Acid Tolerance Response Research Articles

Physiological, Structural, and Functional Analysis of the Paralogous Cation-Proton Antiporters of NhaP Type from Vibrio cholerae.

The transmembrane K+/H+ antiporters of NhaP type of Vibrio cholerae (Vc-NhaP1, 2, and 3) are critical for maintenance of K+ homeostasis in the cytoplasm. The entire functional NhaP group is indispensable for the survival of V. cholerae at low pHs suggesting their possible role in the acid tolerance response (ATR) of V. cholerae. Our findings suggest that the Vc-NhaP123 group, and especially its major component, Vc-NhaP2, might be a promising target for the development of novel antimicrobials by narrowly targeting V. cholerae and other NhaP-expressing pathogens. On the basis of Vc-NhaP2 in silico structure modeling, Molecular Dynamics Simulations, and extensive mutagenesis studies, we suggest that the ion-motive module of Vc-NhaP2 is comprised of two functional regions: (i) a putative cation-binding pocket that is formed by antiparallel unfolded regions of two transmembrane segments (TMSs V/XII) crossing each other in the middle of the membrane, known as the NhaA fold; and (ii) a cluster of amino acids determining the ion selectivity.

Response to Acid Adaptation in Salmonella enterica Serovar Enteritidis.

Acid adaptation in Salmonella Enteritidis was characterized by phenotypic and gene-expression analyses. S. Enteritidis cells at log-phase and stationary-phase were kept at pH 4.5 to 6.0 for 1 to 4 hours. All treatments induced various levels of acid tolerance response that were dependent on pH, exposure time and growth phase. This acid adaptation resulted in tolerance to 50 °C and 8% NaCl regardless of the growth phase. However, the tolerance of log-phase and stationary-phase cells to low temperatures (4 and -20 °C) was increased and decreased, respectively. RT-qPCR analysis revealed that genes involved in tolerance to acid (SEN1564A and cfa), heat (rpoH, uspB, and htrA), salt (proP, proV, and osmW), and cold (cspA, cspC, and csdA) stress were generally upregulated after acid adaptation. These results provide an initial insight into mechanisms of acid adaptation and induced cross protection in S. Enteritidis. PRACTICAL APPLICATION: Stress tolerance acquisition resulting from acid adaptation in foodborne pathogens poses a great threat to food safety. The current work showed that acid adaptation induced direct tolerance and cross-tolerance to high temperature, low temperature, and salt in Salmonella Enteritidis, possibly due to the upregulation of stress tolerance-related genes. These results provide key insights into acid adaptation mechanisms and efficient control of S. Enteritidis.

Silence is golden: gene silencing of V. cholerae during intestinal colonization delivers new aspects to the acid tolerance response

ABSTRACTBacterial pathogens of the gastrointestinal tract alter their expression profile upon ingestion by the host and activate a variety of factors enhancing colonization and virulence. However, gene silencing during infection might be as important as gene activation to achieve full colonization fitness. Thus, we developed and successfully applied a reporter technology to identify 101 in vivo repressed (ivr) genes of the bacterial pathogen Vibrio cholerae. In depth analysis of the in vivo repressed H+/Cl− transporter ClcA revealed an inverse requirement along gastrointestinal colonization. ClcA could be linked to acid tolerance response required during stomach passage, but ClcA expression is detrimental during subsequent colonization of the lower intestinal tract as it exploits the proton-motive force in alkaline environments. The study summarized in this addendum demonstrates that constitutive expression of ivr genes can reduce intestinal colonization fitness of V. cholerae, highlighting the necessity to downregulate these genes in vivo.

Crosstalk between the serine/threonine kinase StkP and the response regulator ComE controls the stress response and intracellular survival of Streptococcus pneumoniae.

