Acid Resistance Systems Required for Survival of Escherichia coli O157:H7 in the Bovine Gastrointestinal Tract and in Apple Cider Are Different (original) (raw)
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Journal of Applied Microbiology, 2007
Aims: To compare survival of enterohaemorrhagic Escherichia coli (EHEC) strains of two clonal groups in a simulated gastric environment and to quantify the effect of storage in an acidic food, apple juice, on subsequent survival of EHEC in the simulated gastric environment. Methods and Results: To characterize acid resistance of EHEC under conditions simulating the gastric environment, survival was measured in a model stomach system (MSS) for two clonal groups of EHEC: 14 EHEC 1 strains of serotype O157:H7 and 12 EHEC 2 strains of serotypes O26:H11 and O111:H8. There were significant differences between the two EHEC groups, with the average survival rate of O157 strains in the MSS twice as great as the O26/ O111 strains. Strains of the two groups also differed in the quantity of injured cells in MSS and in the transcript levels of the glutamate decarboxylase genes (measured by quantitative PCR) in stationary phase before cultures entered the MSS.
Characterization of Enterohemorrhagic Escherichia coli Strains Based on Acid Resistance Phenotypes
Infection and Immunity, 2005
Acid resistance is perceived to be an important property of enterohemorrhagic Escherichia coli strains, enabling the organisms to survive passage through the acidic environment of the stomach so that they may colonize the mammalian gastrointestinal tract and cause disease. Accordingly, the organism has developed at least three genetically and physiologically distinct acid resistance systems which provide different levels of protection. The glutamate-dependent acid resistance (GDAR) system utilizes extracellular glutamate to protect cells during extreme acid challenges and is believed to provide the highest protection from stomach acidity. In this study, the GDAR system of 82 pathogenic E. coli isolates from 34 countries and 23 states within the United States was examined. Twenty-nine isolates were found to be defective in inducing GDAR under aerobic growth conditions, while five other isolates were defective in GDAR under aerobic, as well as fermentative, growth conditions. We introduced rpoS on a low-copy-number plasmid into 26 isolates and were able to restore GDAR in 20 acid-sensitive isolates under aerobic growth conditions. Four isolates were found to be defective in the newly discovered LuxR-like regulator GadE (formerly YhiE). Defects in other isolates could be due to a mutation(s) in a gene(s) with an as yet undefined role in acid resistance since GadE and/or RpoS could not restore acid resistance. These results show that in addition to mutant alleles of rpoS, mutations in gadE exist in natural populations of pathogenic E. coli. Such mutations most likely alter the infectivity of individual isolates and may play a significant role in determining the infective dose of enterohemorrhagic E. coli.
Journal of Food Protection, 2011
Outbreaks of disease due to vegetative bacterial pathogens associated with acid foods (such as apple cider) have raised concerns about acidified vegetables and related products that have a similar pH (3.2 to 4.0). Escherichia coli O157:H7 and related strains of enterohemorrhagic E. coli (EHEC) have been identified as the most acid resistant vegetative pathogens in these products. Previous research has shown that the lack of dissolved oxygen in many hermetically sealed acid or acidified food products can enhance survival of EHEC compared with their survival under aerobic conditions. We compared the antimicrobial effects of several food acids (acetic, malic, lactic, fumaric, benzoic, and sorbic acids and sulfite) on a cocktail of EHEC strains under conditions representative of non–heat-processed acidified vegetables in hermetically sealed jars, holding the pH (3.2) and ionic strength (0.342) constant under anaerobic conditions. The overall antimicrobial effectiveness of weak acids use...
