Regulatory aspects of alkali tolerance induction in Escherichia coli

Studies on the effect of temperature and pH on the inactivation of fish viral and bacterial pathogens

Disposal of fish by-products in the European Community must comply with Regulation (EC) No 1069/2009 which categorizes animal by-products according to risk, and specifies methods of disposal of by-products according to that risk. There is provision under the regulation for composting or ensiling to be used for by-products from aquatic animals. Biosecurity considerations require knowledge of the parameters of time and temperature, or time and pH, required to inactivate any fish pathogens that may be present. To provide those data, we undertook laboratory studies on the inactivation of a number of fish pathogenic viruses and bacteria at 60 °C, pH 4.0 and pH 12.0 as a preliminary to conducting subsequent trials with the most resistant viruses and bacteria in fish tissues. The most resistant bacterium to 60 °C, pH 4.0 as well as pH 12.0 was Lactococcus garvieae. Its concentration was reduced to the level of sensitivity of the test after 24-48 h exposure to 60 °C, but it survived for at least 7 days at pH 4.0 and 14 days at pH 12.0. The most resistant virus to 60 °C was infectious pancreatic necrosis virus, and to pH 12.0 was infectious salmon anaemia virus. The majority of the viruses tested survived exposure to pH 4.0 for up to 28 days. The results suggest that the process of acid ensiling alone is not an effective method for the inactivation of many viral and bacterial pathogens, and fish by-products would need further treatment by a method approved under the regulation following ensiling, whereas alkaline or heat treatment are likely to provide an increased degree of biosecurity for on-farm processing of mortalities.

Genomic and proteomic analysis of the Alkali-Tolerance Response (AlTR) in Listeria monocytogenes 10403S

BACKGROUND

Information regarding the Alkali-Tolerance Response (AlTR) in Listeria monocytogenes is very limited. Treatment of alkali-adapted cells with the protein synthesis inhibitor chloramphenicol has revealed that the AlTR is at least partially protein-dependent. In order to gain a more comprehensive perspective on the physiology and regulation of the AlTR, we compared differential gene expression and protein content of cells adapted at pH 9.5 and un-adapted cells (pH 7.0) using complementary DNA (cDNA) microarray and two-dimensional (2D) gel electrophoresis, (combined with mass spectrometry) respectively.

RESULTS

In this study, L. monocytogenes was shown to exhibit a significant AlTR following a 1-h exposure to mild alkali (pH 9.5), which is capable of protecting cells from subsequent lethal alkali stress (pH 12.0). Adaptive intracellular gene expression involved genes that are associated with virulence, the general stress response, cell division, and changes in cell wall structure and included many genes with unknown functions. The observed variability between results of cDNA arrays and 2D gel electrophoresis may be accounted for by posttranslational modifications. Interestingly, several alkali induced genes/proteins can provide a cross protective overlap to other types of stresses.

CONCLUSION

Alkali pH provides therefore L. monocytogenes with nonspecific multiple-stress resistance that may be vital for survival in the human gastrointestinal tract as well as within food processing systems where alkali conditions prevail. This study showed strong evidence that the AlTR in L. monocytogenes functions as to minimize excess alkalisation and energy expenditures while mobilizing available carbon sources.

pH regulates genes for flagellar motility, catabolism, and oxidative stress in Escherichia coli K-12

Gene expression profiles of Escherichia coli K-12 W3110 were compared as a function of steady-state external pH. Cultures were grown to an optical density at 600 nm of 0.3 in potassium-modified Luria-Bertani medium buffered at pH 5.0, 7.0, and 8.7. For each of the three pH conditions, cDNA from RNA of five independent cultures was hybridized to Affymetrix E. coli arrays. Analysis of variance with an alpha level of 0.001 resulted in 98% power to detect genes showing a twofold difference in expression. Normalized expression indices were calculated for each gene and intergenic region (IG). Differential expression among the three pH classes was observed for 763 genes and 353 IGs. Hierarchical clustering yielded six well-defined clusters of pH profiles, designated Acid High (highest expression at pH 5.0), Acid Low (lowest expression at pH 5.0), Base High (highest at pH 8.7), Base Low (lowest at pH 8.7), Neutral High (highest at pH 7.0, lower in acid or base), and Neutral Low (lowest at pH 7.0, higher at both pH extremes). Flagellar and chemotaxis genes were repressed at pH 8.7 (Base Low cluster), where the cell's transmembrane proton potential is diminished by the maintenance of an inverted pH gradient. High pH also repressed the proton pumps cytochrome o (cyo) and NADH dehydrogenases I and II. By contrast, the proton-importing ATP synthase F1Fo and the microaerophilic cytochrome d (cyd), which minimizes proton export, were induced at pH 8.7. These observations are consistent with a model in which high pH represses synthesis of flagella, which expend proton motive force, while stepping up electron transport and ATPase components that keep protons inside the cell. Acid-induced genes, on the other hand, were coinduced by conditions associated with increased metabolic rate, such as oxidative stress. All six pH-dependent clusters included envelope and periplasmic proteins, which directly experience external pH. Overall, this study showed that (i) low pH accelerates acid consumption and proton export, while coinducing oxidative stress and heat shock regulons; (ii) high pH accelerates proton import, while repressing the energy-expensive flagellar and chemotaxis regulons; and (iii) pH differentially regulates a large number of periplasmic and envelope proteins.

