delivery.

Impacts of co-contaminants and dilution on perchlorate biodegradation using various carbon sources

This research investigates the biodegradation of perchlorate in the presence of the co-contaminants nitrate and chlorate using soluble and slow-release carbon sources. In addition, the impact of bio-augmentation and dilution, which results in lower total dissolved salts (TDS) and contaminant levels, is examined. Laboratory microcosms were conducted using actual groundwater and soils from a contaminated aquifer. The results revealed that both soluble and slow-release carbon sources support biodegradation of contaminants in the sequence nitrate > chlorate > perchlorate. Degradation rates, including and excluding lag times, revealed that the overall impact of the presence of co-contaminants depends on degradation kinetics and the relative concentrations of the contaminants. When the lag time caused by the presence of the co-contaminants is considered, the degradation rates for chlorate and perchlorate were two to three times slower. The results also show that dilution causes lower initial contaminant concentrations, and consequently, slower degradation rates, which is not desirable. On the other hand, the dilution resulting from the injection of amendments to support remediation promotes desirably lower salinity levels. However, the salinity associated with the presence of sulfate does not inhibit biodegradation. The naturally occurring bacteria were able to support the degradation of all contaminants. Bio-augmentation was effective only in diluted microcosms. Proteobacteria and Firmicutes were the dominant phyla identified in the microcosms.

PhoPQ Regulates Quinolone and Cephalosporin Resistance Formation in Salmonella Enteritidis at the Transcriptional Level

The two-component system (TCS) PhoPQ has been demonstrated to be crucial for the formation of resistance to quinolones and cephalosporins in Salmonella Enteritidis (S. Enteritidis). However, the mechanism underlying PhoPQ-mediated antibiotic resistance formation remains poorly understood. Here, it was shown that PhoP transcriptionally regulated an assortment of genes associated with envelope homeostasis, the osmotic stress response, and the redox balance to confer resistance to quinolones and cephalosporins in S. Enteritidis. Specifically, cells lacking the PhoP regulator, under nalidixic acid and ceftazidime stress, bore a severely compromised membrane on the aspects of integrity, fluidity, and permeability, with deficiency to withstand osmolarity stress, an increased accumulation of intracellular reactive oxygen species, and dysregulated redox homeostasis, which are unfavorable for bacterial survival. The phosphorylated PhoP elicited transcriptional alterations of resistance-associated genes, including the outer membrane porin ompF and the aconitate hydratase acnA, by directly binding to their promoters, leading to a limited influx of antibiotics and a well-maintained intracellular metabolism. Importantly, it was demonstrated that the cavity of the PhoQ sensor domain bound to and sensed quinolones/cephalosporins via the crucial surrounding residues, as their mutations abrogated the binding and PhoQ autophosphorylation. This recognition mode promoted signal transduction that activated PhoP, thereby modulating the transcription of downstream genes to accommodate cells to antibiotic stress. These findings have revealed how bacteria employ a specific TCS to sense antibiotics and combat them, suggesting PhoPQ as a potential drug target with which to surmount S. Enteritidis. IMPORTANCE The prevalence of quinolone and cephalosporin-resistant S. Enteritidis is of increasing clinical concern. Thus, it is imperative to identify novel therapeutic targets with which to treat S. Enteritidis-associated infections. The PhoPQ two-component system is conserved across a variety of Gram-negative pathogens, by which bacteria adapt to a range of environmental stimuli. Our earlier work has demonstrated the importance of PhoPQ in the resistance formation in S. Enteritidis to quinolones and cephalosporins. In the current work, we identified a global profile of genes that are regulated by PhoP under antibiotic stresses, with a focus on how PhoP regulated downstream genes, either positively or negatively. Additionally, we established that PhoQ sensed quinolones and cephalosporins in a manner of directly binding to them. These identified genes and pathways that are mediated by PhoPQ represent promising targets for the development of a drug potentiator with which to neutralize antibiotic resistance in S. Enteritidis.

Regulation of phosphate starvation-specific responses in Escherichia coli

Toxic agents added into the medium of rapidly growing Escherichia coli induce specific stress responses through the activation of specialized transcription factors. Each transcription factor and downstream regulon (e.g. SoxR) are linked to a unique stress (e.g. superoxide stress). Cells starved of phosphate induce several specific stress regulons during the transition to stationary phase when the growth rate is steadily declining. Whereas the regulatory cascades leading to the expression of specific stress regulons are well known in rapidly growing cells stressed by toxic products, they are poorly understood in cells starved of phosphate. The intent of this review is to both describe the unique mechanisms of activation of specialized transcription factors and discuss signalling cascades leading to the induction of specific stress regulons in phosphate-starved cells. Finally, I discuss unique defence mechanisms that could be induced in cells starved of ammonium and glucose.

