Phosphate starvation is accompanied by disturbance of intracellular cysteine homeostasis in Escherichia coli

Metabolic rearrangements that occur during depletion of essential nutrients can lead to accumulation of potentially dangerous metabolites. Here we showed that depletion of phosphate (Pi), accompanied by a sharp inhibition of growth and respiration, caused a transient excess of intracellular cysteine due to a decrease in the rate of protein synthesis. High cysteine level can be dangerous due to its ability to produce ROS and reduce Fe3+ to Fenton-reactive Fe2+. To prevent these negative effects, excess cysteine was mainly incorporated into glutathione (GSH), the intracellular level of which increased by 3 times, and was also exported to the medium and partially degraded to form H2S with participation of 3-mercaptopyruvate sulfotransferase (3MST). The addition of Pi to starving cells led to a sharp recovery of respiration and growth, GSH efflux into the medium and K+ influx into the cells. A pronounced coupling of Pi, GSH, and K+ fluxes was shown upon Pi depletion and addition, which may be necessary to maintain the ionic balance in the cytoplasm. We suggest that processes aimed at restoring cysteine homeostasis may be an integral part of the universal response to stress under different types of stress and for different types of bacteria.

[Changes in the Activity of Antioxidant Systems of Escherichia coli under Phosphate Starvation]

Changes in the activity of antioxidant systems in Escherichia coli during phosphate starvation have been studied. It is shown that starvation was accompanied by a decrease in the intensity of respiration, an increase in the rate of superoxide production, and a decrease in the level of ATP. Simultaneously, there was a decrease in H2O2 in the medium and a significant increase in the expression of the katG and katE genes which encode the HPI and HPII catalases, respectively. At the same time, there was no drop in the membrane potential, which may indicate the retention of normal membrane activity in starving cells. It has been shown for the first time that the transition of E. coli to phosphate starvation is accompanied by significant changes in the status of glutathione. The most important of these are associated with a decrease in the level of reduced glutathione in the medium (GSHout) and with a simultaneous increase in its content in the cytoplasm (GSHin), as well as a shift in the GSHin to oxidized glutathione form (GSSGin) ratio towards reductive values, and GSHout/GSSGout towards oxidative values. Among the mutants used in the work, the gor trxB double mutant, which is deficient in the synthesis of glutathione reductase and thioredoxin reductase, showed the most pronounced distinctive features. Compared to the parental strain, this mutant showed a multiple higher expression of katG::lacZ, the highest level of oxidized intra- and extracellular glutathione, and, accordingly, the lowest GSH/GSSG ratio in both compartments. In general, the data we obtained indicate that during phosphate starvation the interaction of the glutathione redox-system and regulons that control protection against reactive oxygen species creates conditions that allow maintaining the concentration of ROS below the toxic level. As a result, phosphate-starved E. coli cells can maintain high viability for a long period of time, which allows them to quickly resume growth after the addition of phosphate.

Effect of Changes in the Redox Status on Biofilm Formation in Escherichia coli

Changes in the redox balance in the medium and in Escherichia coli cells significantly affect the ability of bacteria to form biofilms. An increase in the level of aeration in the culture of wild-type bacteria led to a 3-fold decrease in the mass of biofilms. Mutants lacking components of the glutathione and thioredoxin redox systems, as well as transporters involved in the transmembrane cycling of glutathione, demonstrated increased biofilm formation ability. The effect of exogenous glutathione on biofilm formation depended on the culturing conditions. The addition of 0.1-1 mM Trolox (a water-soluble analog of vitamin E) was accompanied by a 30-40% reduction in biofilm formation.