Extracellular sensors and extracellular alarmones, which permit cross-talk between organisms, determine the levels of alkali tolerance and trigger alkaliinduced acid sensitivity in Escherichia coli

For several stress responses in Escherichia coli, switching on involves conversion by the stress of an extracellular stress sensor (an extracellular sensing component, ESC) to an extracellular induction component (EIC), the latter functioning as an alarmone and inducing the response. The aim of this study was to establish whether alkali tolerance induction at pH 9.0, alkali sensitisation induced at pH 5.5 and the acid sensitisation induced at pH 9.0 involve sensing of pH changes by ESCs. The techniques involved made use of studies with cell-free culture filtrates. With respect to the inducible responses under test, these filtrates were prepared either from induced or uninduced cultures and filtrates from uninduced cultures were also activated in vitro, by the pH stress, in the absence of bacteria. Tests were then made to examine whether EICs (known to be needed for all these systems) are formed by activation, at the appropriate pH values, of filtrates from pH 7.0-grown cultures (i.e. uninduced culture filtrates); appearance of an EIC on activation would indicate the presence in the uninduced culture filtrate of an ESC. The studies showed that all three systems use ESCs to detect pH changes. Tests involving attempted enzymic and physical inactivation of the ESCs, and attempted removal of the ESCs by dialysis, showed that the ESC involved in alkali sensitisation is a small very heat-resistant protein. Strikingly, protease only partially inactivated the ESCs needed for alkali tolerance induction and for acid sensitisation; each system may be complex, involving both protein and non-protein (RNA?) ESCs, although other explanations are possible. It was also established that appropriate killed cultures can induce all three responses when incubated with pH 7.0-grown living cultures. The occurrence of ESC/EIC pairs for these three responses has led to the evolution of early warning systems for each, the diffusibility of the EICs, and their interaction with non-producers, allowing them to act pheromonally, inducing sensitive organisms to stress tolerance, prior to exposure to stressor.

Treatment of Salmonella enterica serovar Enteritidis with a sublethal concentration of trisodium phosphate or alkaline pH induces thermotolerance

The responses of Salmonella enterica serovar Enteritidis to a sublethal dose of trisodium phosphate (TSP) and its equivalent alkaline pH made with NaOH were examined. Pretreatment of S. enterica serovar Enteritidis cells with 1.5% TSP or pH 10.0 solutions resulted in a significant increase in thermotolerance, resistance to 2.5% TSP, resistance to high pH, and sensitivity to acid and H(2)O(2). Protein inhibition studies with chloramphenicol revealed that thermotolerance, unlike resistance to high pH, was dependent on de novo protein synthesis. Two-dimensional polyacrylamide gel electrophoresis (PAGE) of total cellular proteins from untreated control cells resolved as many as 232 proteins, of which 22 and 15% were absent in TSP- or alkaline pH-pretreated cells, respectively. More than 50% of the proteins that were either up- or down-regulated by TSP pretreatment were also up- or down-regulated by alkaline pH pretreatment. Sodium dodecyl sulfate-PAGE analysis of detergent-insoluble outer membrane proteins revealed the up-regulation of at least four proteins. Mass spectrometric analysis showed the up-regulated proteins to include those involved in the transport of small hydrophilic molecules across the cytoplasmic membrane and those that act as chaperones and aid in the export of newly synthesized proteins by keeping them in open conformation. Other up-regulated proteins included common housekeeping proteins like those involved in amino acid biosynthesis, nucleotide metabolism, and aminoacyl-tRNA biosynthesis. In addition to the differential expression of proteins following TSP or alkaline pH treatment, changes in membrane fatty acid composition were also observed. Alkaline pH- or TSP-pretreated cells showed a higher saturated and cyclic to unsaturated fatty acid ratio than did the untreated control cells. These results suggest that the cytoplasmic membrane could play a significant role in the induction of thermotolerance and resistance to other stresses following TSP or alkaline pH treatment.