The rpoS gene confers resistance to low osmolarity conditions in Salmonella enterica serovar Typhi

Salmonella enterica serovars Typhimurium and Typhi are enteropathogens that differ in host range and the diseases that they cause. We found that exposure to a combination of hypotonicity and the detergent Triton X-100 significantly reduced the viability of the S. Typhi strain Ty2 but had no effect on the S. Typhimurium strain SL1344. Further analysis revealed that hypotonicity was the critical factor: incubation in distilled water alone was sufficient to kill Ty2, while the addition of sodium chloride inhibited killing in a dose-dependent manner. Ty2's loss of viability in water was modified by culture conditions: bacteria grown in well-aerated shaking cultures were more susceptible than bacteria grown under less aerated static conditions. Ty2, like many S. Typhi clinical isolates, has an inactivating mutation in the rpoS gene, a transcriptional regulator of stress responses, whereas most S. Typhimurium strains, including SL1344, have the wild-type gene. Transformation of Ty2 with a plasmid expressing wild-type rpoS, but not the empty vector, significantly increased survival in distilled water. Moreover, an S. Typhi strain with wild-type rpoS had unimpaired survival in water. Inactivation of the wild-type gene in this strain significantly reduced survival, while replacement with an arabinose-inducible allele of rpoS restored viability in water under inducing conditions. Our observations on rpoS-dependent differences in susceptibility to hypotonic conditions may be relevant to the ability of S. Typhi and S. Typhimurium to tolerate the various environments they encounter during the infectious cycle. They also have implications for the handling of these organisms during experimental manipulations.

Genomic Analysis of Halotolerant Bacterial Strains Martelella soudanensis NC18T and NC20

Two novel, halotolerant strains of Martelella soudanensis, NC18^T and NC20, were isolated from deep subsurface sediment, deeply sequenced, and comparatively analyzed with related strains. Based on a phylogenetic analysis using 16S rRNA gene sequences, the two strains grouped with members of the genus Martelella. Here, we sequenced the complete genomes of NC18^T and NC20 to understand the mechanisms of their halotolerance. The genome sizes and G+C content of the strains were 6.1 Mb and 61.8 mol%, respectively. Moreover, NC18^T and NC20 were predicted to contain 5,849 and 5,830 genes, and 5,502 and 5,585 protein-coding genes, respectively. Both strains contain the identically predicted 6 rRNAs and 48 tRNAs. The harboring of halotolerant-associated genes revealed that strains NC18^T and NC20 might tolerate high salinity through the accumulation of potassium ions in a "salt-in" strategy induced by K⁺ uptake protein (kup) and the K⁺ transport system (trkAH and kdpFABC). These two strains also use the ectoine transport system (dctPQM), the glycine betaine transport system (proVWX), and glycine betaine uptake protein (opu) to accumulate "compatible solutes," such as ectoine and glycine betaine, to protect cells from salt stress. This study reveals the halotolerance mechanism of strains NC18^T and NC20 in high salt environments and suggests potential applications for these halotolerant and halophilic strains in environmental biotechnology.

Origin, accumulation and fate of dissolved organic matter in an extreme hypersaline shallow lake

Hypersaline endorheic aquatic systems (H-SEAS) are lakes/shallow playas in arid and semiarid regions that undergo extreme oscillations in salinity and severe drought episodes. Although their geochemical uniqueness and microbiome have been deeply studied, very little is known about the availability and quality of dissolved organic matter (DOM) in the water column.. A H-SEAS from the Monegros Desert (Zaragoza, NE Spain) was studied during a hydrological wetting-drying-rewetting cycle. DOM analysis included: (i) a dissolved organic carbon (DOC) mass balance; (ii) spectroscopy (absorbance and fluorescence) and (iii) a molecular characterization with Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The studied system stored a large amount of DOC and under the highest salinity conditions, salt-saturated waters (i.e., brines with salinity > 30%) accumulated a disproportionate quantity of DOC, indicating a significant in-situ net DOM production. Simultaneously, during the hydrological transition from wet to dry, the DOM pool showed strong alterations of it molecular composition. Spectroscopic methods indicated that aromatic and degraded DOM was rapidly replaced by fresher, relatively small, microbial-derived moieties with a large C/N ratio. FT-ICR-MS highlighted the accumulation of small, saturated and oxidized molecules (molecular O/C > 0.5), with a remarkable increase in the relative contribution of highly oxygenated (molecular O/C>0.9) compounds and a decrease of aliphatic and carboxyl-rich alicyclic moleculesThese results indicated that H-SEAS are extremely active in accumulating and processing DOM, with the notable release of organic solutes probably originated from decaying microplankton under large osmotic stress at extremely high salinities.

Three Microbial Musketeers of the Seas: Shewanella baltica, Aliivibrio fischeri and Vibrio harveyi, and Their Adaptation to Different Salinity Probed by a Proteomic Approach

Osmotic changes are common challenges for marine microorganisms. Bacteria have developed numerous ways of dealing with this stress, including reprogramming of global cellular processes. However, specific molecular adaptation mechanisms to osmotic stress have mainly been investigated in terrestrial model bacteria. In this work, we aimed to elucidate the basis of adjustment to prolonged salinity challenges at the proteome level in marine bacteria. The objects of our studies were three representatives of bacteria inhabiting various marine environments, Shewanella baltica, Vibrio harveyi and Aliivibrio fischeri. The proteomic studies were performed with bacteria cultivated in increased and decreased salinity, followed by proteolytic digestion of samples which were then subjected to liquid chromatography with tandem mass spectrometry analysis. We show that bacteria adjust at all levels of their biological processes, from DNA topology through gene expression regulation and proteasome assembly, to transport and cellular metabolism. The finding that many similar adaptation strategies were observed for both low- and high-salinity conditions is particularly striking. The results show that adaptation to salinity challenge involves the accumulation of DNA-binding proteins and increased polyamine uptake. We hypothesize that their function is to coat and protect the nucleoid to counteract adverse changes in DNA topology due to ionic shifts.