Study of the early response of Escherichia coli lpcA and ompF mutants to ciprofloxacin

In most previous studies the sensitivity of Escherichia coli outer membrane mutants to ciprofloxacin (CF) was studied by MIC method. In the present work, the early response of these mutants to CF was studied using physiological and biochemical methods and electrochemical sensors. The use of sensors made it possible to monitor dissolved oxygen, potassium and extracellular sulfide continuously directly in growing cultures in real time. In the absence of CF, no significant differences were found between the mutants deficient in porin OmpF and lipopolysaccharide (LPS) and the parent. The only exception was 5-6 times higher extracellular glutathione and 1.5-3 times lower intracellular glutathione in the lpcA compared to the parent and the ompF. Ciprofloxacin inhibited growth, respiration, membrane potential and K⁺ consumption, which was less pronounced in both mutants compared to the parent. Changes in these parameters correlated with each other, but not with survival. A reversible increase in sulfide level was observed at 3 μg ml^-1 CF in the parent, at 20 μg ml^-1 CF in ompF and was absent in lpcA at all concentrations. The data obtained show that the use of electrochemical sensors can provide a more complete understanding of the early response of bacteria to CF.

Study of the contribution of active defense mechanisms to ciprofloxacin tolerance in Escherichia coli growing at different rates

Using rpoS, tolC, ompF, and recA knockouts, we investigated their effect on the physiological response and lethality of ciprofloxacin in E. coli growing at different rates on glucose, succinate or acetate. We have shown that, regardless of the strain, the degree of changes in respiration, membrane potential, NAD⁺/NADH ratio, ATP and glutathione (GSH) strongly depends on the initial growth rate and the degree of its inhibition. The deletion of the regulator of the general stress response RpoS, although it influenced the expression of antioxidant genes, did not significantly affect the tolerance to ciprofloxacin at all growth rates. The mutant lacking TolC, which is a component of many E. coli efflux pumps, showed the same sensitivity to ciprofloxacin as the parent. The absence of porin OmpF slowed down the entry of ciprofloxacin into cells, prolonged growth and shifted the optimal bactericidal concentration towards higher values. Deficiency of RecA, a regulator of the SOS response, dramatically altered the late phase of the SOS response (SOS-dependent cell death), preventing respiratory inhibition and a drop in membrane potential. The recA mutation inverted GSH fluxes across the membrane and abolished ciprofloxacin-induced H₂S production. All studied mutants showed an inverse linear relationship between logCFU ml^-1 and the specific growth rate. Mutations shifted the plot of this dependence relative to the parental strain according to their significance for ciprofloxacin tolerance. The crucial role of the SOS system is confirmed by dramatic shift down of this plot in the recA mutant.

Effect of resveratrol and quercetin on the susceptibility of Escherichia coli to antibiotics

Activities of plant polyphenols (PPs), resveratrol and quercetin, alone or in combination with four conventional antibiotics against Escherichia coli have been investigated. In medium without antibiotics, both polyphenols caused a dose-dependent growth inhibition. However, pretreatment with resveratrol (40 and 100 μg ml^-1) and quercetin (40 μg ml^-1) reduced the bacteriostatic effect of kanamycin, streptomycin, cefotaxime and partially of ciprofloxacin. With few exceptions, both PPs also reduced the bactericidal effect of tested antibiotics. Paradoxically, low doses of PPs enhanced the bactericidal effect of kanamycin and partially ciprofloxacin. Compared to quercetin, resveratrol showed a weaker effect on the induction of antioxidant genes and the resistance of E. coli to the oxidative stress generated by hydrogen peroxide treatment. Both polyphenols at high doses reduced membrane potential. Altogether, these findings suggest that the decrease in the bactericidal effect of antibiotics by high doses of polyphenols is mostly due to bacteriostatic action of the latter. In the case of quercetin, the contribution of its antioxidant activity for antibiotic protection may be significant. There is a growing interest in the use of plant-derived compounds to enhance the toxicity of traditional antibiotics. This and other studies show that, under certain conditions, the use of polyphenols as adjuvants may not exert the expected therapeutic effect, but rather to decrease antimicrobial activity of antibiotics.