Sensitivity of Escherichia coli O157:H7 to commercially available alkaline cleaners and subsequent resistance to heat and sanitizers

The effects of seven commercially available alkaline cleaners used in the food processing industry, 0.025 M NaOH, and 0.025 M KOH on viability of wild-type (EDL 933) and rpoS-deficient (FRIK 816-3) strains of Escherichia coli O157:H7 in logarithmic and stationary phases of growth were determined. Cells were treated at 4 or 23 degrees C for 2, 10, or 30 min. Cleaners 2, 4, 6, and 7, which contained hypochlorite and <11% NaOH and/or KOH (pH 11.2 to 11.7), killed significantly higher numbers of cells than treatment with cleaner 3, containing sodium metasilicate (pH 11.4) and <10% KOH, and cleaner 5, containing ethylene glycol monobutyl ether (pH 10.4). There were no differences in the sensitivities of logarithmic and stationary-phase cells to the alkaline cleaners. Treatment with KOH or NaOH (pH 12.2) was not as effective as four out of seven commercial cleaners in killing E. coli O157:H7, indicating that chlorine and other cleaner components have bactericidal activity at high pH. Stationary-phase cells of strain EDL 933 that had been exposed to cleaner 7 at 4 or 23 degrees C and strain FRIK 816-3 exposed to cleaner 7 at 23 degrees C had significantly higher D(55 degrees C) (decimal reduction time, minutes at 55 degrees C) values than control cells or cells exposed to cleaner 5, indicating that exposure to cleaner 7 confers cross-protection to heat. Cells of EDL 933 treated with cleaner 7 at 12 degrees C showed significantly higher D(55 degrees C) values than cells of FRIK 816-3, indicating that rpoS may play a role in cross-protection. Stationary-phase cells treated with cleaner 5 or cleaner 7 at 4 or 12 degrees C were not cross-protected against subsequent exposure to sanitizers containing quaternary ammonium compounds or sodium hypochlorite, or to cetylpyridinium chloride and benzalkonium chloride.

Survival and heat resistance of Listeria monocytogenes after exposure to alkali and chlorine

A strain of Listeria monocytogenes isolated from a drain in a food-processing plant was demonstrated, by determination of D values, to be more resistant to the lethal effect of heat at 56 or 59 degrees C following incubation for 45 min in tryptose phosphate broth (TPB) at pH 12.0 than to that of incubation for the same time in TPB at pH 7.3. Cells survived for at least 6 days when they were suspended in TPB at pHs 9.0, 10.0, and 11.0 and stored at 4 or 21 degrees C. Cells of L. monocytogenes incubated at 37 degrees C for 45 min and then stored for 48 or 144 h in TPB at pH 10.0 were more resistant to heat treatment at 56 degrees C than were cells stored in TPB at pH 7.3. The alkaline-stress response in L. monocytogenes may induce resistance to otherwise lethal thermal-processing conditions. Treatment of cells in 0.05 M potassium phosphate buffer (pH 7.00 +/- 0.05) containing 2.0 or 2.4 mg of free chlorine per liter reduced populations by as much as 1.3 log(10) CFU/ml, while treatment with 6.0 mg of free chlorine per liter reduced populations by as much as 4.02 log(10) CFU/ml. Remaining subpopulations of chlorine-treated cells exhibited some injury, and cells treated with chlorine for 10 min were more sensitive to heating at 56 degrees C than cells treated for 5 min. Contamination of foods by L. monocytogenes cells that have survived exposure to processing environments ineffectively cleaned or sanitized with alkaline detergents or disinfectants may have more severe implications than previously recognized. Alkaline-pH-induced cross-protection of L. monocytogenes against heat has the potential to enhance survival in minimally processed as well as in heat-and-serve foods and in foods on holding tables, in food service facilities, and in the home. Cells surviving exposure to chlorine, in contrast, are more sensitive to heat; thus, the effectiveness of thermal processing in achieving desired log(10)-unit reductions is not compromised in these cells.

