Importance of RpoS and Dps in survival of exposure of both exponential- and stationary-phase Escherichia coli cells to the electrophile N-ethylmaleimide

RpoS-Regulated Genes and Phenotypes in the Phytopathogenic Bacterium Pectobacterium atrosepticum

The alternative sigma factor RpoS is considered to be one of the major regulators providing stress resistance and cross-protection in bacteria. In phytopathogenic bacteria, the effects of RpoS have not been analyzed with regard to cross-protection, and genes whose expression is directly or indirectly controlled by RpoS have not been determined at the whole-transcriptome level. Our study aimed to determine RpoS-regulated genes and phenotypes in the phytopathogenic bacterium Pectobacterium atrosepticum. Knockout of the rpoS gene in P. atrosepticum affected the long-term starvation response, cross-protection, and virulence toward plants with enhanced immune status. The whole-transcriptome profiles of the wild-type P. atrosepticum strain and its ΔrpoS mutant were compared under different experimental conditions, and functional gene groups whose expression was affected by RpoS were determined. The RpoS promoter motif was inferred within the promoter regions of the genes affected by rpoS deletion, and the P. atrosepticum RpoS regulon was predicted. Based on RpoS-controlled phenotypes, transcriptome profiles, and RpoS regulon composition, the regulatory role of RpoS in P. atrosepticum is discussed.

Insertion of prpoD_rpoS fragment enhances expression of recombinant protein by dps auto-inducible promoter in Escherichia coli

BACKGROUND

Nowadays, recombinant therapeutic proteins have been widely produced and consumed. For the safety and effectiveness of the protein production, an auto-inducible expression vector is required to replace inducer interference, which is uneconomic and could be harmful. In this research, an auto-inducible expression plasmid, pCAD₂_sod (a pBR322 derivate plasmid), which was under dps (RpoS-dependent gene) promoter control, was modified to provide RpoS at earlier phase. Hence, accumulates more target protein and resulting a new plasmid, pCAD₂⁺_sod. pCAD₂_sod had been constructed to automatically induces the expression of recombinant superoxide dismutase (SOD) from Staphylococcus equorum (rMnSODSeq) in the stationary growth phase of Escherichia coli. This work aimed to obtain pCAD₂⁺_sod and determine the expression level of rMnSODSeq on mRNA and protein level.

METHOD AND RESULTS

A synthetic rpoS coding region under rpoD promoter control (prpoD_rpoS) was inserted to pCAD₂_sod and generated pCAD₂⁺_sod. The rMnSODSeq (24.3 kDa) produced from pCAD₂⁺_sod was ~ 1.5 fold higher at 37 °C and more intense at 43 °C compared to that from pCAD₂_sod, likewise shifted to earlier phase (after 1 h of incubation), as shown in the SDS-PAGE. The dismutase activity was also retained after zymography assay. The mRNA level from pCAD₂⁺_sod was determined by qPCR and gave quantification cycle (Cq) values of cDNA lowest among others. It made the relative quantification (RQ) of the mRNA expression towards rho reference gene were high.

CONCLUSIONS

The prpoD_rpoS insertion shifts and increases the rMnSODSeq production from stationary to exponential phase. The pCAD₂⁺_sod plasmid is potential for further recombinant protein productions.

Characterizing the mechanosensitive response of Paraburkholderia graminis membranes