Streptococcus pneumoniae is an opportunistic human bacterial pathogen that usually colonizes the upper respiratory tract, but the invasion and survival mechanism in respiratory epithelial cells remains elusive. Previously, we described that acidic stress-induced lysis (ASIL) and intracellular survival are controlled by ComE through a yet unknown activation mechanism under acidic conditions, which is independent of the ComD histidine kinase that activates this response regulator for competence development at pH 7.8. Here, we demonstrate that the serine/threonine kinase StkP is essential for ASIL, and show that StkP phosphorylates ComE at Thr128. Molecular dynamic simulations predicted that Thr128-phosphorylation induces conformational changes on ComE’s DNA-binding domain. Using nonphosphorylatable (ComET128A) and phosphomimetic (ComET128E) proteins, we confirmed that Thr128-phosphorylation increased the DNA-binding affinity of ComE. The non-phosphorylated form of ComE interacted more strongly with StkP than the phosphomimetic form at acidic pH, suggesting that pH facilitated crosstalk. To identify the ComE-regulated genes under acidic conditions, a comparative transcriptomic analysis was performed between the comET128A and wt strains, and differential expression of 104 genes involved in different cellular processes was detected, suggesting that the StkP/ComE pathway induced global changes in response to acidic stress. In the comET128A mutant, the repression of spxB and sodA correlated with decreased H2O2 production, whereas the reduced expression of murN correlated with an increased resistance to cell wall antibiotic-induced lysis, compatible with cell wall alterations. In the comET128A mutant, ASIL was blocked and acid tolerance response was higher compared to the wt strain. These phenotypes, accompanied with low H2O2 production, are likely responsible for the increased survival in pneumocytes of the comET128A mutant. We propose that the StkP/ComE pathway controls the stress response, thus affecting the intracellular survival of S. pneumoniae in pneumocytes, one of the first barriers that this pathogen must cross to establish an infection.

Biological and regulatory roles of acid-induced small RNA RyeC in Salmonella Typhimurium

Salmonella Typhimurium is an enteric pathogen that has evolved masterful strategies to enable survival under stress conditions both within and outside a host. The acid tolerance response (ATR) is one such mechanism that enhances the viability of acid adapted bacteria to lethal pH levels. While numerous studies exist on the protein coding components of this response, there is very little data on the roles of small RNAs (sRNAs). These non-coding RNA molecules have recently been shown to play roles as regulators of bacterial stress response and virulence pathways. They function through complementary base pairing interactions with target mRNAs and affect their translation and/or stability. There are also a few that directly bind to proteins by mimicking their respective targets. Here, we identify several sRNAs expressed during the ATR of S. Typhimurium and characterize one highly induced candidate, RyeC. Further, we identify ptsI as a trans-encoded target that is directly regulated by this sRNA. From a functional perspective, over-expression of RyeC in Salmonella produced a general attenuation of several in vitro phenotypes including acid survival, motility, adhesion and invasion of epithelial cell lines as well as replication within macrophages. Together, this study highlights the diverse roles played by sRNAs in acid tolerance and virulence of S. Typhimurium.

The acid tolerance response and pH adaptation of Enterococcus faecalis in extract of lime Citrus aurantiifolia from Aceh Indonesia.

Background:The objective of the present study was to evaluate the acid tolerance response and pH adaptation when Enterococcusfaecalis interacted with extract of lime ( Citrus aurantiifolia). Methods: We used E. faecalis ATCC 29212 and lime extract from Aceh, Indonesia. The microbe was analyzed for its pH adaptation, acid tolerance response, and adhesion assay using a light microscope with a magnification of x1000. Further, statistical tests were performed to analyze both correlation and significance of the acid tolerance and pH adaptation as well as the interaction activity. Results:E. faecalis was able to adapt to a very acidic environment (pH 2.9), which was characterized by an increase in its pH (reaching 4.2) at all concentrations of the lime extract (p < 0.05). E. faecalis was also able to provide acid tolerance response to lime extract based on spectrophotometric data (595 nm) (p < 0.05). Also, the interaction activity of E. faecalis in different concentrations of lime extract was relatively stable within 6 up to 12 hours (p < 0.05), but it became unstable within 24-72 hours (p > 0.05) based on the mass profiles of its interaction activity. Conclusions:E. faecalis can adapt to acidic environments (pH 2.9-4.2); it is also able to tolerate acid generated by Citrus aurantiifolia extract, revealing a stable interaction in the first 6-12 hours.