Study of the relationship between extracellular superoxide and glutathione production in batch cultures of Escherichia coli

Aerobically growing Escherichia coli generates superoxide flux into the periplasm via the oxidation of dihydromenaquinone and simultaneously carries out continuous transmembrane cycling of glutathione (GSH). Here we have shown that, under the conditions of a gradual decrease in dissolved oxygen (dO₂), characteristic of batch culture, the global regulatory system ArcB/ArcA can play an important role in the coordinated control of extracellular superoxide and GSH fluxes and their interaction with intracellular antioxidant systems. The lowest superoxide production was observed in the menA and arcB mutants, while the atpA, atpC and atpE mutants generated superoxide 1.3-1.5 times faster than the parent. The share of exported glutathione in the ubiC, atpA, atpC, and atpE mutants was 2-3 times higher compared to the parent. A high direct correlation (r = 0.87, p = 0.01) between extracellular superoxide and GSH was revealed. The menA and arcB mutants, as well as the cydD mutant lacking the GSH export system CydDC, were not capable of GSH excretion with a decrease in dO₂, which indicates a positive control of GSH export by ArcB. In contrast, ArcB downregulates sodA, therefore, an inverse correlation (r = -0.86, p = 0.013) between superoxide production and sodA expression was observed.

Cysteine homeostasis under inhibition of protein synthesis in Escherichia coli cells

Increased intracellular cysteine poses a potential danger to cells due to the high ability of cysteine to reduce free iron and promote the Fenton reaction. Here, we studied ways to maintain cysteine homeostasis in E. coli cells while inhibiting protein synthesis with valine or chloramphenicol. When growing wild-type bacteria on minimal medium with sulfate, an excess of cysteine resulting from the inhibition of protein synthesis is mainly incorporated into glutathione (up to 90%), which, therefore, can be considered as cysteine buffer. The share of hydrogen sulfide, which is the product of cysteine degradation by cysteine synthase B (CysM), does not exceed 1-3%, the rest falls on free cysteine, exported from cells. As a result, intracellular free cysteine is maintained at a low level (about 0.1 mM). The lack of glutathione in the gshA mutant increases H₂S production and excretion of cysteine and leads to a threefold increase in the level of intracellular cysteine in response to valine and chloramphenicol. The relA mutants, exposed to valine, produce more H₂S, dramatically accelerate the export of glutathione and accumulate more cysteine in the cytoplasm than their parent, which indicates that the regulatory nucleotide (p)ppGpp is involved in maintaining cysteine homeostasis. Disruption of cysteine homeostasis in gshA and relA mutants increases their sensitivity to peroxide stress.

Relationship between Escherichia coli growth rate and bacterial susceptibility to ciprofloxacin

The effect of Escherichia coli growth rate on its susceptibility to ciprofloxacin was investigated using bacteria grown on different carbon sources and harboring mutations in genes encoding tricarboxylic acid cycle enzymes. A 1-h treatment of the wild type (wt) grown on glucose, succinate, malate, α-ketoglutarate or acetate with 0.3 μg ml-1 ciprofloxacin decreased the number of surviving cells (CFU ml-1), 560, 110, 74, 62 and 5 times, respectively. Among the mutants tested, sucB strain, which grew 1.75 times slower than wt, was 7.4-fold more tolerant to 0.3 μg ml-1 of ciprofloxacin than wt. Strong inverse correlations between log(CFU ml-1) after 1-h exposure to 0.3 and 3.0 μg ml-1 ciprofloxacin and the specific growth rate prior to antibiotic treatment (r = - 0.93 and -0.96, respectively) were observed. Data from the current and previous studies on the inhibitory effect of ciprofloxacin on cultures exhibiting a wide range of growth rates (0.01-1.3 h-1) were collated. Statistical analysis revealed a significant inverse correlation between log(CFU ml-1) after exposure to 3.0 μg ml-1 of ciprofloxacin and the specific bacterial growth rate prior to antibiotic exposure (r = -0.92). These data may be used in a design of antibiotic treatment protocols.