Japanese Journal of Food Microbiology, 2003
Acid injured bacteria are sensitive to selective agents. Enumeration of such bacteria in selective media therefore results in lower estimation of viable cell counts. In this study, several antibiotic resistant Escherichia coli O157:H7 strains were constructed and their recovery in acidic conditions on several agar media was validated. Introduction of nalidixic acid or rifampicin-resistant strains did not affect either their growth rate or sensitivity against acidic challenge. Recovery of such resistant strains on tryptic-soy agar (TSA) supplemented with corresponding antibiotic was higher than that of other selective media used, and the use of antibiotic-resistant strains or TSA medium supplemented with corresponding antibiotic did not affect the estimation of recovery. These results suggested that the antibiotic resistant strains could be useful for the challenge tests of acidic food.
Journal of Food …, 2004
Twenty-five strains of Escherichia coli O157:H7 isolated from humans, cattle, and pigs were maintained in HCl (pH 2.5) and in a volatile fatty acid (VFA) mixture (pH 4.0) for up to 6 h at 37ЊC to assess their ability to survive in acidic conditions that simulate those of the stomach and ileum, respectively. In HCl, the average group survival of bovine strains was significantly higher than that of porcine and human strains, whereas in VFAs, porcine strains were significantly more resistant than bovine and human strains. Bovine strains exhibited significantly higher average survival in HCl than in VFAs. The average survival of strains classified as octamer-based genome scanning (OBGS) lineage II was significantly superior to that of strains classified as OBGS lineage I in HCl. The group of lineage I strains was more resistant in VFAs compared with lineage II, but only after 6 h of challenge. The possible involvement of urease in acid resistance of E. coli O157:H7 was also examined. Although the strains possessed the ureC gene, as shown by PCR, this gene did not appear to contribute to acid resistance under the conditions tested. The data indicate that there is a relationship between acid resistance and source or lineage of O157:H7 strains.
Food Microbiology, 2007
This study assessed the acid tolerance response (ATR) of stationary phase, acid-adapted (tryptic soy broth [TSB]+1% glucose) or nonacid-adapted (glucose-free TSB) Escherichia coli O157:H7 strains (ATCC43889, ATCC43895, ATCC51658 and EO139), grown individually or in a mixed culture, prior to inoculation of beef or meat decontamination runoff (washings) fluids (acidic [pH 4.95] or nonacidic [pH 7.01]). The inoculated beef was left untreated or treated by dipping for 30 s in hot water (75 1C) followed by 2% lactic acid (55 1C). Inoculated beef samples and washings were stored aerobically at 4 or 15 1C for 6 d, and at set intervals (0, 2, and 6 d) were exposed (for 0, 60, 120, and 180 min) to pH 3.5 (adjusted with lactic acid) TSB plus 0.6% yeast extract. Overall, there were no significant (PX0:05) differences in responses of cultures prepared as individual or mixed strains. Decontamination of meat did not affect the subsequent ATR of E. coli O157:H7 other than resulting in lower initial pathogen levels exposed to acidic conditions. In this study, E. coli O157:H7 appeared to become more tolerant to acid following incubation in acidic washings of sublethal pH (4.89-5.22) compared to nonacidic washings (pH 6.97-7.41) at 4 1C or in both types of washings incubated at 15 1C. The ATR of the pathogen inoculated into washings was enhanced when cells were previously acid-adapted and incubated at 4 1C. Similarly, the ATR on meat was increased by previous acid-adaptation of the inoculum in broth and enhanced by storage at 4 1C. Populations on treated meat were consistently lower than those on untreated meat during storage and following exposure to acid. Although on day-0 there were no significant (PX0:05) differences in ATR between acid-adapted and nonacid-adapted populations on meat, acid-adapted cells displayed consistently higher resistance through day-6. This suggests that acid-adapted E. coli O157:H7 introduced on meat may become resistant to subsequent lactic acid exposure following storage at 4 1C. r
Survival and characterization of Escherichia coli strains in a typical Mexican acid-fermented food
International Journal of Food Microbiology, 2001
In this study, the acid resistance and the changes in outer membrane protein (Omps) profiles of Escherichia coli strains isolated from pozol, an acid-fermented maize beverage consumed in Southeastern Mexico, were determined. Results showed that adaptation to acid by these E. coli strains significantly enhances their survival in acid conditions. Changes in Omp profiles were found in non-adapted acid challenged cells compared with non-challenged cells that had not been adapted to acid. Challenged adapted cells showed no significant changes in these profiles when compared with the acid adapted non-challenged strains. N-terminal sequences of some of the Omps were determined. The intensity of the main porins OmpC and OmpA was lower in the acid challenged strains, than in the non-challenged ones. The OmpF porin was identified in non-challenged K12 strain, but did not appear in adapted or non-adapted pozol strains nor in E. coli O157:H7.
