Adaptive Acid Tolerance Response Research Articles

Adaptive acid tolerance response in Lactobacillus acidophilus

Stationary phase cultures of Lactobacillus acidophilus CRL 639 are naturally acid resistant to pH 3.0 while exponential phase cells induce an acid tolerance response upon exposure to sublethal pH (3.8–6.0). Maximal response was achieved after adaptation at pH 4.2 and pH 5.0. Protein synthesis was required in the latter case. © Rapid Science Ltd. 1998

Response of Brucella suis 1330 and B. canis RM6/66 to growth at acid pH and induction of an adaptive acid tolerance response

Acid pH is an environmental stress often encountered by Brucella during both the "environmental" and the "pathogenic" stages of its life. We have investigated the behaviour of B. suis biovar 1 and B. canis in acid conditions. Growth at suboptimal pH was characterized by a dramatic reduction in growth yield due to an early onset of stationary phase. B. suis was more resistant to low pH than B. canis, which lysed at pH 4.6. Viable counts measured after a 4-h acid shock at pH 3.2 showed that the relative survival of B. suis was 1,000-fold greater than that of B. canis. An adaptive acid tolerance response (ATR) was induced in both species by culture at pH 5.8; however, while the acid-sensitive B. canis had more than a 2,000-fold increase in survival following acid shock at pH 3.2, the increase in survival of B. suis was only around 50-fold. The kinetics of the induction of ATR were followed: for B. suis, 1-2 h (1 generation) at pH 5.8 were required to induce acid tolerance (50-fold protection), and these levels remained constant over 24 h. B. canis became relatively acid-resistant after only 30-min exposure to pH 5.8. Levels of acid tolerance continued to increase and were maximal at 24 h. Stationary phase pH 7.2 cultures of either species did not exhibit acid resistance, suggesting that, like Salmonella, Brucella does not have an rpoS-controlled stationary phase acid resistance.

Acid tolerance in Listeria monocytogenes: the adaptive acid tolerance response (ATR) and growth-phase-dependent acid resistance.

Listeria monocytogenes acquired increased acid tolerance during exponential growth upon exposure to sublethal acid stress, a response designated the acid tolerance response (ATR). Maximal acid resistance was seen when the organism was exposed to pH 5.0 for 1 h prior to challenge at pH 3.0, although intermediate levels of protection were afforded by exposure to pH values ranging from 4.0 to 6.0. A 60 min adaptive period was required for the development of maximal acid tolerance; during this period the level of acid tolerance increased gradually. Full expression of the ATR required de novo protein synthesis; chloramphenicol, a protein synthesis inhibitor, prevented full induction of acid tolerance. Analysis of protein expression during the adaptive period by two-dimensional gel electrophoresis revealed a change in the expression of at least 23 proteins compared to the non-adapted culture. Eleven proteins showed induced expression while 12 were repressed, implying that the ATR is a complex response involving a modulation in the expression of a large number of genes. In addition to the exponential phase ATR, L.monocytogenes also developed increased acid resistance upon entry into the stationary phase; this response appeared to be independent of the pH-dependent ATR seen during exponential growth.

Acid tolerance in Listeria monocytogenes: the adaptive acid tolerance response (ATR) and growth-phase-dependent acid resistance

Acid tolerance in Listeria monocytogenes: the adaptive acid tolerance response (ATR) and growth-phase-dependent acid resistance

Adaptive acid tolerance response in Listeria monocytogenes: isolation of an acid-tolerant mutant which demonstrates increased virulence.

The ability of Listeria monocytogenes to tolerate low-pH environments is of particular importance because the pathogen encounters such environments in vivo, both during passage through the stomach and within the macrophage phagosome. In our study, L. monocytogenes was shown to exhibit a significant adaptive acid tolerance response following a 1-h exposure to mild acid (pH 5.5), which is capable of protecting cells from severe acid stress (pH 3.5). Susceptibility to pH 3.5 acid is growth phase dependent. Stationary-phase Listeria cultures are naturally resistant to the challenge pH (pH 3.5), while exponential-phase cultures require adaptation at pH 5.5 to induce acid tolerance. Adaptation requires protein synthesis, since treatment with chloramphenicol prevents the development of acid tolerance. Induction of the acid tolerance response also protects L. monocytogenes against the effect of other environmental stresses. Acid-adapted cells demonstrate increased tolerance toward thermal stress, osmotic stress, crystal violet, and ethanol. Following prolonged exposure of L. monocytogenes to pH 3.5, we isolated mutants which constitutively demonstrate increased acid tolerance at all stages of the growth cycle. These mutants do not display full acid tolerance, but their resistance to low pH can be further increased following adaptation to mild-acid conditions. The mutants demonstrated increased lethality for mice relative to that of the wild type when inoculated by the intraperitoneal route. When administered as lower inocula, the mutants reached higher levels in the spleens of infected mice than did the wild type. The data suggest that low-pH conditions may have the potential to select for L. monocytogenes mutants with increased natural acid tolerance and increased virulence.