Bacterial mechanosensitive channels gate in response to membrane tension, driven by shifts in environmental osmolarity. The mechanosensitive channels of small conductance (MscS) and large conductance (MscL) from Escherichia coli (Ec) gate in response to mechanical force applied to the membrane. Ec-MscS is the foundational member of the MscS superfamily of ion channels, a diverse family with at least fifteen subfamilies identified by homology to the pore lining helix of Ec-MscS, as well as significant diversity on the N- and C-termini. The MscL family of channels are homologous to Ec-MscL. In a rhizosphere associated bacterium, Paraburkholderia graminis C4D1M, mechanosensitive channels are essential for cell survival during changing osmotic environments such as a rainstorm. Utilizing bioinformatics, we predicted six MscS superfamily members and a single MscL homologue. The MscS superfamily members fall into at least three subfamilies: bacterial cyclic nucleotide gated, multi-TM, and extended N-terminus. Osmotic downshock experiments show that wildtype P. graminis cells contain a survival mechanism that prevents cell lysis in response to hypoosmotic shock. To determine if this rescue is due to mechanosensitive channels, we developed a method to create giant spheroplasts of P. graminis to explore the single channel response to applied mechanical tension. Patch clamp electrophysiology on these spheroplasts shows two unique conductances: MscL-like and MscS-like. These conductances are due to likely three unique proteins. This indicates that channels that gate in response to mechanical tension are present in the membrane. Here, we report the first single channel evidence of mechanosensitive ion channels from P. graminis membranes.

Production of acetol from glycerol using engineered Escherichia coli

Escherichia coli Lin43 is a strain which has some mutations in glycerol kinase (GlpK) and the repressor for the glycerol 3-phosphate regulon (GlpR). When exposed to glycerol, it quickly accumulates lethal levels of methylglyoxal, which is a precursor of acetol; acetol is important for the manufacture of polyols, acrolein, dyes, and skin tanning agents. This work reports the engineering of E. coli Lin 43 for the conversion of glycerol into acetol. First, the glyoxalase system was interrupted by deleting the gloA gene, which increased the acetol yield by 32%. In addition, the aldehyde reductase YqhD was overexpressed which led to an increase of acetol production by 11.4-fold. Acetol production was optimized by varying the cell density, glycerol concentration, supplemental carbon source, pH and temperature. Under the optimal conditions (OD600=20, 20 g/L glycerol, 2g/L succinate, pH 7.0, and 28°C), we obtained 5.4 g/L acetol in 21 h.

Coevolution of aah: a dps-like gene with the host bacterium revealed by comparative genomic analysis

A protein named AAH was isolated from the bacterium Microbacterium arborescens SE14, a gut commensal of the lepidopteran larvae. It showed not only a high sequence similarity to Dps-like proteins (DNA-binding proteins from starved cell) but also reversible hydrolase activity. A comparative genomic analysis was performed to gain more insights into its evolution. The GC profile of the aah gene indicated that it was evolved from a low GC ancestor. Its stop codon usage was also different from the general pattern of Actinobacterial genomes. The phylogeny of dps-like proteins showed strong correlation with the phylogeny of host bacteria. A conserved genomic synteny was identified in some taxonomically related Actinobacteria, suggesting that the ancestor genes had incorporated into the genome before the divergence of Micrococcineae from other families. The aah gene had evolved new function but still retained the typical dodecameric structure.

The unusual antibacterial activity of medical-grade Leptospermum honey: antibacterial spectrum, resistance and transcriptome analysis

There is an urgent need for new, effective agents in topical wound care, and selected honeys show potential in this regard. Using a medical-grade honey, eight species of problematic wound pathogens, including those with high levels of innate or acquired antibiotic resistance, were killed by 4.0-14.8% honey, which is a concentration that can be maintained in the wound environment. Resistance to honey could not be induced under conditions that rapidly induced resistance to antibiotics. Escherichia coli macroarrays were used to determine the response of bacterial cells to a sub-lethal dose of honey. The pattern of gene expression differed to that reported for other antimicrobial agents, indicating that honey acts in a unique and multifactorial way; 78 (2%) genes were upregulated and 46 (1%) genes were downregulated more than two-fold upon exposure to the medical-grade honey. Most of the upregulated genes clustered into distinct functional regulatory groups, with many involved in stress responses, and the majority of downregulated genes encoded for products involved in protein synthesis. Taken together, these data indicate that honey is an effective topical antimicrobial agent that could help reduce some of the current pressures that are promoting antibiotic resistance.