The acid tolerance response and pH adaptation of Enterococcus faecalis in extract of lime Citrus aurantiifolia from Aceh Indonesia

Background: The objective of the present study was to evaluate the acid tolerance response and pH adaptation when Enterococcus faecalis interacted with extract of lime ( Citrus aurant iifolia). Methods : We used E. faecalis ATCC 29212 and lime extract from Aceh, Indonesia. The microbe was analyzed for its pH adaptation, acid tolerance response, and adhesion assay using a light microscope with a magnification of x1000. Further, statistical tests were performed to analyze both correlation and significance of the acid tolerance and pH adaptation as well as the interaction activity. Results : E. faecalis was able to adapt to a very acidic environment (pH 2.9), which was characterized by an increase in its pH (reaching 4.2) at all concentrations of the lime extract (p < 0.05). E. faecalis was also able to provide acid tolerance response to lime extract based on spectrophotometric data (595 nm) (p < 0.05). Also, the interaction activity of E. faecalis in different concentrations of lime extract was relatively stable within 6 up to 12 hours (p < 0.05), but it became unstable within 24–72 hours (p > 0.05) based on the mass profiles of its interaction activity. Conclusions : E. faecalis can adapt to acidic environments (pH 2.9–4.2); it is also able to tolerate acid generated by Citrus auranti ifolia extract, revealing a stable interaction in the first 6–12 hours.

The acid tolerance response and pH adaptation of Enterococcus faecalis in extract of lime Citrus aurantiifolia from Aceh Indonesia.

Background: The objective of the present study was to evaluate the acid tolerance response and pH adaptation when Enterococcusfaecalis interacted with extract of lime ( Citrus aurantiifolia). Methods: We used E. faecalis ATCC 29212 and lime extract from Aceh, Indonesia. Both materials were analyzed for their pH adaptation, acid tolerance response, adhesion assay, and mass profiles using a light microscope with a magnification of x1000. Further, statistical tests were performed to analyze both correlation and significance of the acid tolerance and pH adaptation also the interaction activity. Results:E. faecalis was able to adapt to a very acidic environment (pH 2.9), which was characterized by an increase in its pH (reaching 4.2) at all concentrations of the lime extract (p < 0.05). E. faecalis was also able to provide acid tolerance response to lime extract based on spectrophotometric data (595 nm) (p < 0.05). Also, the interaction activity of E. faecalis and the lime extract was relatively stable within 6 up to 12 hours (p < 0.05), but it became unstable within 24-72 hours (p > 0.05) based on the mass profiles of its interaction activity. Conclusions:E. faecalis can adapt to acidic environments (pH 2.9-4.2); it is also able to tolerate acid generated by Citrus aurantiifolia extract, revealing a stable interaction in the first 6-12 hours.

Global transcriptomic Acid Tolerance Response in Salmonella Enteritidis

Salmonella is a major foodborne pathogen throughout the world and able to resist extreme acid stress such as stomach gastric juice after mild acid adaptation, which makes it a potential hazard in food industry. We conducted a comparative transcriptomic analysis by high-throughout RNA-seq to elucidate the Acid Tolerance Response (ATR) in Salmonella Enteritidis. Among these 554 differentially expressed genes (DEGs) identified in this study, the expression of 280 DEGs were up-regulated and 274 DEGs were down-regulated. First, S. Enteritidis tried to survive the adverse environment by reducing the energy-consuming metabolisms and maintaining essential processes as an energy conservation strategy. Second, ATR S. Enteritidis underwent damages in DNA, an oxidization damage result in an iron-lacking circumstance and so on. Thus DEGs related to Fe/S cluster biogenesis, stress response regulating proteins and transport proteins were responsive to acid stress to repair damage caused by acid and ROS. Last, to balance extra protons, by increasing the intracellular NAD (+)/NADH ratio, S. Enteritidis could also decline the protein acetylation level by promoting the consumption of acetyl coenzyme A via TCA, to prevent the intracellular pH from further decline under acid stress.