Role of the acid tolerance response in virulence of Salmonella typhimurium

During its life cycle, Salmonella typhimurium is exposed to a variety of acidic conditions. Survival in the acidic environments within the host may require the adaptive acid tolerance response (ATR), which is characterized by the induction of several Salmonella proteins upon exposure to mildly acidic conditions. These induced proteins protect the bacterium from death under severe acid challenge. The goal of this study was to examine the role of ATR in Salmonella pathogenesis. Initially, we observed that differences exist between the virulent S. typhimurium strains and the laboratory S. typhimurium strain LT2 with respect to their ATR. Mutations affecting the ATR of S. typhimurium LT2, including atrB, atrC (polA), atrD, atbR, and fur, were crossed into virulent Salmonella strains, and the resultant transductants were screened for virulence in mice and acid sensitivity. Surprisingly, with the exception of the fur mutation, none of the muatations had a major effect on acid resistance or virulence in the pathogenic strains. The fur mutants showed a 1-to 3-log increase in the 50% lethal dose; however, the magnitude of its effect was dependent on the strain background. Strains containing two or three different atr mutations were constructed, and these were also examined for acid sensitivity and virulence. The double and triple mutants that contained an atrC mutation no longer displayed an ATR. Those mutants which were more acid sensitive were also highly attenuated, suggesting a strong correlation between the ability to mount and ATR and virulence in S. typhimurium. Comparison of the ability of the various atr single, double, and triple mutants to survive within macrophages showed that strains containing an atrC mutation survived much less than the wild type in bone marrow-derived macrophages. No difference in survival within J774 macrophage like cells were detected.

The life of root nodule bacteria in the acidic underground

The effects of low pH on the growth and survival of root nodule bacteria are important in determining the success of legume inoculation in practical agriculture. The mechanisms for maintenance of intracellular pH (pHi) are crucial, but little is yet known about them in root nodule bacteria. Like other Gram-negative bacteria, root nodule bacteria show an adaptive acid tolerance response, with growth at moderately acidic conditions protecting against an extreme acid shock. Variations in acid tolerance within species of root nodule bacteria imply a genetic basis to low pH tolerance and studies of acid-sensitive mutants suggest that as many as 20 genes could be involved. A general model for acid tolerance in root nodule bacteria is presented.

The life of root nodule bacteria in the acidic underground

The effects of low pH on the growth and survival of root nodule bacteria are important in determining the success of legume inoculation in practical agriculture. The mechanisms for maintenance of intracellular pH (pHi) are crucial, but little is yet known about them in root nodule bacteria. Like other Gram-negative bacteria, root nodule bacteria show an adaptive acid tolerance response, with growth at moderately acidic conditions protecting against an extreme acid shock. Variations in acid tolerance within species of root nodule bacteria imply a genetic basis to low pH tolerance and studies of acid-sensitive mutants suggest that as many as 20 genes could be involved. A general model for acid tolerance in root nodule bacteria is presented.

Adaptive acid tolerance response (ATR) in Aeromonas hydrophila.

Aeromonas hydrophila, a gastrointestinal pathogen of humans, was shown to exhibit a significant adaptive acid tolerance response (ATR) capable of protecting cells from severe acid at a pH of 3.5. The ATR was induced by exposure to a relatively mild pH level of 5.0 for 20 min. Adaptation required protein synthesis since treatment with chloramphenicol during adaptation to pH 5.0 prevented the development of acid tolerance. The adaptation to acid environment was found to be a non-transient phenomenon. Also, iron was not required for acid adaptation in A. hydrophila. Two-dimensional protein analyses revealed an increased production of 28 proteins and decreased synthesis of 10 following pH shifts from 7.2 to 5.0. The mild pH treatment must act as a signal to A. hydrophila to adapt and survive in acid environments by producing 'protective' proteins. The adaptation and survival of this pathogen in low pH may provide valuable information about its ability to withstand acid environments in nature and in the human gastrointestinal tract.

The adaptive acid tolerance response in root nodule bacteria and Escherichia coli

The adaptive acid tolerance response in root nodule bacteria and Escherichia coli

Adaptive acid tolerance response by Salmonella typhi and candidate live oral typhoid vaccine strains.