What do we know about osmoadaptation of Yersinia pestis?

The plague agent Yersinia pestis mainly spreads among mammalian hosts and their associated fleas. Production of a successful mammal-flea-mammal life cycle implies that Y. pestis senses and responds to distinct cues in both host and vector. Among these cues, osmolarity is a fundamental parameter. The plague bacillus lives in a tightly regulated environment in the mammalian host, while osmolarity fluctuates in the flea gut (300-550 mOsM). Here, we review the mechanisms that enable Y. pestis to perceive fluctuations in osmolarity, as well as genomic plasticity and physiological adaptation of the bacterium to this stress.

Cryopreservation of Infant Gut Microbiota with Natural Cryoprotectants

A growing body of evidence has demonstrated the importance of the gut microbiome in human health. In general, fecal microbial samples are used to study the mechanisms of relevant diseases. In this context, it is worth mentioning that an optimized cryopreservation method is urgently needed to successfully perform clinical diagnosis, therapy, and scientific investigations of the gut microbiome without affecting its viability and biological activity. In this study, we aimed to test the relative cryopreservation efficiency of different nontoxic natural cryoprotectants using infant fecal and meconium samples. First, we selected two facultative and two obligate anaerobic bacteria as the experimental gut microbial strain to compare these cryoprotectants' toxicity and concentration-dependent bacteria viabilities after cryopreservation, then the viabilities and bacterial diversity of mixed facultative and obligate anaerobic bacteria. Finally, we explored the effects of optimized cryoprotectants for meconium and infant feces after cryopreservation using 16S rRNA sequencing analyses. In addition, to better understand the effectiveness of these cryoprotectants, we used different freeze-thaw conditions mimicking real-life situations in the process of distribution. We found that the better choice for the infant fecal sample's cryopreservation was 100 mg/mL trehalose, whereas 200 mg/mL trehalose/betaine was the optimum choice for meconium cryopreservation. We hope that our results will shed light on the importance of natural cryoprotectants toward the long-term and stable viability of invaluable human gut microbial specimens.

Structural basis for potassium transport in prokaryotes by KdpFABC

KdpFABC is an oligomeric K⁺ transport complex in prokaryotes that maintains ionic homeostasis under stress conditions. The complex comprises a channel-like subunit (KdpA) from the superfamily of K⁺ transporters and a pump-like subunit (KdpB) from the superfamily of P-type ATPases. Recent structural work has defined the architecture and generated contradictory hypotheses for the transport mechanism. Here, we use substrate analogs to stabilize four key intermediates in the reaction cycle and determine the corresponding structures by cryogenic electron microscopy. We find that KdpB undergoes conformational changes consistent with other representatives from the P-type superfamily, whereas KdpA, KdpC, and KdpF remain static. We observe a series of spherical densities that we assign as K⁺ or water and which define a pathway for K⁺ transport. This pathway runs through an intramembrane tunnel in KdpA and delivers ions to sites in the membrane domain of KdpB. Our structures suggest a mechanism where ATP hydrolysis is coupled to K⁺ transfer between alternative sites in KdpB, ultimately reaching a low-affinity site where a water-filled pathway allows release of K⁺ to the cytoplasm.

Fluorescent protein-based reporters reveal stress response of intracellular Salmonella enterica at level of single bacterial cells

Intracellular bacteria such as Salmonella enterica are confronted with a broad array of defence mechanisms of their mammalian host cells. The ability to sense host cell-imposed damages, and to mount efficient stress responses are crucial for survival and proliferation of intracellular pathogens. The various combinations of host defence mechanisms acting on intracellular bacteria and their individual response also explain the occurrence of distinct subpopulations of intracellular S. enterica such as dormant or persisting, slowly or rapidly replicating cells. Here we describe a set of fluorescence protein (FP)-based reporter strains that were used to monitor the expression of cytoplasmic or periplasmic stress response systems of single bacterial cells. This is mediated by a fast-maturing FP as reporter for induction of stress response genes. We evaluated slower maturing FPs for a second function, that is, the analysis of the status of intracellular proliferation of pathogens. The combination of two FPs allows, at level of single bacterial cells, the interrogation of stress response and intracellular proliferation. Application of these reporters to S. enterica allowed us to detect and quantify distinct intracellular subpopulations with different levels of stress response and proliferation.