Impact of rpoS deletion on the proteome of Escherichia coli grown planktonically and as biofilm

To investigate the role of rpoS in gene expression of Escherichia coli cells grown as biofilms, we compared the proteomes of a rpoS mutant and the wild-type strain. Experiments were performed on planktonic cells (in exponential or stationary growth phase) and biofilms developed on glass wool. Spot-by-spot comparison of gels obtained from biofilm and planktonic wild-type organisms showed that the intensity of between 22 and 30% of detected spots was affected by the growth mode, depending of the control used. Principal component analysis, used to interpret the variations in protein spot densities, discriminated exponential-phase cells (wild-type and mutant) from the other incubation conditions and secondarily 72-old cultures. The statistical analysis demonstrated that the rpoS mutation did not significantly modify the proteome of exponential-growth phase cells, the differences involving only 3% of the proteome. However, increasing the incubation time from 8 to 72 h noticeably increased the number of changed proteins. A cluster analysis showed that RpoS plays a role in the special nature of the gene expression of biofilm cells but lower than in stationary-phase bacteria. We identified 35 rpoS-regulated proteins that were already or not described as controlled by this sigma factor. For some of them, the mode of regulation by RpoS was obviously dependent on the culture condition (planktonic vs biofilm).

Selective repression by Fis and H-NS at the Escherichia coli dps promoter

Dps is a nucleoid-associated protein that plays a major role in condensation of the Escherichia coli chromosome in stationary phase. Here we show that two other nucleoid-associated proteins, Fis and H-NS, can bind at the dps gene promoter and downregulate its activity. Both Fis and H-NS selectively repress the dps promoter, preventing transcription initiation by RNA polymerase containing sigma(70), the housekeeping sigma factor, but not by RNA polymerase containing sigma(38), the stationary-phase sigma factor. Fis represses by trapping RNA polymerase containing sigma(70) at the promoter. In contrast, H-NS functions by displacing RNA polymerase containing sigma(70), but not RNA polymerase containing sigma(38). Dps levels are known to be very low in exponentially growing cells and rise sharply as cells enter stationary phase. Conversely, Fis levels are high in growing cells but fall to nearly zero in stationary-phase cells. Our data suggest a simple model to explain how the Dps-dependent super-compaction of the folded chromosome is triggered as cell growth ceases.

RpoS regulation of gene expression during exponential growth of Escherichia coli K12

RpoS is a major regulator of genes required for adaptation to stationary phase in E. coli. However, the exponential phase expression of some genes is affected by rpoS mutation, suggesting RpoS may also have an important physiological role in growing cells. To test this hypothesis, we examined the regulatory role of RpoS in exponential phase using both genomic and biochemical approaches. Microarray expression data revealed that, in the rpoS mutant, the expression of 268 genes was attenuated while the expression of 24 genes was enhanced. Genes responsible for carbon source transport (the mal operon for maltose), protein folding (dnaK and mopAB), and iron acquisition (fepBD, entCBA, fecI, and exbBD) were positively controlled by RpoS. The importance of RpoS-mediated control of iron acquisition was confirmed by cellular metal analysis which revealed that the intracellular iron content of wild type cells was two-fold higher than in rpoS mutant cells. Surprisingly, many previously identified RpoS stationary-phase dependent genes were not controlled by RpoS in exponential phase and several genes were RpoS-regulated only in exponential phase, suggesting the involvement of other regulators. The expression of RpoS-dependent genes osmY, tnaA and malK was controlled by Crl, a transcriptional regulator that modulates RpoS activity. In summary, the identification of a group of exponential phase genes controlled by RpoS reveals a novel aspect of RpoS function.

Formation of N-ethylmaleimide (NEM)-glutathione conjugate and N-ethylmaleamic acid revealed by mass spectral characterization of intracellular and extracellular microbial metabolites of NEM

The extracellular and intracellular metabolites formed upon exposure of activated sludge microorganisms to a sublethal concentration of N-ethylmaleimide were monitored by liquid chromatography with ion trap mass spectrometry. The metabolite N-ethylsuccinimido-S-glutathione (m/z 433) was converted rapidly to N-(2-oxoethyl)-2,2-(propionylamino)propanamide (m/z 187) and N-ethylmaleamic acid (m/z 144).