The effect of 20-hydroxyecdysone on the susceptibility of Escherichia coli to different antibiotics

AIMS

The aim of this study was to investigate the influence of 20-hydroxyecdysone (20E) on the susceptibility of growing Escherichia coli to antibiotics.

METHODS AND RESULTS

Susceptibility of E. coli to antibiotics in the presence of 20E was estimated by determination of the colony-forming ability and the specific growth rate. Pretreatment with 20E decreased the bactericidal effect of ciprofloxacin (0·3 and 3·0 μg ml^-1 ), streptomycin (10 and 40 μg ml^-1 ) and kanamycin (10 μg ml^-1 ) and increased the bactericidal action of 0·03 μg ml^-1 ciprofloxacin. To study the influence of 20E on gene expression, we used strains of E. coli carrying fusions of promoters of relevant genes with the structural gene of β-galactosidase. 20E had no marked effect on the expression of antioxidant genes katG, katE, sodA and the rpoS gene (general stress response), while it (alone or combined with ciprofloxacin) markedly stimulated expression of the sulA gene belonging to the SOS regulon (DNA damage response).

CONCLUSION

In growing E. coli, 20E modulated the bactericidal action of antibiotics and stimulated the SOS response.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study may be used to enhance the efficiency of antibacterial therapies.

Ciprofloxacin provokes SOS-dependent changes in respiration and membrane potential and causes alterations in the redox status of Escherichia coli

An in-depth understanding of the physiological response of bacteria to antibiotic-induced stress is needed for development of new approaches to combatting microbial infections. Fluoroquinolone ciprofloxacin causes phase alterations in Escherichia coli respiration and membrane potential that strongly depend on its concentration. Concentrations lower than the optimal bactericidal concentration (OBC) do not inhibit respiration during the first phase. A dose higher than the OBC provokes immediate SOS-independent inhibition of respiration and growth that can contribute to a decreased SOS response and lowered susceptibility to high concentrations of ciprofloxacin. Cells retain their metabolic activity, membrane potential and accelerated K⁺ uptake and produce low levels of superoxide and H₂O₂ during the first phase. The time before initiation of the second phase is inversely correlated with the ciprofloxacin concentration. The second phase is SOS-dependent and characterized by respiratory inhibition, membrane depolarization, K⁺ and glutathione leakage and cessation of glucose consumption and may be considered as cell death. atpA, gshA and kefBkefC knockouts, which perturb fluxes of protons and K⁺, can modify the degree and duration of respiratory inhibition and potassium retention. Loss of K⁺ efflux channels KefB and KefC enhances the susceptibility of E. coli to ciprofloxacin.

[Role of Thiol Redox Systems in Escherichia coli Response to Thermal and Antibiotic Stresses]

Isogenic knockout mutants of Escherichia coli deficient in components of the glutathione and thioredoxin redox systems and growing at various temperatures (20-46°C) exhibited considerable differences in growth rate and survival, as well as in expression of the antioxidant genes. In the parent strain E. coli BW25113 (wt) treated with ciprofloxacin, ampicillin, or streptomycin, dependence of survival from growth temperature was a V-shaped curve with the maximum sensitivity within the range corresponding to high growth rates (40-44°C). Significant inverse correlation was observed between log CFU at different temperatures and specific growth rate prior to antibiotic addition. This applied to most of the mutants, which exhibited higher resistance to the three antibiotics tested at nonoptimal temperatures (20 and 46°C) than at 37 and 40°C. No correlation was found between resistance to stress and antibiotics and expression of the antioxidant genes. The role of global regulators ppGpp and σ(s) in E. coli resistance to antibiotics and nonoptimal temperatures was shown.