Bacterial Communities Are More Sensitive to Water Addition Than Fungal Communities Due to Higher Soil K and Na in a Degraded Karst Ecosystem of Southwestern China

Precipitation is predicted to become more intense in Southern China in the context of climate change; however, the responses of microbial communities to variations in soil moisture have not been well documented for karst areas. The climate is typically in a subtropical monsoon category with two different seasons: a dry season (December-May) and a wet season (June-November). Based on a randomized complete block design (RCBD), a water addition experiment (0, +20, +40, and +60% relative to local precipitation) was established in April 2017, with five replicates, in a degraded grass-shrub community. Sampling was performed in May and at the end of August of 2017. Macroelements (C, H, N, P, K, Ca, Mg, and S), microelements (Mn, Fe, Zn, and Cu), and non-essential elements (Na, Al, and Si) were quantified in the soil. The total DNA of the soil samples was analyzed through 16S rRNA amplicon by Illumina Miseq. Subsequent to the addition of water during both the dry and wet seasons, the concentrations of non-metal elements (C, H, N, S, and P, except for Si) in the soil remained relatively stable; however, metal elements (K, Na, Fe, and Mg, along with Si) increased significantly, whereas Zn and Ca decreased. During the dry season, fungal and bacterial communities were significantly distinct from those during the wet season along the PC axis 1 (p < 0.001). Water addition did not alter the compositions of bacterial or fungal communities during the dry season. However, during the wet season, water addition altered the compositions of bacterial rather than fungal community based on principal component analysis. At the phylum level, the relative abundance of Actinobacteria increased with water addition and had a significantly positive correlation with K+ (r 2 = 0.70, p < 0.001) and Na+ (r 2 = 0.36, p < 0.01) contents, whereas that of Acidobacteria, Planctomycetes, and Verrucomicrobia decreased and showed negative correlation with soil K and Na content, and no changes were observed for the fungal phyla. This suggests that the karst bacterial communities can be influenced by the addition of water during the wet season likely linked to changes in soil K and Na contents. These findings implied that increased rainfall might alter the elemental compositions of karst soils, and bacterial communities are likely to be more sensitive to variations in soil moisture in contrast to their fungal counterparts.

Serine phosphorylation regulates the P-type potassium pump KdpFABC

KdpFABC is an ATP-dependent K⁺ pump that ensures bacterial survival in K⁺-deficient environments. Whereas transcriptional activation of kdpFABC expression is well studied, a mechanism for down-regulation when K⁺ levels are restored has not been described. Here, we show that KdpFABC is inhibited when cells return to a K⁺-rich environment. The mechanism of inhibition involves phosphorylation of Ser162 on KdpB, which can be reversed in vitro by treatment with serine phosphatase. Mutating Ser162 to Alanine produces constitutive activity, whereas the phosphomimetic Ser162Asp mutation inactivates the pump. Analyses of the transport cycle show that serine phosphorylation abolishes the K⁺-dependence of ATP hydrolysis and blocks the catalytic cycle after formation of the aspartyl phosphate intermediate (E1~P). This regulatory mechanism is unique amongst P-type pumps and this study furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.

The KdpFABC complex - K+ transport against all odds

In bacteria, K⁺ is used to maintain cell volume and osmotic potential. Homeostasis normally involves a network of constitutively expressed transport systems, but in K⁺ deficient environments, the KdpFABC complex uses ATP to pump K⁺ into the cell. This complex appears to be a hybrid of two types of transporters, with KdpA descending from the superfamily of K⁺ transporters and KdpB belonging to the superfamily of P-type ATPases. Studies of enzymatic activity documented a catalytic cycle with hallmarks of classical P-type ATPases and studies of ion transport indicated that K⁺ import into the cytosol occurred in the second half of this cycle in conjunction with hydrolysis of an aspartyl phosphate intermediate. Atomic structures of the KdpFABC complex from X-ray crystallography and cryo-EM have recently revealed conformations before and after formation of this aspartyl phosphate that appear to contradict the functional studies. Specifically, structural comparisons with the archetypal P-type ATPase, SERCA, suggest that K⁺ transport occurs in the first half of the cycle, accompanying formation of the aspartyl phosphate. Further controversy has arisen regarding the path by which K⁺ crosses the membrane. The X-ray structure supports the conventional view that KdpA provides the conduit, whereas cryo-EM structures suggest that K⁺ moves from KdpA through a long, intramembrane tunnel to reach canonical ion binding sites in KdpB from which they are released to the cytosol. This review discusses evidence supporting these contradictory models and identifies key experiments needed to resolve discrepancies and produce a unified model for this fascinating mechanistic hybrid.

Cultivation at high osmotic pressure confers ubiquinone 8-independent protection of respiration on Escherichia coli

Ubiquinone 8 (coenzyme Q8 or Q8) mediates electron transfer within the aerobic respiratory chain, mitigates oxidative stress, and contributes to gene expression in Escherichia coli In addition, Q8 was proposed to confer bacterial osmotolerance by accumulating during growth at high osmotic pressure and altering membrane stability. The osmolyte trehalose and membrane lipid cardiolipin accumulate in E. coli cells cultivated at high osmotic pressure. Here, Q8 deficiency impaired E. coli growth at low osmotic pressure and rendered growth osmotically sensitive. The Q8 deficiency impeded cellular O₂ uptake and also inhibited the activities of two proton symporters, the osmosensing transporter ProP and the lactose transporter LacY. Q8 supplementation decreased membrane fluidity in liposomes, but did not affect ProP activity in proteoliposomes, which is respiration-independent. Liposomes and proteoliposomes prepared with E. coli lipids were used for these experiments. Similar oxygen uptake rates were observed for bacteria cultivated at low and high osmotic pressures. In contrast, respiration was dramatically inhibited when bacteria grown at the same low osmotic pressure were shifted to high osmotic pressure. Thus, respiration was restored during prolonged growth of E. coli at high osmotic pressure. Of note, bacteria cultivated at low and high osmotic pressures had similar Q8 concentrations. The protection of respiration was neither diminished by cardiolipin deficiency nor conferred by trehalose overproduction during growth at low osmotic pressure, but rather might be achieved by Q8-independent respiratory chain remodeling. We conclude that osmotolerance is conferred through Q8-independent protection of respiration, not by altering physical properties of the membrane.