The data presented here demonstrate that Salmonella typhi is capable of expressing an acid tolerance response (ATR) and that effective induction of this response (in nutrient-rich medium) occurs at pH 5.0 in anaerobic conditions. The candidate live oral S. typhi vaccine strains made by precise genetic methods and which carry auxotrophic mutations were CVD 906 (carries defined attenuating deletion mutations: delta aroC, delta aroD), CVD 908 (carries defined attenuating deletion mutations: delta aroC, delta aroD), 541Ty (carries attenuating deletion mutations: aroA, purA), and galE, Vi-negative (via) strain EX462. All generate an effective ATR. In contrast, nitrosoguanidine-derived live oral typhoid vaccine strain Ty21a only weakly expresses acid tolerance. This further demonstrates that the non-specific mutagenesis process used to produce Ty21a affects genetic loci outside the intended target genes for mutagenesis, galE and via, and further emphasizes the importance of using precise genetic techniques when developing live oral S. typhi vaccines.

Effect of Salmonella typhimurium ferric uptake regulator (fur) mutations on iron- and pH-regulated protein synthesis.

Fur is an important regulatory protein known to function in the presence of iron as a repressor of iron-controlled genes. It was recently discovered that Fur is also essential to Salmonella typhimurium for mounting an adaptive acid tolerance response (J. W. Foster, J. Bacteriol 173:6896-6902, 1991). Because little is known about the effect of Fur on the physiology of this enteric pathogen, a systematic two-dimensional polyacrylamide gel electrophoresis (PAGE) analysis was conducted to identify proteins whose synthesis is linked to iron levels. Mutations in the fur locus were identified and used to classify which proteins are controlled by Fur. Thirty-six proteins were overtly affected by iron availability, most of which were clearly under the control of Fur. Although most of the Fur-dependent proteins were under negative control, a significant portion (15 of 34) appeared to be under a form of positive control. Nine of the positively controlled proteins required Fur and iron for expression. However, Fur lacking iron was also required for the induction of six gene products. Surprisingly, not all iron-regulated proteins were controlled by Fur and not all Fur-dependent proteins were obviously regulated by iron status. Because fur mutants fail to mount an effective acid tolerance response, we made a comparative two-dimensional PAGE analysis of 100 total acid- and iron-regulated gene products. Production of most of these proteins was regulated by only one of the two stresses, yet a clear subset of seven genes were influenced by both acid and iron and were also controlled by fur. These proteins were also members of the acid tolerance response modulon. Consistent with the fur effect on pH-regulated protein synthesis, fur mutants lacked the inducible pH homeostasis system associated with the acid tolerance response. The results provide further evidence that Fur has an extensive impact on gene expression and cellular physiology and suggest an explanation for the acid-sensitive nature of fur mutants.

Salmonella acid shock proteins are required for the adaptive acid tolerance response

Salmonella typhimurium, as well as other enteric bacteria, experiences significant fluctuations in H+ ion concentrations during growth in diverse ecological niches. In fact, some pH conditions which should kill cells rapidly, such as stomach acidity, are nevertheless tolerated. The complete mechanism for this tolerance is unknown. However, I have recently demonstrated that S. typhimurium has the ability to survive extreme low pH (pH 3.0 to 4.0) if first adapted to mild pH (pH 5.5 to 6.0). This phenomenon has been referred to as the acidification tolerance response (ATR). The exposure to mild acid is referred to as preshock, and the proteins involved are called preshock ATR proteins. A second type of encounter with acid, called acid shock, involves shifting cells directly from alkaline conditions (pH 7.7) to acid conditions (pH 4.5 or below). During acid shock, the organism immediately ceases reproduction and dramatically changes the expression of at least 52 proteins. All but four are distinct from the preshock ATR proteins. Surprisingly, acid shock alone did not afford significant protection against strong acid challenge in minimal medium. Furthermore, inhibiting protein synthesis prior to acid shock revealed that the acid shock proteins do not appear to contribute to acid survival in minimal medium even at pH 4.3. Constitutive cellular pH homeostatic mechanisms seem sufficient to protect cells at this pH. The data suggest that the induction of acid shock and preshock ATR proteins are separate processes requiring separate signals. However, for S. typhimurium to survive extreme acid conditions, it must induce both the preshock and acid shock systems. Preventing the expression of one or the other eliminates acid tolerance. I propose a two-stage process that allows S. typhimurium to phase in acid tolerance as the environmental pH becomes progressively more acidic.