Influence of polyphenols on Escherichia coli resistance to oxidative stress

Among all polyphenols tested (tannic acid and flavonoids belonging to different subclasses) only tannin and quercetin significantly enhanced resistance of Escherichia coli to peroxide stress. Pretreatment of the cells with quercetin and tannin resulted in a decrease in the growth arrest duration under moderate H(2)O(2) concentration (2 mM) and an increase in survival under high (10 mM) doses. The shorter growth recovery period in pretreated cells was connected with more rapid H(2)O(2) elimination because of induced activity of scavenging enzymes. This effect was absent in the Delta oxyR mutant, which was unable to induce genes responding to peroxide stress. The data obtained suggest that the observed protection was a result of two overlapping effects: induction of OxyR regulon by low concentrations of H(2)O(2), accumulated during extracellular autoxidation of quercetin and tannin, and protection of synthesis of OxyR-regulated antioxidant enzymes during H(2)O(2) stress because of intracellular binding of iron by quercetin and tannin and suppressing Fenton chemistry.

Redox regulation of cellular functions

Maintenance of normal intracellular redox status plays an important role in such processes as DNA synthesis, gene expression, enzymatic activity, and others. In addition, it is clear that changes in the redox status of intracellular content and individual molecules, resulting from stress or intrinsic cellular activity, are involved in the regulation of different processes in cells. Small changes in intracellular levels of reactive oxygen species participate in intracellular signaling. Thiol-containing molecules, such as glutathione, thioredoxins, glutaredoxins, and peroxiredoxins, also play an important role in maintaining redox homeostasis and redox regulation. This review attempts to summarize the current knowledge about redox regulation in different cell types.

Glutathione in bacteria

Glutathione metabolism and its role in vital functions of bacterial cells are considered, as well as common features and differences between the functions of glutathione in prokaryotic and eukaryotic cells. Particular attention is given to the recent data for the role of glutathione in bacterial redox-regulation and adaptation to stresses.

Effects of Cystine and Hydrogen Peroxide on Glutathione Status and Expression of Antioxidant Genes in Escherichia coli

Cysteine or cystine was earlier shown to multiply enhance the toxic effect of hydrogen peroxide on Escherichia coli cells. In the present work, the treatment of E. coli with H2O2 in the presence of cystine increased fivefold the level of extracellular oxidized glutathione (GSSG(out)) and decreased fivefold the GSH/GSSG(out) ratio (from 16.8 to 3.6). The same treatment of cells with deficiency in glutathione oxidoreductase (GOR) resulted in even more severe oxidation of GSH(out), so that the level of oxidized glutathione exceeded that of reduced glutathione and the GSH/GSSG(out) ratio decreased to 0.4. Addition of cystine to the GOR deficient cells resulted in significant oxidation of extracellular glutathione even in the absence of oxidant and in tenfold increase in intracellular oxidized glutathione along with a decrease in the GSH/GSSG(out) ratio from 282 to 26. However, in the cytoplasm of wild type cells, the level of oxidized glutathione (GSSG(in)) was changed insignificantly and the GSH/GSSG(in) ratio increased by 26% (from 330 to 415). Data on glutathione status and cystine reduction in the E. coli gsh and gor mutants suggested that exogenous cystine at first should be reduced with extracellular GSH outside the cells and then imported into them. The high toxicity of H2O2 in the presence of cystine resulted in disorders of membrane functions and inhibition of the expression of genes including those responsible for neutralization of oxidants and DNA repair.

Role of glutathione in the response of Escherichia coli to osmotic stress

The growth of Escherichia coli mutants deficient in glutathione synthesis (gshA) and in glutathione reductase (gor) was suppressed in medium of elevated osmolarity. A mutant in gamma-glutamyl transpeptidase (ggt) displayed better ability for osmoadaptation than the parental strain. The unfavorable effect of the gsh mutation on osmoadaptation of growing E. coli cells was more pronounced at low concentrations of K+ in the medium. An increase in osmolarity caused an increase in the intracellular content of glutathione. Changes in the extracellular glutathione level were biphasic: the glutathione level rapidly decreased during the first stage of the response and increased during the second stage. The changes in glutathione levels suggest that under hyperosmotic shock the glutathione transport from the medium into the cell can contribute to the intracellular glutathione accumulation. Changes in the level of intracellular K+ were similarly biphasic: a rapid increase in the K+ level during the first stage of the response to hyperosmotic shock changed to a gradual decrease during the second stage. In mutant gshA cells adapted to osmotic shock, the intracellular K+ level was markedly higher than in the parental strain cells. The possible role of glutathione in the response of E. coli to osmotic shock is discussed.