Label-Free Bacterial Toxin Detection in Water Supplies Using Porous Silicon Nanochannel Sensors

Lipopolysaccharides (LPS) are the major component of the outer membrane of all Gram-negative bacteria and some cyanobacteria and are released during growth and cell death. LPS pose a potential health risk in water, causing acute respiratory illnesses, inhalation fever, and gastrointestinal disorders. The need for rapid and accurate detection of LPS has become a major priority to facilitate more timely and efficacious intervention and, hence, avoid unsafe water distribution. In this context, a porous silicon membrane (pSiM)-based electrochemical biosensor was developed for direct and sensitive detection of LPS. pSiM, featuring arrays of nanochannels, was modified with polymyxin B (PmB), an antimicrobial peptide with strong affinity to LPS. Detection of LPS was based on measuring the changes in the diffusion through the nanochannels of an electroactive species added in solution, caused by the nanochannel blockage upon LPS binding to PmB. Results showed a limit of detection of 1.8 ng/mL, and a linear response up to 10,000 ng/mL spiked in buffer. Selectivity of the sensor toward potential interfering species in water supplies was also assessed. Sensor performance was then evaluated in water samples from a water treatment plant (WTP), and detection of LPS well below the levels encountered in episodes of water contamination and in humidifiers was demonstrated. The same platform was also tested for bacterial detection including Pseudomonas aeruginosa and Escherichia coli spiked in water samples from a WTP. Considering its performance characteristics, this platform represents a promising screening tool to identify the presence of LPS in water supplies and provide early warning of contamination events.

GREACE-assisted adaptive laboratory evolution in endpoint fermentation broth enhances lysine production by Escherichia coli

BACKGROUND

Late-stage fermentation broth contains high concentrations of target chemicals. Additionally, it contains various cellular metabolites which have leaked from lysed cells, which would exert multifactorial stress to industrial hyperproducers and perturb both cellular metabolism and product formation. Although adaptive laboratory evolution (ALE) has been wildly used to improve stress tolerance of microbial cell factories, single-factor stress condition (i.e. target product or sodium chloride at a high concentration) is currently provided. In order to enhance bacterial stress tolerance to actual industrial production conditions, ALE in late-stage fermentation broth is desired. Genome replication engineering assisted continuous evolution (GREACE) employs mutants of the proofreading element of DNA polymerase complex (DnaQ) to facilitate mutagenesis. Application of GREACE coupled-with selection under stress conditions is expected to accelerate the ALE process.

RESULTS

In this study, GREACE was first modified by expressing a DnaQ mutant KR5-2 using an arabinose inducible promoter on a temperature-sensitive plasmid, which resulted in timed mutagenesis introduction. Using this method, tolerance of a lysine hyperproducer E. coli MU-1 was improved by enriching mutants in a lysine endpoint fermentation broth. Afterwards, the KR5-2 expressing plasmid was cured to stabilize acquired genotypes. By subsequent fermentation evaluation, a mutant RS3 with significantly improved lysine production capacity was selected. The final titer, yield and total amount of lysine produced by RS3 in a 5-L batch fermentation reached 155.0 ± 1.4 g/L, 0.59 ± 0.02 g lysine/g glucose, and 605.6 ± 23.5 g, with improvements of 14.8%, 9.3%, and 16.7%, respectively. Further metabolomics and genomics analyses, coupled with molecular biology studies revealed that mutations SpeB^A302V, AtpB^S165N and SecY^M145V mainly contributed both to improved cell integrity under stress conditions and enhanced metabolic flux into lysine synthesis.

CONCLUSIONS

Our present study indicates that improving a lysine hyperproducer by GREACE-assisted ALE in its stressful living environment is efficient and effective. Accordingly, this is a promising method for improving other valuable chemical hyperproducers.

Transcriptional Sequencing Uncovers Survival Mechanisms of Salmonella enterica Serovar Enteritidis in Antibacterial Egg White

Salmonella enterica serovar Enteritidis is a major foodborne pathogen that causes salmonellosis mainly through contaminated chicken eggs or egg products and has been a worldwide public health threat since 1980. Frequent outbreaks of this serotype through eggs correlate significantly with its exceptional survival ability in the antibacterial egg white. Research on the survival mechanism of S. Enteritidis in egg white will help to further understand the complex and highly effective antibacterial mechanisms of egg white and lay the foundation for the development of safe and effective vaccines to prevent egg contamination by this Salmonella serotype. Key pathways and genes that were previously overlooked under bactericidal conditions were characterized as being induced in egg white, and synergistic effects between different antimicrobial factors appear to exist according to the gene expression changes. Our work provides new insights into the survival mechanism of S. Enteritidis in egg white.