Structural studies on the second Mycobacterium smegmatis Dps: invariant and variable features of structure, assembly and function

A second DNA binding protein from stationary-phase cells of Mycobacterium smegmatis (MsDps2) has been identified from the bacterial genome. It was cloned, expressed and characterised and its crystal structure was determined. The core dodecameric structure of MsDps2 is the same as that of the Dps from the organism described earlier (MsDps1). However, MsDps2 possesses a long N-terminal tail instead of the C-terminal tail in MsDps1. This tail appears to be involved in DNA binding. It is also intimately involved in stabilizing the dodecamer. Partly on account of this factor, MsDps2 assembles straightway into the dodecamer, while MsDps1 does so on incubation after going through an intermediate trimeric stage. The ferroxidation centre is similar in the two proteins, while the pores leading to it exhibit some difference. The mode of sequestration of DNA in the crystalline array of molecules, as evidenced by the crystal structures, appears to be different in MsDps1 and MsDps2, highlighting the variability in the mode of Dps-DNA complexation. A sequence search led to the identification of 300 Dps molecules in bacteria with known genome sequences. Fifty bacteria contain two or more types of Dps molecules each, while 195 contain only one type. Some bacteria, notably some pathogenic ones, do not contain Dps. A sequence signature for Dps could also be derived from the analysis.

Activated sludge inhibition by chemical stressors--a comprehensive study

The effects of shock loads of 1-chloro-2,4-dinitrobenzene (CDNB); cadmium; 1-octanol; 2,4-dinitrophenol (DNP); weakly complexed cyanide; pH 5, 9, and 11; and high ammonia levels on activated sludge biomass growth, respiration rate, flocculation, chemical oxygen demand removal, dewaterability, and settleability were studied. For all chemical shocks, except ammonia and pH, concentrations that caused 15, 25, and 50% respiration inhibition were used to provide a single pulse shock to sequencing batch reactor systems containing a nitrifying or non-nitrifying biomass. Cadmium and pH 11 shocks were most detrimental to all processes, followed by CDNB. The DNP and cyanide primarily affected respiration, while pH 5, pH 9, octanol, and ammonia did not affect the treatment process to a significant extent. A chemical source-process effect matrix is provided, which we believe will aid in the development of methods that prevent and/or attenuate the effects of toxic shock loads on activated sludge systems.

Role of N and C-terminal Tails in DNA Binding and Assembly in Dps: Structural Studies of Mycobacterium smegmatis Dps Deletion Mutants

Mycobacterium smegmatis Dps degrades spontaneously into a species in which 16 C-terminal residues are cleaved away. A second species, in which all 26 residues constituting the tail were deleted, was cloned, expressed and purified. The first did not bind DNA but formed dodecamers like the native protein, while the second did not bind to DNA and failed to assemble into dodecamers, indicating a role in assembly also for the tail. In the crystal structure of the species without the entire C-terminal tail the molecule has an unusual open decameric structure resulting from the removal of two adjacent subunits from the original dodecameric structure of the native form. A Dps dodecamer could assemble with a dimer or one of two trimers (trimer-A and trimer-B) as intermediate. Trimer-A is the intermediate species in the M. smegmatis protein. Estimation of the surface area buried on trimerization indicates that association within trimer-B is weak. It weakens further when the C-terminal tail is removed, leading to the disruption of the dodecameric structure. Thus, the C-terminal tail has a dual role, one in DNA binding and the other in the assembly of the dodecamer. M. smegmatis Dps also has a short N-terminal tail. A species with nine N-terminal residues deleted formed trimers but not dodecamers in solution, unlike wild-type M. smegmatis Dps, under the same conditions. Unlike in solution, the N-terminal mutant forms dodecamers in the crystal. In native Dps, the N-terminal stretch of one subunit and the C-terminal stretch of a neighboring subunit lock each other into ordered positions. The deletion of one stretch results in the disorder of the other. This disorder appears to result in the formation of a trimeric species of the N-terminal deletion mutant contrary to the indication provided by the native structure. The ferroxidation site is intact in the mutants.