An extracellular sensor and an extracellular induction component are required for alkali induction of alkyl hydroperoxide tolerance in Escherichia coli

Escherichia coli K12 transferred from pH 7.0 to pH 9.0 gains alkylhydroperoxide (AHP) tolerance. The aim here was to establish whether extracellular components (ECs) are needed for such induction. Therefore, the effects of removing ECs during incubation at pH 9.0 were tested and the abilities of culture filtrates to induce tolerance were examined. First, AHP tolerance did not appear, at pH 9.0, if cultures were subjected to continuous filtration or dialysis, against the same medium, suggesting that an EC might be needed. Second, neutralized filtrates from pH 9.0-grown cultures induced tolerance at pH 7.0, and these filtrates were inactivated by dialysis, filtration or heating but not by protease. Thus, pH 9.0 filtrates have a small non-protein extracellular induction component (EIC), which acts as an alarmone, 'warning' cells of stress and preparing them to resist it. Filtrates from pH 7.0-grown cultures did not induce AHP tolerance at pH 7.0 but if incubated at pH 9.0 without organisms, gained such ability. It is proposed that pH 7.0 filtrates have an EIC precursor (termed an extracellular sensing component, ESC), which senses alkaline pH, and is converted by it to the EIC. The ESC in pH 6.0 filtrates was distinct from that in pH 7.0 filtrates; there may be several oligomeric (or conformational) forms of this ESC. As the EIC is small, it can diffuse away from the alkalinized region and induce tolerance in unstressed organisms.

Effect of osmotic, alkaline, acid or thermal stresses on the growth and inhibition of Listeria monocytogenes

Five strains of Listeria monocytogenes (a, b, c, d and e) isolated from industrial plants have been subjected to different osmotic, alkaline, acid or thermal stresses. The effects of these treatments on lag-phase (L) and growth rate (mu) of cells in mid-log phase have been followed using an automated optical density monitoring system. Increasing the osmotic pressure by the addition of different amounts of NaCl increased the lag phase and decreased the growth rate. The same phenomena were observed after decreasing the pH of the medium to 5.8, 5.6 or 5.4 by addition of acetic, lactic or hydrochloric acids. The inhibitory effect was: acetic acid > lactic acid > hydrochloric acid. The addition of NaOH to attain pH values of 9.5, 10.0, 10.5 or 11.0 in the medium produced a dramatic increase of the lag phase at pH 10.5 and 11. Growth rates were also decreased while the maximal population increased with high pH values. These effects varied according to strains. Strains d and e were the most resistant to acidic and alkaline stresses, and e was the most affected by the addition of NaCl. A cold shock of 30 min at 0 degree C had limited effects on growth parameters. On the other hand, hyperthermal shocks (55 or 63 degrees C, 30 min) led to similar increased lag phases and to significant increases of the maximal population in all five strains.

Properties of an L-glutamate-induced acid tolerance response which involves the functioning of extracellular induction components

Escherichia coli became more acid tolerant following incubation for 60 min in a medium containing L-glutamate at pH 7.0, 7.5 or 8.5. Several agents, including cAMP, NaCl, sucrose, SDS and DOC, prevented tolerance appearing if present with L-glutamate. Lesions in cysB, hns, fur, himA and relA, which frequently affect pH responses, failed to prevent L-glutamate-induced acid tolerance but a lesion in L-glutamate decarboxylase abolished the response. Induction of acid tolerance by L-glutamate was associated with the accumulation in the growth medium of a protein (or proteins) which was able to convert pH 7.0-grown cultures to acid tolerance, and the original L-glutamate-induced tolerance response was dependent on this component(s). Acid tolerance was also induced by L-aspartate at pH 7.0 and induction of such tolerance was dependent on an extracellular protein (or proteins). The L-glutamate and L-aspartate acid tolerance induction processes are further examples of a number of stress tolerance responses which differ from most inductions in that extracellular components, including extracellular sensors, are required.

Alkaline stress response in Enterococcus faecalis: adaptation, cross-protection, and changes in protein synthesis

The alkaline shock response in Enterococcus faecalis was studied in this work. Cells adapted to an optimum pH of 10.5 were tolerate to pH 11.9 conditions but acquired sensitivity to acid damage. An analysis of stress proteins revealed that 37 polypeptides were amplified. Two of these are DnaK and GroEL. The combined results show that bile salts and alkaline stress responses are closely related.