The survival mechanism of Salmonella enterica serovar Enteritidis in antibacterial egg white is not fully understood. In our lab, an egg white-resistant strain, S. Enteritidis SJTUF 10978, was identified. Cell envelope damage and osmotic stress response (separation of cell wall and inner membrane as well as cytoplasmic shrinkage) of this strain surviving in egg white were identified through microscopic observation. RNA-Seq analysis of the transcriptome of Salmonella survival in egg white showed that a considerable number of genes involved in DNA damage repair, alkaline pH adaptation, osmotic stress adaptation, envelope damage repair, Salmonella pathogenicity island 2 (SPI-2), iron absorption, and biotin synthesis were significantly upregulated (fold change ≥ 2) in egg white, indicating that these pathways or genes might be critical for bacterial survival. RNA-Seq results were confirmed by qRT-PCR, and the survival analysis of six gene deletion mutants confirmed their importance in the survival of bacteria in egg white. The importance of alkaline pH adaptation and envelope damage repair for Salmonella to survive in egg white were further confirmed by analysis of nhaA, cpxR, waaH, and eco deletion mutants. According to the RNA-Seq results, we propose that alkaline pH adaptation might be the cause of bacterial osmotic stress phenotype and that the synergistic effect between alkaline pH and other inhibitory factors can enhance the bacteriostatic effect of egg white. Moreover, cpxR and sigE were recognized as the central regulators that coordinate bacterial metabolism to adapt to envelope damage and alkaline pH.

IMPORTANCESalmonella enterica serovar Enteritidis is a major foodborne pathogen that causes salmonellosis mainly through contaminated chicken eggs or egg products and has been a worldwide public health threat since 1980. Frequent outbreaks of this serotype through eggs correlate significantly with its exceptional survival ability in the antibacterial egg white. Research on the survival mechanism of S. Enteritidis in egg white will help to further understand the complex and highly effective antibacterial mechanisms of egg white and lay the foundation for the development of safe and effective vaccines to prevent egg contamination by this Salmonella serotype. Key pathways and genes that were previously overlooked under bactericidal conditions were characterized as being induced in egg white, and synergistic effects between different antimicrobial factors appear to exist according to the gene expression changes. Our work provides new insights into the survival mechanism of S. Enteritidis in egg white.

Role of Proline in Pathogen and Host Interactions

SIGNIFICANCE

Proline metabolism has complex roles in a variety of biological processes, including cell signaling, stress protection, and energy production. Proline also contributes to the pathogenesis of various disease-causing organisms. Understanding the mechanisms of how pathogens utilize proline is important for developing new strategies against infectious diseases. Recent Advances: The ability of pathogens to acquire amino acids is critical during infection. Besides protein biosynthesis, some amino acids, such as proline, serve as a carbon, nitrogen, or energy source in bacterial and protozoa pathogens. The role of proline during infection depends on the physiology of the host/pathogen interactions. Some pathogens rely on proline as a critical respiratory substrate, whereas others exploit proline for stress protection.

CRITICAL ISSUES

Disruption of proline metabolism and uptake has been shown to significantly attenuate virulence of certain pathogens, whereas in other pathogens the importance of proline during infection is not known. Inhibiting proline metabolism and transport may be a useful therapeutic strategy against some pathogens. Developing specific inhibitors to avoid off-target effects in the host, however, will be challenging. Also, potential treatments that target proline metabolism should consider the impact on intracellular levels of Δ1-pyrroline-5-carboxylate, a metabolite intermediate that can have opposing effects on pathogenesis.

FUTURE DIRECTIONS

Further characterization of how proline metabolism is regulated during infection would provide new insights into the role of proline in pathogenesis. Biochemical and structural characterization of proline metabolic enzymes from different pathogens could lead to new tools for exploring proline metabolism during infection and possibly new therapeutic compounds.

Role of CsrA in stress responses and metabolism important for Salmonella virulence revealed by integrated transcriptomics

To cause infection, Salmonella must survive and replicate in host niches that present dramatically different environmental conditions. This requires a flexible metabolism and physiology, responsive to conditions of the local milieu. The sequence specific RNA binding protein CsrA serves as a global regulator that governs gene expression required for pathogenicity, metabolism, biofilm formation, and motility in response to nutritional conditions. Its activity is determined by two noncoding small RNAs (sRNA), CsrB and CsrC, which sequester and antagonize this protein. Here, we used ribosome profiling and RNA-seq analysis to comprehensively examine the effects of CsrA on mRNA occupancy with ribosomes, a measure of translation, transcript stability, and the steady state levels of transcripts under in vitro SPI-1 inducing conditions, to simulate growth in the intestinal lumen, and under in vitro SPI-2-inducing conditions, to simulate growth in the Salmonella containing vacuole (SCV) of the macrophage. Our findings uncovered new roles for CsrA in controlling the expression of structural and regulatory genes involved in stress responses, metabolism, and virulence systems required for infection. We observed substantial variation in the CsrA regulon under the two growth conditions. In addition, CsrB/C sRNA levels were greatly reduced under the simulated intracellular conditions and were responsive to nutritional factors that distinguish the intracellular and luminal environments. Altogether, our results reveal CsrA to be a flexible regulator, which is inferred to be intimately involved in maintaining the distinct gene expression patterns associated with growth in the intestine and the macrophage.