Role of glutathione in regulation of hydroperoxidase I in growing Escherichia coli

To examine role of glutathione in regulation of catalases in growing Escherichia coli, katG::lacZ and katE::lacZ fusions were transformed into a glutathione-deficient Escherichia coli strain and wild-type parent. In the absence of H2O2 and in the presence of the low H2O2 concentrations (0.1-3 mM), the gshA mutation stimulated katG::lacZ expression and the total catalase activity in exponential phase. In the absence of H2O2, the mutation in gshA also stimulated katE::lacZ expression. At higher H2O2 concentrations, the gshA mutation suppressed katG::lacZ expression and catalase activity. In stationary and mid-exponential phases, the intracellular concentrations of H2O2 in the gshA mutant were markedly increased compared to those in the wild type. These results suggest that glutathione may be involved in regulation of catalases.

The role of antioxidant enzymes in response of Escherichia coli to osmotic upshift

Aerobic growth of Escherichia coli sodAsodB and katE mutants lacking cytosolic superoxide dismutases and catalase hydroperoxidase II was inhibited by osmotic upshift to a greater extent than of their wild-type parent strains. The fur mutation leading to an intracellular overload of iron also increased sensitivity of growing E. coli cells to osmotic upshift. Using lacZ fusions, it was shown that expression of antioxidant genes soxS and katE was stimulated by an increase in osmolarity. These data suggest that in aerobically growing E. coli cells, moderate osmotic upshift causes activation of certain antioxidant systems.

Effects of menadione and hydrogen peroxide on glutathione status in growing Escherichia coli

Menadione (MD) and H2O2 caused distinct effects on glutathione status in growing Escherichia coli. Treatment of E. coli AB1157 with 1-25 mM H2O2 did not result in an appreciable decrease in intracellular total glutathione (reduced glutathione [GSH] + oxidized glutathione [GSSG]). Only when cells were treated with 25 mM H2O2 an increase in GSSG and a decrease in the GSH:GSSG ratio were observed. In cells deficient in catalase HPI, such effect was observed even at 10 mM H2O2. The exposure of E. coli AB1157 to MD caused a dose-dependent decrease in intracellular total glutathione, an increase in GSSG, and a decrease in the ratio of GSH:GSSG. In E. coli deficient in cytosolic superoxide dismutase activity, a decrease in total glutathione after incubation with 0.2 mM MD was not accompanied by an increase in GSSGin, and the ratio of GSHin:GSSGin was three times higher than in the wild-type cells. The changes in the redox status of extracellular glutathione under the action of both oxidants were similar. Although the catalase activity increased several times after exposure to both oxidants, there were little or no changes in the activity of enzymes related to glutathione metabolism. A possible role of changes in redox status of glutathione under oxidative stress is discussed.

Oxidative stress resistance of Escherichia coli strains deficient in glutathione biosynthesis

Sensitivity to various oxidants was determined for Escherichia coli strains JTG10 and 821 deficient in biosynthesis of glutathione (gsh-) and their common parental strain AB1157 (gsh+). The three strains showed identical sensitivity to H2O2. E. coli 821 was more resistant than AB1157 and JTG10 to menadione, cumene hydroperoxide, and N-ethylmaleimide. This resistance was not related to the gsh mutation because the other gsh- mutant and the parental strain showed similar sensitivity to these oxidants. The measured activities of NADPH:menadione diaphorase and glucose-6-phosphate dehydrogenase and the extracellular level of menadione suggested that the enhanced resistance of E. coli 821 to menadione might be due to decreased diaphorase activity, but not to a lowered rate of menadione uptake.