Implicating the glutathione-gated potassium efflux system as a cause of electrophile-induced activated sludge deflocculation

The glutathione-gated K(+) efflux (GGKE) system represents a protective microbial stress response that is activated by electrophilic or thiol-reactive stressors. It was hypothesized that efflux of cytoplasmic K(+) occurs in activated sludge communities in response to shock loads of industrially relevant electrophilic chemicals and results in significant deflocculation. Novosphingobium capsulatum, a bacterium consistent with others found in activated sludge treatment systems, responded to electrophilic thiol reactants with rapid efflux of up to 80% of its cytoplasmic K(+) pool. Furthermore, N. capsulatum and activated sludge cultures exhibited dynamic efflux-uptake-efflux responses very similar to those observed by others in Escherichia coli K-12 exposed to the electrophilic stressors N-ethylmaleimide and 1-chloro-2,4-dinitrobenzene and the reducing agent dithiothreitol. Fluorescent LIVE/DEAD stains were used to show that cell lysis was not the cause of electrophile-induced K(+) efflux. Nigericin was used to artificially stimulate K(+) efflux from N. capsulatum and activated sludge cultures as a comparison to electrophile-induced K(+) efflux and showed that cytoplasmic K(+) efflux by both means corresponded with activated sludge deflocculation. These results parallel those of previous studies with pure cultures in which GGKE was shown to cause cytoplasmic K(+) efflux and implicate the GGKE system as a probable causal mechanism for electrophile-induced, activated sludge deflocculation. Calculations support the notion that shock loads of electrophilic chemicals result in very high K(+) concentrations within the activated sludge floc structure, and these K(+) levels are comparable to that which caused deflocculation by external (nonphysiological) KCl addition.

DpsA protects the human pathogen Burkholderia pseudomallei against organic hydroperoxide

The human pathogen, Burkholderia pseudomalle, is able to survive and multiply in hostile environments such as within macrophages. In an attempt to understand its strategy to cope with oxidative stress, the physiological role and gene regulation of a nonspecific DNA-binding protein (DpsA) was investigated. Expression of dpsA increases in response to oxidative stress through increased transcription from the upstream katG (catalase-peroxidase) promoter, which is OxyR dependent. dpsA is also transcribed from its own promoter, which is activated by osmotic stress in an OxyR-independent manner. DpsA-deficient mutants are hypersensitive to tert-butyl hydroperoxide, while overexpression of DpsA leads to increased resistance to organic oxidants. B. pseudomallei DpsA can also protect Escherichia coli against organic hydroperoxide toxicity. The mechanism of DpsA-mediated resistance to organic hydroperoxides was shown to differ from that of alkyl hydroperoxide reductase.

A role for mechanosensitive channels in survival of stationary phase: regulation of channel expression by RpoS

The mechanosensitive (MS) channels MscS and MscL are essential for the survival of hypoosmotic shock by Escherichia coli cells. We demonstrate that MscS and MscL are induced by osmotic stress and by entry into stationary phase. Reduced levels of MS proteins and reduced expression of mscL- and mscS-LacZ fusions in an rpoS mutant strain suggested that the RNA polymerase holoenzyme containing sigmaS is responsible, at least in part, for regulating production of MS channel proteins. Consistent with the model that the effect of sigmaS is direct, the MscS and MscL promoters both use RNA polymerase containing sigmaS in vitro. Conversely, clpP or rssB mutations, which cause enhanced levels of sigmaS, show increased MS channel protein synthesis. RpoS null mutants are sensitive to hypoosmotic shock upon entry into stationary phase. These data suggest that MscS and MscL are components of the RpoS regulon and play an important role in ensuring structural integrity in stationary phase bacteria.