Rhodobacter sp. Rb3, an aerobic anoxygenic phototroph which thrives in the polyextreme ecosystem of the Salar de Huasco, in the Chilean Altiplano

The Salar de Huasco is an evaporitic basin located in the Chilean Altiplano, which presents extreme environmental conditions for life, i.e. high altitude (3800 m.a.s.l.), negative water balance, a wide salinity range, high daily temperature changes and the occurrence of the highest registered solar radiation on the planet (> 1200 W m^-2). This ecosystem is considered as a natural laboratory to understand different adaptations of microorganisms to extreme conditions. Rhodobacter, an anoxygenic aerobic phototrophic bacterial genus, represents one of the most abundant groups reported based on taxonomic diversity surveys in this ecosystem. The bacterial mat isolate Rhodobacter sp. strain Rb3 was used to study adaptation mechanisms to stress-inducing factors potentially explaining its success in a polyextreme ecosystem. We found that the Rhodobacter sp. Rb3 genome was characterized by a high abundance of genes involved in stress tolerance and adaptation strategies, among which DNA repair and oxidative stress were the most conspicuous. Moreover, many other molecular mechanisms associated with oxidative stress, photooxidation and antioxidants; DNA repair and protection; motility, chemotaxis and biofilm synthesis; osmotic stress, metal, metalloid and toxic anions resistance; antimicrobial resistance and multidrug pumps; sporulation; cold shock and heat shock stress; mobile genetic elements and toxin-antitoxin system were detected and identified as potential survival mechanism features in Rhodobacter sp. Rb3. In total, these results reveal a wide set of strategies used by the isolate to adapt and thrive under environmental stress conditions as a model of polyextreme environmental resistome.

Borrelia burgdorferi genes, bb0639-0642, encode a putative putrescine/spermidine transport system, PotABCD, that is spermidine specific and essential for cell survival

Polyamines are an essential class of metabolites found throughout all kingdoms in life. Borrelia burgdorferi harbors no enzymes to synthesize or degrade polyamines yet does contain a polyamine uptake system, potABCD. In this report, we describe the initial characterization of this putative transport system. After several unsuccessful attempts to inactivate potABCD, we placed the operon under the control of an inducible LacI promoter expression system. Analyses of this construct confirmed that potABCD was required for in vitro survival. Additionally, we demonstrated that the potABCD operon were upregulated in vitro by low osmolarity. Previously, we had shown that low osmolarity triggers the activation of the Rrp2/RpoN/RpoS regulatory cascade, which regulates genes essential for the transmission of spirochetes from ticks to mammalian hosts. Interestingly, induction of the pot operon was only affected in an rpoS mutant but not in a rpoN mutant, suggesting that the genes were RpoS dependent and RpoN independent. Furthermore, potABCD was upregulated during tick feeding concomitant with the initiation of spirochete replication. Finally, uptake experiments determined the specificity of B. burgdorferi's PotABCD for spermidine.

Sodium Chloride Does Not Ensure Microbiological Safety of Foods: Cases and Solutions

Addition of salt or salt-containing water to food is one of the oldest and most effective preservation methods in history; indeed, salt-cured foods are generally recognized as microbiologically safe due to their high salinity. However, a number of microbiological risks remain. The microbiological hazards and risks associated with salt-cured foods must be addressed more in-depth as they are likely to be underestimated by previous studies. This review examined a number of scientific reports and articles about the microbiological safety of salt-cured foods, which included salted, brined, pickled, and/or marinated vegetables, meat, and seafood. The following subjects are covered in order: (1) clinical cases and outbreaks attributed to salt-cured foods; (2) the prevalence of foodborne pathogens in such foods; (3) the molecular, physiological, and virulent responses of the pathogens to the presence of NaCl in both laboratory media and food matrices; (4) the survival and fate of microorganisms in salt-cured foods (in the presence/absence of additional processes); and (5) the interaction between NaCl and other stressors in food processes (e.g., acidification, antimicrobials, drying, and heating). The review provides a comprehensive overview of potentially hazardous pathogens associated with salt-cured foods and suggests further research into effective intervention techniques that will reduce their levels in the food chain.

Osmoregulated Periplasmic Glucans

Among all the systems developed by enterobacteria to face osmotic stress, only osmoregulated periplasmic glucans (OPGs) were found to be modulated during osmotic fluxes. First detected in 1973 by E.P. Kennedy's group in a study of phospholipid turnover in Escherichia coli, OPGs have been shown across alpha, beta, and gamma subdivisions of the proteobacteria. Discovery of OPG-like compounds in the epsilon subdivision strongly suggested that the presence of periplasmic glucans is essential for almost all proteobacteria. This article offers an overview of the different classes of OPGs. Then, the biosynthesis of OPGs and their regulation in E. coli and other species are discussed. Finally, the biological role of OPGs is developed. Beyond structural function, OPGs are involved in pathogenicity, in particular, by playing a role in signal transduction pathways. Recently, OPG synthesis proteins have been suggested to control cell division and growth rate.

