Effects of Acid Adaptation of Escherichia coli O157:H7 on Efficacy of Acetic Acid Spray Washes To Decontaminate Beef Carcass Tissue

Exposure to low pH and organic acids in the bovine gastrointestinal tract may result in the induced acid resistance of Escherichia coli O157:H7 and other pathogens that may subsequently contaminate beef carcasses. The effect of acid adaptation of E. coli O157:H7 on the ability of acetic acid spray washing to reduce populations of this organism on beef carcass tissue was examined. Stationary-phase acid resistance and the ability to induce acid tolerance were determined for a collection of E. coli O157:H7 strains by testing the survival of acid-adapted and unadapted cells in HCl-acidified tryptic soy broth (pH 2.5). Three E. coli O157:H7 strains that were categorized as acid resistant (ATCC 43895) or acid sensitive (ATCC 43890) or that demonstrated inducible acid tolerance (ATCC 43889) were used in spray wash studies. Prerigor beef carcass surface tissue was inoculated with bovine feces containing either acid-adapted or unadapted E. coli O157:H7. The beef tissue was subjected to spray washing treatments with water or 2% acetic acid or left untreated. For strains ATCC 43895 and 43889, larger populations of acid-adapted cells than of unadapted cells remained on beef tissue following 2% acetic acid treatments and these differences remained throughout 14 days of 4°C storage. For both strains, numbers of acid-adapted cells remaining on tissue following 2% acetic acid treatments were similar to numbers of both acid-adapted and unadapted cells remaining on tissue following water treatments. For strain ATCC 43890, there was no difference between populations of acid-adapted and unadapted cells remaining on beef tissue immediately following 2% acetic acid treatments. These data indicate that adaptation to acidic conditions by E. coli O157:H7 can negatively influence the effectiveness of 2% acetic acid spray washing in reducing the numbers of this organism on carcasses.     

Survival or growth of Escherichia coli O157:H7 in a model system of fresh meat decontamination runoff waste fluids and its resistance to subsequent lactic acid stress

A potential may exist for survival of and resistance development by Escherichia coli O157:H7 in environmental niches of meat plants applying carcass decontamination interventions. This study evaluated (i) survival or growth of acid-adapted and nonadapted E. coli O157:H7 strain ATCC 43895 in acetic acid (pH 3.6 +/- 0.1) or in water (pH 7.2 +/- 0.2) fresh beef decontamination runoff fluids (washings) stored at 4, 10, 15, or 25 degrees C and (ii) resistance of cells recovered from the washings after 2 or 7 days of storage to a subsequent lactic acid (pH 3.5) stress. Corresponding cultures in sterile saline or in heat-sterilized water washings were used as controls. In acetic acid washings, acid-adapted cultures survived better than nonadapted cultures, with survival being greatest at 4 degrees C and lowest at 25 degrees C. The pathogen survived without growth in water washings at 4 and 10 degrees C, while it grew by 0.8 to 2.7 log cycles at 15 and 25 degrees C, and more in the absence of natural flora. E. coli O157:H7 cells habituated without growth in water washings at 4 or 10 degrees C were the most sensitive to pH 3.5, while cells grown in water washings at 15 or 25 degrees C were relatively the most resistant, irrespective of previous acid adaptation. Resistance to pH 3.5 of E. coli O157:H7 cells habituated in acetic acid washings for 7 days increased in the order 15 degrees C > 10 degrees C > 4 degrees C, while at 25 degrees C cells died off. These results indicate that growth inhibition by storage at low temperatures may be more important than competition by natural flora in inducing acid sensitization of E. coli O157:H7 in fresh meat environments. At ambient temperatures in meat plants, E. coli O157:H7 may grow to restore acid resistance, unless acid interventions are applied to inhibit growth and minimize survival of the pathogen. Acid-habituated E. coli O157:H7 at 10 to 15 degrees C may maintain a higher acid resistance than when acid habituated at 4 degrees C. These responses should be evaluated with fresh meat and may be useful for the optimization of decontamination programs and postdecontamination conditions of meat handling.     