In vivo repressed genes of Vibrio cholerae reveal inverse requirements of an H+/Cl− transporter along the gastrointestinal passage

The facultative human pathogen Vibrio cholerae changes its transcriptional profile upon oral ingestion by the host to facilitate survival and colonization fitness. Here, we used a modified version of recombination-based in vivo expression technology to investigate gene silencing during the in vivo passage, which has been understudied. Using a murine model of cholera, we screened a V. cholerae transposon library composed of 10,000 randomly generated reporter fusions and identified 101 in vivo repressed (ivr) genes. Our data indicate that constitutive expression of ivr genes reduces colonization fitness, highlighting the necessity to down-regulate these genes in vivo. For example, the ivr gene clcA, encoding an H+/Cl- transporter, could be linked to the acid tolerance response against hydrochloric acid. In a chloride-dependent manner, ClcA facilitates survival under low pH (e.g., the stomach), but its presence becomes detrimental under alkaline conditions (e.g., lower gastrointestinal tract). This pH-dependent clcA expression is controlled by the LysR-type activator AphB, which acts in concert with AphA to initiate the virulence cascade in V. cholerae after oral ingestion. Thus, transcriptional networks dictating induction of virulence factors and the repression of ivr genes overlap to regulate in vivo colonization dynamics. Overall, the results presented herein highlight the impact of spatiotemporal gene silencing in vivo. The molecular characterization of the underlying mechanisms can provide important insights into in vivo physiology and virulence network regulation.

DECONTAMINATION EFFECT OF ELECTROLYZED WATER WASHING ON FRUITS AND VEGETABLES

The use of electrolyzed water in the washing of fruits and vegetables is a promising alternative treatment to chlorine washing. Electrolyzed water washing, is safer, healthier, reduces cleaning times, and is ready to handle. In recent years, food poisoning outbreaks which are caused by bacteria with acid tolerance response in fruits and vegetables has increased. In addition, pathogen produce cases and outbreaks linked to fresh fruits and vegetables, such as cantaloupes, strawberries, fruit salads, spinach, lettuce, celery, and tomatoes has been encountered. Nowadays, the necessity of effective and healthy decontamination processes has gained more importance. The aim of this review is to offer a complete view about electrolyzed water, its classifications and applications. Decontamination results of extant literature of electrolyzed water were also presented. Also, the effects and results of electrolyzed water decontamination on the microbial counts of fresh fruits and vegetables compared with different sanitizing agents have been summarized.

Plasticity of the Pyruvate Node Modulates Hydrogen Peroxide Production and Acid Tolerance in Multiple Oral Streptococci

Commensal Streptococcus sanguinis and Streptococcus gordonii are pioneer oral biofilm colonizers. Characteristic for both is the SpxB-dependent production of H2O2, which is crucial for inhibiting competing biofilm members, especially the cariogenic species Streptococcus mutans H2O2 production is strongly affected by environmental conditions, but few mechanisms are known. Dental plaque pH is one of the key parameters dictating dental plaque ecology and ultimately oral health status. Therefore, the objective of the current study was to characterize the effects of environmental pH on H2O2 production by S. sanguinis and S. gordoniiS. sanguinis H2O2 production was not found to be affected by moderate changes in environmental pH, whereas S. gordonii H2O2 production declined markedly in response to lower pH. Further investigation into the pyruvate node, the central metabolic switch modulating H2O2 or lactic acid production, revealed increased lactic acid levels for S. gordonii at pH 6. The bias for lactic acid production at pH 6 resulted in concomitant improvement in the survival of S. gordonii at low pH and seems to constitute part of the acid tolerance response of S. gordonii Differential responses to pH similarly affect other oral streptococcal species, suggesting that the observed results are part of a larger phenomenon linking environmental pH, central metabolism, and the capacity to produce antagonistic amounts of H2O2IMPORTANCE Oral biofilms are subject to frequent and dramatic changes in pH. S. sanguinis and S. gordonii can compete with caries- and periodontitis-associated pathogens by generating H2O2 Therefore, it is crucial to understand how S. sanguinis and S. gordonii adapt to low pH and maintain their competitiveness under acid stress. The present study provides evidence that certain oral bacteria respond to environmental pH changes by tuning their metabolic output in favor of lactic acid production, to increase their acid survival, while others maintain their H2O2 production at a constant level. The differential control of H2O2 production provides important insights into the role of environmental conditions for growth competition of the oral flora.