Bimodal protection of DNA by Mycobacterium smegmatis DNA-binding protein from stationary phase cells

Some members of the DNA-binding protein from stationary phase cells (Dps) family of proteins have been shown to play an important role in protecting microorganisms from oxidative or nutritional stress. Dps homologs have been identified in various bacteria such as Escherichia coli, Bacillus subtilis, and Listeria innocua. Recently we have reported the presence of a Dps homolog, Ms-Dps, in Mycobacterium smegmatis. Ms-Dps was found to have a nonspecific DNA binding ability. Here we have detected two stable oligomeric forms of Ms-Dps in vitro, a trimeric and a dodecameric form. Interestingly, the conversion of Dps from a trimeric to a dodecameric form takes place upon incubation at 37 degrees C for 12 h. These two oligomeric forms differ in their DNA binding properties. The dodecameric form is capable of DNA binding and forming large crystalline arrays with DNA, whereas the trimeric form cannot do so. However, even in the absence of DNA binding, the trimeric form has the capacity to protect the DNA against Fenton's-mediated damage. The protection is afforded by the ferroxidase activity of the trimer. However, the trimeric form cannot protect DNA from DNaseI attack, for which a direct physical shielding of DNA by the dodecamer is required. Thus we suggest that Ms-Dps provides a bimodal protection of DNA by its two different oligomeric forms.

Biofilm formation by acid-adapted and nonadapted Listeria monocytogenes in fresh beef decontamination washings and its subsequent inactivation with sanitizers

The antimicrobial effects of sodium hypochlorite (SH, 200 ppm, at an adjusted pH of 6.80 +/- 0.20 and at an unadjusted pH of 10.35 +/- 0.25), quaternary ammonium compound (pH 10.20 +/- 0.12, 200 ppm), and peroxyacetic acid (PAA, pH 3.45 +/- 0.20, 150 ppm) on previously acid-adapted or nonadapted Listeria monocytogenes inoculated (10(5) CFU/ml) into beef decontamination water washings were evaluated. The effects of the sanitizers on suspended cells (planktonic or deattached) and on cells attached to stainless steel coupons obtained from inoculated washings stored at 15 degrees C for up to 14 days were studied. Cells were exposed to sanitizers on days 2, 7, and 14. The pathogen had formed a biofilm of 5.3 log CFU/cm2 by day 2 of storage (which was reduced to 4.6 log CFU/cm2 by day 14), while the total microbial populations showed more extensive attachment (6.1 to 6.6 log CFU/cm2). The sanitizers were more effective in reducing populations of cells in suspension than in reducing populations of attached cells. Overall, there were no differences between previously acid-adapted and nonadapted L monocytogenes with regard to sensitivity to sanitizers. The total microbial biofilms were the most sensitive to all of the sanitizers on day 2, but their resistance increased during storage, and they were at their most resistant on day 14. Listeria monocytogenes displayed stronger resistance to the effects of the sanitizers on day 7 than on day 2 but had become sensitized to all sanitizers by day 14. SH at the adjusted pH (6.80) (ASH) was generally more effective in reducing bacterial populations than was SH at the unadjusted pH. PAA generally killed attached cells faster at 30 to 300 s of exposure than did the other sanitizers, except for ASH on day 2. PAA was more effective in killing attached cells than in killing cells treated in suspension, in contrast to the other sanitizers.

Cell immobilization induces changes in the protein response of Escherichia coli K-12 to a cold shock

We have compared the protein expression of gel-entrapped Escherichia coli cells submitted to a cold shock at 4 degrees C with those of exponential- and stationary-phase free-floating counterparts. Autoradiograms of two-dimensional gel electrophoresis patterns of proteins radiolabeled with L-[35S]methionine were compared using computing scanning densitometry. The levels of 203 proteins synthesized during the temperature shift were significantly and reproducibly higher than those corresponding to synthesis at 37 degrees C. A principal component analysis (PCA) was performed on the synthesis levels of these 203 proteins in the different incubation conditions tested. This study showed that the protein response of immobilized cells after the cold shock was significantly different from those of exponential- and stationary-phase free-floating organisms. For instance, protein SSB was specifically overexpressed by shocked immobilized organisms. Such induction of specific molecular mechanisms in immobilized bacteria might explain the high resistance of sessile-like organisms to stresses.