A systems biology approach sheds new light on Escherichia coli acid resistance
Nucleic Acids Research, 2011
Escherichia coli has to pass through the stomach, where the pH can be as low as 1. Mechanisms that enable E. coli to survive in low pH are thus potentially relevant for pathogenicity. Four acid response systems involved in reducing the concentration of intracellular protons have been identified so far. However, it is still unclear to what extent the regulation of other important cellular functions may be required for survival in acid conditions. Here, we have combined molecular and phenotypic analysis of wild-type and mutant strains with computational network inference to identify molecular pathways underlying E. coli response to mild and strong acid conditions. The interpretative model we have developed led to the hypothesis that a complex transcriptional programme, dependent on the twocomponent system regulator OmpR and involving a switch between aerobic and anaerobic metabolism, may be key for survival. Experimental validation has shown that the OmpR is responsible for controlling a sizeable component of the transcriptional programme to acid exposure. Moreover, we found that a "ompR strain was unable to mount any transcriptional response to acid exposure and had one of the strongest acid sensitive phenotype observed.
Applied and Environmental Microbiology, 1996
Prior growth of seven enterohemorrhagic and one nonenterohemorrhagic strains of Escherichia coli in tryptic soy broth with (TSB؉G) and without (TSB؊G) 1% glucose was evaluated for its effect on acid tolerance. The final pHs of 18-h TSB؉G and TSB-G cultures were 4.6 to 5.2 and 6.9 to 7.0, respectively. Cells were then transferred to brain heart infusion broth adjusted to pH 2.5 or 3.0 with HCl, incubated at 37؇C for up to 7 h, and assayed periodically for viable populations with brain heart infusion and MacConkey agars. All enterohemorrhagic strains were acid resistant (<0.5 log decline after 7 h) when initially cultured in TSB؉G, but substantial differences in acid tolerance were observed among strains cultured in TSB؊G (log declines ranged from <0.3 to >3.8). The results indicated that prior growth in a medium with and without a fermentable carbohydrate is a convenient way of studying the induction of acid tolerance, that acid inactivation is preceded by a period of acid injury, and that pH-independent and pH-dependent stationary-phase acid tolerance phenotypes may exist among strains of enterohemorrhagic E. coli.
Food Microbiology, 1999
The ability of pH-dependent, stationary phase acid resistance to cross-protect Escherichia coli O157:H7 against a subsequent lethal thermal stress was evaluated using microbiological media and three liquid foods. Three strains were grown for 18 h at 378C in acidogenic (TSB+G, ¢nal pH 4Á6±4Á7) and non-acidogenic (TSB-G, ¢nal pH 7Á0±7Á2) media to provide stationary phase cells with and without induction of pH-dependent acid resistance. The cells were then heated in BHI broth (pH 6Á0) at 588C, using a submerged coil apparatus. The TSB+G grown strains had greatly increased heat resistance, with the heating time needed to achieve a ¢ve-log inactivation, being increased two-to fourfold. The z-values of TSB+G and TSB-G grown cells were 4Á78C and 4Á38C, respectively. Increases in heat resistance withTSB+G-grown E. coli O157:H7 were also observed using milk and chicken broth, but not with apple juice. However, cross-protection was restored if the pH of the apple juice was increased from 3Á5 to 4Á5. The data indicate that pH-dependent acid resistance provides E. coli O157:H7 with cross-protection against heat treatments, and that this factor must be considered to estimate this pathogen's thermal tolerance accurately.