Direct interaction between the transcription factors CadC and OmpR involved in the acid stress response of Salmonella enterica.

In Salmonella enterica serovar Typhimurium, the acid-sensing regulator CadC activates transcription of the cadBA operon which contributes to the acid tolerance response. The DNA-binding response regulator OmpR in two-component regulatory system with EnvZ binds to its own promoter for autoinduction. We previously reported that CadC exerts a negative influence on ompR transcription during acid adaptation. However, its underlying mechanisms remain to be elucidated. Here we show that the level of OmpR protein is gradually reduced by a gradual increase in the CadC level using an arabinose-inducible expression system, indicating there exists a negative correlation between the expression levels of two transcription factors. To explore the molecular basis for OmpR repression by CadC, we performed in vitro binding assays and determined that CadC directly interacts with OmpR. We further show that inactivation of cadC inhibits transcription of the fliC gene, which encodes the major flagellar subunit, resulting in impaired flagellar motility under acid-adaptation conditions. Together, our findings suggest that CadC may repress autoinduction of the OmpR response regulator through their direct interaction.

ACID TOLERANCE RESPONCE OF CARIOGENIC MICROORGANISMS AND MALOLACTIC FERMENTATION

Dental caries is an infectious disease which occurs by the metabolism of bacteria acids released to dental enviroment which results hard tissue resolutions. Becouse of the oxygen-free structures of mature plaques complex and deep layers,cariojenic bacteries which have the ability of fermentation come forward. Strong acids like lactic acid, formic acid and pürivat deminish ph of the plaque and the acidty of the plaque causes demineralization of enamel during caries evolution. Existed plaque acidification is not only causes losing minerals from enamel but also threats microorganisms living in the biofilm of the plaque. So most of the microorganisms can’t survive under the ph value of 2.5. The ability of bacteria to survive in this acidic environment depends on the acid tolerance responses they have. Protection against acidity is possible by the production of glicoses,lactic acid and ATP(Adenosine triphosphate) by bacteries. Malolactic fermentation is the most important system that provides these productions in acidic environment. In order to better understand the anti-caries treatment protocols used in current preventive dental practice, the role of bacteria in the fermentation process needs to be known. İn this review we examined: chemical reactions of fermentation, which acids has been occured by the result of these reactions, ph changes in dental plaque, acidojenic and aciduric properties of bacteries which realise fermentation, how can microorganisms survive in acidic environment, what are the advantages propable inhibition of acid tolerance responces for guest. So we tried to attract attention to the anti-cariojenic strategies such as flour, chitosan, α-mangostin and gene studies which are used in the inhibition of acid tolerance systems of bacteria.

Genotypic diversity and phenotypic traits of Streptococcus mutans isolates and their relation to severity of early childhood caries

BackgroundEarly childhood caries (ECC) is an aggressive condition that can affect teeth of young children. This study aimed to evaluate genotypic diversity and phenotypic traits of S. mutans isolated from dental biofilms of children with different caries status in comparison with caries free (CF) children.MethodsStreptococcus mutans strains were isolated from supragingival biofilm samples of CF, ECC and severe-ECC (S-ECC) children and genotyped by arbitrary-primer polymerase chain reaction - AP-PCR. S. mutans genotypes were tested for their ability to reduce the suspension pH through glycolysis, to tolerate extreme acid challenge and by their ability to form biofilm. Response variables were analyzed by ANOVA/Tukey or Kruskal-Wallis/Mann-Whitney tests at a 5% of significance.ResultsThere was an increase in the prevalence of Streptococcus mutans in biofilms with the severity of dental caries. No differences in genotypic diversity and in acidogenicity of genotypes were found among CF, ECC and S-ECC children. S mutans strains with genotypes more characteristic for ECC and S-ECC children formed more biofilms than those identified in CF children. The strains isolated from S-ECC children were highly acid tolerant.ConclusionAlthough S. mutans genotypic diversity was similar among the groups of children, phenotypic traits of S. mutans, especially the acid tolerance response, could explain the severity of early childhood caries.