Catalase has a novel protective role against electrophile killing of Xanthomonas

The ability of XANTHOMONAS: campestris pv. phaseoli to protect itself against lethal concentrations of man-made (N:-ethylmaleimide, NEM) and endogenously produced (methylglyoxal, MG) electrophiles was investigated. Pretreatment of X. c. pv. phaseoli with a low concentration of NEM induced protection against lethal concentrations of NEM and MG. MG pretreatment weakly induced protection against NEM but not against MG itself. NEM-induced protection against electrophile killing required new protein synthesis and was abolished by the addition of a protein synthesis inhibitor. By contrast, MG-induced protection against NEM killing was independent of de novo protein synthesis. X. c. pv. phaseoli harbouring an expression vector carrying a catalase gene was over 100-fold more resistant to MG and NEM killing. High expression levels of genes for other peroxide-protective enzymes, such as those for alkyl hydroperoxide reductase (ahpC and ahpF) and ohr, failed to protect against electrophile killing. Thus, catalase appears to have a novel protective role(s) against electrophile toxicity. This finding suggests that in X. c. pv. phaseoli NEM and MG toxicity might involve accumulation and/or increased production of H(2)O(2). This idea was supported by the observation that addition of 10 mM sodium pyruvate, a compound that can react chemically with peroxide or hydroxyl radical scavengers (DMSO and glycerol), was found to protect XANTHOMONAS: from electrophile killing. The protective role of catalase and the role of H(2)O(2) in electrophile toxicity are novel observations and could be generally important in other bacteria. In addition, unlike other bacteria, XANTHOMONAS: in stationary phase was more susceptible to electrophile killing compared to cells in exponential phase.

Contribution of dps to acid stress tolerance and oxidative stress tolerance in Escherichia coli O157:H7

An Escherichia coli O157:H7 dps::nptI mutant (FRIK 47991) was generated, and its survival was compared to that of the parent in HCl (synthetic gastric fluid, pH 1.8) and hydrogen peroxide (15 mM) challenges. The survival of the mutant in log phase (5-h culture) was significantly impaired (4-log(10)-CFU/ml reduction) compared to that of the parent strain (ca. 1.0-log(10)-CFU/ml reduction) after a standard 3-h acid challenge. Early-stationary-phase cells (12-h culture) of the mutant decreased by ca. 4 log(10) CFU/ml while the parent strain decreased by approximately 2 log(10) CFU/ml. No significant differences in the survival of late-stationary-phase cells (24-h culture) between the parent strain and the mutant were observed, although numbers of the parent strain declined less in the initial 1 h of acid challenge. FRIK 47991 was more sensitive to hydrogen peroxide challenge than was the parent strain, although survival improved in stationary phase. Complementation of the mutant with a functional dps gene restored acid and hydrogen peroxide tolerance to levels equal to or greater than those exhibited by the parent strain. These results demonstrate that decreases in survival were from the absence of Dps or a protein regulated by Dps. The results from this study establish that Dps contributes to acid tolerance in E. coli O157:H7 and confirm the importance of Dps in oxidative stress protection.

Glutathione-dependent conversion of N-ethylmaleimide to the maleamic acid by Escherichia coli: an intracellular detoxification process

The electrophile N-ethylmaleimide (NEM) elicits rapid K(+) efflux from Escherichia coli cells consequent upon reaction with cytoplasmic glutathione to form an adduct, N-ethylsuccinimido-S-glutathione (ESG) that is a strong activator of the KefB and KefC glutathione-gated K(+) efflux systems. The fate of the ESG has not previously been investigated. In this report we demonstrate that NEM and N-phenylmaleimide (NPM) are rapidly detoxified by E. coli. The detoxification occurs through the formation of the glutathione adduct of NEM or NPM, followed by the hydrolysis of the imide bond after which N-substituted maleamic acids are released. N-ethylmaleamic acid is not toxic to E. coli cells even at high concentrations. The glutathione adducts are not released from cells, and this allows glutathione to be recycled in the cytoplasm. The detoxification is independent of new protein synthesis and NAD(+)-dependent dehydrogenase activity and entirely dependent upon glutathione. The time course of the detoxification of low concentrations of NEM parallels the transient activation of the KefB and KefC glutathione-gated K(+) efflux systems.