Effects of penetrating and non-penetrating oxidants on Escherichia coli

Treatment of Escherichia coli K-12 cells aerobically grown in M9 glucose salt medium (H2O2) and non-penetrating (ferricyanide) oxidants resulted in similar inhibition of growth and decrease in intracellular K+ pool by 15%. Only H2O2 inhibited growth of auxotrophic strains grown in M9 medium supplemented with protein hydrolysate. Ferricyanide reduction was associated with decrease in low-molecular-weight thiols, whereas the treatment of cells with H2O2 increased their level. Pretreatment of cells with ferricyanide enhanced the H2O2-induced expression of katG gene encoding for catalase HPI; this gene is a member of the gene family controlled by the oxyR gene. Pretreatment with ferricyanide inhibited H2O2-induced expression of the sfiA gene which is the member of the gene family controlled by the recA and lexA genes. Glutathione is the major low-molecular-weight thiol in E. coli, and it can play different roles in cellular responses to H2O2 and ferricyanide.

Induction of the alkylation-inducible aidB gene of Escherichia coli by cytoplasmic acidification and N-ethylmaleimide

We have studied the effects of changes in intracellular pH and the influence of thiol reagents on the induction of the DNA damage-inducible genes of Escherichia coli, aidB, alkA, and alkB. Under aerobic conditions in the absence of alkylating agents aidB, but not alkA or alkB, was induced by an acidification of cytoplasm or by treatment with the sulfhydryl reagent N-ethylmaleimide. Alkaline shift and thiosalicyclic acid did not affect the induction of aidB and alkB. The induction of alkA increased under the alkaline shift but not in the case of treatment with reducing agents. Compared with the aidB gene, a component of the SOS system, the sulA (sfiA) gene, responded to changes in cytoplasmic pH and in the level of intracellular thiols in an opposite way. SulA induction was observed under alkaline shift and after treatment with thiosalicylic acid.

Changes in intracellular potassium and thiol levels in Escherichia coli K12 under various stresses

The levels of intracellular K+ and thiols were measured synchronously in E. coli cells under starvation stress and osmotic upshock,alkaline shift and cytoplasm acidification. The level of nonprotein thiols increased in response to osmotic upshock and alkaline shift but decreased under cytoplasm acidification. A positive correlation between the changes in the K+ pool and the level of nonprotein thiols was found under osmotic shock and alkaline shift. Inverse correlation between these parameters was observed after the acidification of cytoplasm. It is supposed that there is a tight relationship between the levels of K+, nonprotein thiols and intracellular pH.

Acidification of Escherichia coli and Salmonella typhimurium cytoplasm reduces the mutagenic effect of N-methyl-N'-nitro-N-nitrosoguanidine

Preliminary acidification of the cytoplasm of E. coli cells growing at pH 6.9 by adding to the medium 50 mM of sodium acetate or propionate reduced the mutagenic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) to almost the spontaneous level. In experiments with S. typhimurium the protective effects of cytoplasm acidification against the mutagenic effect of MNNG was observed at pH 5.5 and was absent at a medium pH of 6.9. Alkalinization of the cytoplasm by adding 80 mM of methylamine to the growth medium at pH 8.1 did not modify the effect of MNNG on the cells of E. coli and S. typhimurium. Alkalinization of the cytoplasm of E. coli B/r and K12 was followed by a reduction of the intracellular non-protein SH group level by 25 and 50%, respectively. It is supposed that the protective effect of acidification may be due to a decline in the productivity of mutagenically more active intermediates of MNNG when the pH is reduced and the associated fall of the level of intracellular non-protein thiols occurs. The above situation may serve as a model for studying the effects of MNNG and other alkylating agents on cells differing in physiological status.