Structural studies on the decameric S. typhimurium arginine decarboxylase (ADC): Pyridoxal 5′-phosphate binding induces conformational changes

Enteric pathogens such as Salmonella typhimurium colonize the human gut in spite of the lethal acidic pH environment (pH < 2.5) due to the activation of inducible acid tolerance response (ATR) systems. The pyridoxal 5′-phosphate (PLP)-dependent enzyme, biodegradative arginine decarboxylase (ADC, encoded by AdiA), is a component of an ATR system. The enzyme consumes a cytoplasmic proton in the process of arginine degradation to agmatine. Arginine-agmatine antiporter (AdiC) exchanges the product agmatine for arginine. In this manuscript, we describe the structure of Salmonella typhimurium ADC (StADC). The decameric structure assembled from five dimers related by a non crystallographic 5-fold symmetry represents the first apo-form of the enzyme. The structure suggests that PLP-binding is not a prerequisite for oligomerization. Comparison with E. coli ADC reveals that PLP-binding is accompanied by the movement and ordering of two loops (residues 150–159 and 191–197) and a few active site residues such as His256 and Lys257. A number of residues important for substrate binding are disordered in the apo-StADC structure indicating that PLP binding is important for substrate binding. Unlike the interactions between 5-fold related protomers, interactions that stabilize the dimeric structure are not pH dependent.

A metabolomic approach to characterize the acid-tolerance response in Sinorhizobium meliloti

Sinorhizobium meliloti establishes a symbiosis with Medicago species where the bacterium fixes atmospheric nitrogen for plant nutrition. To achieve a successful symbiosis, however, both partners need to withstand biotic and abiotic stresses within the soil, especially that of excess acid, to which the Medicago-Sinorhizobium symbiotic system is widely recognized as being highly sensitive. To cope with low pH, S. meliloti can undergo an acid-tolerance response (ATR(+)) that not only enables a better survival but also constitutes a more competitive phenotype for Medicago sativa nodulation under acid and neutral conditions. To characterize this phenotype, we employed metabolomics to investigate the biochemical changes operating in ATR(+) cells. A gas chromatography/mass spectrometry approach was used on S. meliloti 2011 cultures showing ATR(+) and ATR(−) phenotypes. After an univariate and multivariate statistical analysis, enzymatic activities and/or reserve carbohydrates characterizing ATR(+) phenotypes were determined. Two distinctive populations were clearly defined in cultures grown in acid and neutral pH based on the metabolites present. A shift occurred in the carbon-catabolic pathways, potentially supplying NAD(P)H equivalents for use in other metabolic reactions and/or for maintaining intracellular-pH homeostasis. Furthermore, among the mechanisms related to acid resistance, the ATR(+) phenotype was also characterized by lactate production, envelope modification, and carbon-overflow metabolism. Acid-challenged S. meliloti exhibited several changes in different metabolic pathways that, in specific instances, could be identified and related to responses observed in other bacteria under various abiotic stresses. Some of the observed changes included modifications in the pentose-phosphate pathway (PPP), the exopolysaccharide biosynthesis, and in the myo-inositol degradation intermediates. Such modifications are part of a metabolic adaptation in the rhizobia that, as previously reported, is associated to improved phenotypes of acid tolerance and nodulation competitiveness.