Cell-Biological Studies of Osmotic Shock Response in Streptomyces spp

Most bacteria are likely to face osmotic challenges, but there is yet much to learn about how such environmental changes affect the architecture of bacterial cells. Here, we report a cell-biological study in model organisms of the genus Streptomyces, which are actinobacteria that grow in a highly polarized fashion to form branching hyphae. The characteristic apical growth of Streptomyces hyphae is orchestrated by protein assemblies, called polarisomes, which contain coiled-coil proteins DivIVA and Scy, and recruit cell wall synthesis complexes and the stress-bearing cytoskeleton of FilP to the tip regions of the hyphae. We monitored cell growth and cell-architectural changes by time-lapse microscopy in osmotic upshift experiments. Hyperosmotic shock caused arrest of growth, loss of turgor, and hypercondensation of chromosomes. The recovery period was protracted, presumably due to the dehydrated state of the cytoplasm, before hyphae could restore their turgor and start to grow again. In most hyphae, this regrowth did not take place at the original hyphal tips. Instead, cell polarity was reprogrammed, and polarisomes were redistributed to new sites, leading to the emergence of multiple lateral branches from which growth occurred. Factors known to regulate the branching pattern of Streptomyces hyphae, such as the serine/threonine kinase AfsK and Scy, were not involved in reprogramming of cell polarity, indicating that different mechanisms may act under different environmental conditions to control hyphal branching. Our observations of hyphal morphology during the stress response indicate that turgor and sufficient hydration of cytoplasm are required for Streptomyces tip growth.

IMPORTANCE

Polar growth is an intricate manner of growth for accomplishing a complicated morphology, employed by a wide range of organisms across the kingdoms of life. The tip extension of Streptomyces hyphae is one of the most pronounced examples of polar growth among bacteria. The expansion of the cell wall by tip extension is thought to be facilitated by the turgor pressure, but it was unknown how external osmotic change influences Streptomyces tip growth. We report here that severe hyperosmotic stress causes cessation of growth, followed by reprogramming of cell polarity and rearrangement of growth zones to promote lateral hyphal branching. This phenomenon may represent a strategy of hyphal organisms to avoid osmotic stress encountered by the growing hyphal tip.

Salinity-dependent impacts of ProQ, Prc, and Spr deficiencies on Escherichia coli cell structure

ProQ is a cytoplasmic protein with RNA chaperone activities that reside in FinO- and Hfq-like domains. Lesions at proQ decrease the level of the osmoregulatory glycine betaine transporter ProP. Lesions at proQ eliminated ProQ and Prc, the periplasmic protease encoded by the downstream gene prc. They dramatically slowed the growth of Escherichia coli populations and altered the morphologies of E. coli cells in high-salinity medium. ProQ and Prc deficiencies were associated with different phenotypes. ProQ-deficient bacteria were elongated unless glycine betaine was provided. High-salinity cultures of Prc-deficient bacteria included spherical cells with an enlarged periplasm and an eccentric nucleoid. The nucleoid-containing compartment was bounded by the cytoplasmic membrane and peptidoglycan. This phenotype was not evident in bacteria cultivated at low or moderate salinity, nor was it associated with murein lipoprotein (Lpp) deficiency, and it differed from those elicited by the MreB inhibitor A-22 or the FtsI inhibitor aztreonam at low or high salinity. It was suppressed by deletion of spr, which encodes one of three murein hydrolases that are redundantly essential for enlargement of the murein sacculus. Prc deficiency may alter bacterial morphology by impairing control of Spr activity at high salinity. ProQ and Prc deficiencies lowered the ProP activity of bacteria cultivated at moderate salinity by approximately 70% and 30%, respectively, but did not affect other osmoregulatory functions. The effects of ProQ and Prc deficiencies on ProP activity are indirect, reflecting their roles in the maintenance of cell structure.

ProQ is an RNA chaperone that controls ProP levels in Escherichia coli

Transporter ProP mediates osmolyte accumulation in Escherichia coli cells exposed to high osmolality media. The cytoplasmic ProQ protein amplifies ProP activity by an unknown mechanism. The N- and C-terminal domains of ProQ are predicted to be structurally similar to known RNA chaperone proteins FinO and Hfq from E. coli. Here we demonstrate that ProQ is an RNA chaperone, binding RNA and facilitating both RNA strand exchange and RNA duplexing. Experiments performed with the isolated ProQ domains showed that the FinO-like domain serves as a high-affinity RNA-binding domain, whereas the Hfq-like domain is largely responsible for RNA strand exchange and duplexing. These data suggest that ProQ may regulate ProP production. Transcription of proP proceeds from RpoD- and RpoS-dependent promoters. Lesions at proQ affected ProP levels in an osmolality- and growth phase-dependent manner, decreasing ProP levels when proP was expressed from its own chromosomal promoters or from a heterologous plasmid-based promoter. Small RNA molecules are known to regulate cellular levels of sigma factor RpoS. ProQ did not act by changing RpoS levels since proQ lesions did not influence RpoS-dependent stationary phase thermotolerance and they affected ProP production and activity similarly in bacteria without and with an rpoS defect. Taken together, these results suggest that ProQ does not regulate proP transcription. It may act as an RNA-binding protein to regulate proP translation.