Protein patterns of gel-entrapped Escherichia coli cells differ from those of free-floating organisms

The two-dimensional electrophoretic patterns of cellular proteins from gel-entrapped Escherichia coli cells were compared to those of exponential- and stationary-phase free-floating organisms. The amounts of several proteins in immobilized cells were significantly different from those in free bacteria. Immobilized organisms rapidly produced a high level of dipeptide permease and a single-strand binding protein, and progressively accumulated an aldehyde dehydrogenase. Immobilization also induced a decrease in the levels of two proteins, i.e., the YFID protein and a DNA-binding, stationary-phase protein. The possible role of these proteins in the high resistance of immobilized bacteria to stresses is discussed.

Increased sensitivity to oxidative challenges associated with topA deletion in Escherichia coli

Deletion of topA in Escherichia coli was found to result in a higher level of killing after treatment with either hydrogen peroxide or N-ethylmaleimide. This effect on oxidative challenge response represents a new role for E. coli DNA topoisomerase I in addition to prevention of excessive negative supercoiling of DNA.

The intrinsic resistance of Escherichia coli to various antimicrobial agents requires ppGpp and sigma s

We have examined the effect of a wide range of antimicrobial compounds (antibiotics and biocides) on the growth of various strains of Escherichia coli which vary in their ability to produce ppGpp and sigma s. We conclude that strains able to synthesize ppGpp, either in a RelA- or SpoT-dependent manner, possess a greater resistance to antimicrobial compounds compared with strains that cannot produce ppGpp. Investigation of an E. coli strain, unable to produce sigma s, and an isogenic parent strain, suggests that there is a requirement for this sigma factor in increased expression of intrinsic resistance. We propose that ppGpp is required to induce production of sigma s, which in turn directs gene expression of intrinsic resistance determinants.

The regulation of intracellular pH in bacteria

The regulation of intracellular pH (pHi) in bacterial cells is achieved through control over cation (and anion) permeability. In addition to the active components of homeostasis there are contributions from essentially passive elements, such as the lipid composition of the membrane and the buffering capacity of the cytoplasm. Active homeostasis involves control over the movement of K+, Na+ and H+. Alterations in the membrane permeability for any of these cations may cause perturbation of homeostasis. In Escherichia coli this is exemplified by the controlled activation of K+ efflux systems by glutathione adducts leading to temporary acidification of the cytoplasm. This is achieved by sophisticated control over the KefB and KefC systems, and is tightly integrated with glutathione-dependent detoxification mechanisms. Such control over pHi facilitates survival of the cell following exposure to toxic electrophiles. The components of pH homeostasis will be reviewed and the molecular mechanisms, and role of, the KefB and KefC systems will be discussed.

Starvation, cessation of growth and bacterial aging

Several components of different Escherichia coli regulons are integrated to prevent premature oxidative deterioration of starving cells. The interconnected regulation of these regulons encompasses oxidation signalling, sigma factor competition, and possibly also the use of sigma factor inhibitors. Recent data demonstrate that stasis-induced oxidation targets both DNA and protein and that some enzymes are specifically susceptible to oxidative attack.

Augmentation of killing of Escherichia coli O157 by combinations of lactate, ethanol, and low-pH conditions

The acid tolerance of Escherichia coli O157:H7 strains can be overcome by addition of lactate, ethanol, or a combination of the two agents. Killing can be increased by as much as 4 log units in the first 5 min of incubation at pH 3 even for the most acid-tolerant isolates. Exponential-phase, habituated, and stationary-phase cells are all sensitive to incubation with lactate and ethanol. Killing correlates with disruption of the capacity for pH homeostasis. Habituated and stationary-phase cells can partially offset the effects of the lowering of cytoplasmic pH.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.