Comparative transcriptomic analysis by RNA-seq of Acid Tolerance Response (ATR) in EHEC O157:H7

ATR in EHEC O157:H7 and the cross-protection effects thus induced lead to the uncertainty of food safety. This study aimed to identify factors that are associated with the ATR in EHEC O157:H7 under stomach acidity using RNA-seq. In total, 223 DEGs in E. coli O157:H7 and 110 DEGs in E. coli after acid treatment were identified, including 118 upregulated and 105 downregulated in EHEC O157:H7, and 89 upregulated and 21 downregulated in E. coli ATCC 25922. According to our results, when facing the ATR environment, protein Asr regulated the whole acid resistance process. In E. coli ATCC 25922, cadA and cadB mediate the lysine decarboxylation consumed the extra protons inside and keep the neutral pH value in cytoplasm to protect the E. coli from death. In contrast, oxidation-reduction mediated by ahpC, which was activated by the depression of oxyR, and hydrogenation mediated by mhpA consumed the protons inside keeping the neutral pH value to protect the EHEC O157:H7 from harm induced by a low pH environment. In addition, EHEC O157:H7 expressed more anti-oxidation enzymes to repair damage caused by acid stress, which enhanced its resistance to acid compared to E. coli ATCC 25922.

Amino acid decarboxylase-dependent acid tolerance, selected phenotypic, and virulence gene expression responses of Salmonella enterica serovar Heidelberg

Salmonella enterica serovar Heidelberg (S. Heidelberg) is one of the pathogens most frequently detected in recent years in food products from animals; here, we examine the acid tolerance of this strain. Mild acid (pH5.5–5.0) induced a strong acid resistance in S. Heidelberg; the induced acid resistance improved within 0.5–6h and resulted in >95% cell survival following challenge in pH3.0 medium. Addition of lysine or arginine to pH2.5 acid challenge medium significantly improved the survival of S. Heidelberg; lysine induced the largest increase in survival. The lysine and arginine decarboxylase-related genes (i.e., cadA, cadB, adiA, and adiY) are acid induced genes, and they play an important role in S. Heidelberg acid resistance. RT-PCR showed that the genes expression levels increased as acid adaptation pH decreased. The increased expression was maintained for at least 4h during adaptation. Moreover, acid adaptation may have increased the production of cellulose and swimming in S. Heidelberg; pH5.5 acid-adapted cells showed a red, dry, and rough morphotype on Congo red plates. The transcriptional levels of Enterotoxin gene (stn) increased three times following acid adaptation; however, the expression of Salmonella pathogenicity island 1 virulence genes significantly decreased.

Variability in the adaptive acid tolerance response phenotype of Salmonella enterica strains

The objective of this study was the assessment of the stationary-phase, low-pH-inducible acid tolerance response (ATR) of different Salmonella enterica strains. For this purpose, 30 strains of the pathogen were grown in tryptone soy broth in the absence (non-adapted cultures) and presence (1% w/v; acid-adapted cultures) of glucose, and then subjected to 4-h acid challenge trials at pH 3.0. Surviving populations of each strain were determined at 1-h intervals, and the Weibull model was fitted to the derived microbiological data. Extensive variability in the acid stress responses of the tested S. enterica strains was observed, with the total population reductions (log CFU/ml) attained in 4 h of acid challenge ranging from 0.9 to 5.5 and from 0.6 to 7.0 for the non-adapted and acid-adapted cultures, respectively. As demonstrated by the model scale parameter δ and shape parameter p, the effect of acid adaptation on the inactivation curves was strain-specific. Although acid adaptation resulted in enhanced acid survival for the majority of the tested strains, there were strains exhibiting similar or decreased acid resistance compared to their non-adapted counterparts. Moreover, acid adaptation appeared to decrease the strain variability of δ whereas increasing the strain variability of p: the coefficient of variation of δ among the tested strains was 97.2 and 54.9% for the non-adapted and acid-adapted cultures, respectively, while the corresponding values for p were 12.7 and 48.1%. The data of the present study, which is the first one to systematically evaluate the adaptive ATR of multiple S. enterica strains, clearly demonstrate that this phenotype (attempted to be induced by growing the pathogen in the presence of glucose) is strain-dependent.