Influence of apple cultivars on inactivation of different strains of Escherichia coli O157:H7 in apple cider by UV irradiation

This study examined the effect of different apple cultivars upon the UV inactivation of Escherichia coli O157:H7 strains within unfiltered apple cider. Apple cider was prepared from eight different apple cultivars, inoculated with approximately 10(6) to 10(7) CFU of three strains of E. coli O157:H7 per ml (933, ATCC 43889, and ATCC 43895), and exposed to 14 mJ of UV irradiation per cm(2). Bacterial populations for treated and untreated samples were then enumerated by using nonselective media. E. coli O157:H7 ATCC 43889 showed the most sensitivity to this disinfection process with an average 6.63-log reduction compared to an average log reduction of 5.93 for both strains 933 and ATCC 43895. The highest log reduction seen, 7.19, occurred for strain ATCC 43889 in Rome cider. The same cider produced the lowest log reductions: 5.33 and 5.25 for strains 933 and ATCC 43895, respectively. Among the apple cultivars, an average log reduction range of 5.78 (Red Delicious) to 6.74 (Empire) was observed, with two statistically significant (alpha < or = 0.05) log reduction groups represented. Within the paired cultivar-strain analysis, five of eight ciders showed statistically significant (alpha < or = 0.05) differences in at least two of the E. coli strains used. Comparison of log reductions among the E. coli strains to the cider parameters of (o)Brix, pH, and malic acid content failed to show any statistically significant relationship (R(2) > or = 0.95). However, the results of this study indicate that regardless of the apple cultivar used, a minimum 5-log reduction is achieved for all of the strains of E. coli O157:H7 tested.     

Tolerance of acid-adapted and non-adapted Escherichia coli O157:H7 cells to reduced pH as affected by type of acidulant

A study was carried out to determine if three strains of Escherichia coli O157:H7 grown (18 h) in Tryptic Soy Broth (TSB) and TSB supplemented with 1.25% glucose (TSBG), i.e. unadapted and acid-adapted cells, respectively, exhibited changes in tolerance to reduced pH when plated on Tryptic Soy Agar (TSA) acidified (pH 3.9, 4.2, 4.5, 4.8, 5.1 and 5.4) with acetic, citric or malic acids. All test strains grew well on TSA acidified with acetic acid at pH > or = 5.4 or malic acid at pH > or = 4.5; two strains grew on TSA acidified with citric acid at pH > or = 4.5, while the third strain grew at pH > or = 4.8. Acid-adapted and control (unadapted) cells differed little in their ability to form visible colonies on TSA containing the same acid at the same pH. However, on plates not showing visible colonies, acid-adapted cells retained higher viability than unadapted cells when plated on acidified TSA. Growth of acid-adapted and control cells of E. coli O157:H7 inoculated into TSB containing acetic acid (pH 5.4 and 5.7) and citric or malic acids (pH 4.2 and 4.5) was also studied. There was essentially no difference in growth characteristics of the two types of cells in TSB acidified at the same pH with a given acid. Tolerance of acid-adapted and control cells on subsequent exposure to low pH is influenced by the type of acidulant. The order of sensitivity at a given pH is acetic > citric > malic acid. When performing acid challenge studies to determine survival and growth characteristics of E. coli O157:H7 in foods, consideration should be given to the type of acid to which cells have been exposed previously, the procedure used to achieve acidic environments and possible differences in response among strains. The use of strains less affected by pH than type of acidulant or vice versa could result in an underestimation of the potential for survival and growth of E. coli O157:H7 in acid foods.     

A chromogenic plating medium for isolating Escherichia coli O157:H7 from beef

There was no significant difference (P > 0.05) in the percentages of Escherichia coli O157:H7 cells recovered on BCM O157:H7 (+) agar (69.7%) and MacConkey sorbitol agar containing 5-bromo-4-chloro-3-indoxyl-beta-D-glucuronic acid (MSA-BCIG) (76.8%) vs Tryptic soy agar. Three E. coli O157:H7 strains (ATCC 35150, 43890 and 43894) were separately inoculated into raw ground beef at low (mean 0.32 cfu g-1) and high (mean 3.12 cfu g-1) levels. Using the United States Department of Agriculture (USDA) m-EC + novobiocin enrichment broth, BCM O157:H7 (+) medium surpassed MSA-BCIG agar with overall percentage sensitivities for BCM O157:H7 (+) of 92.1 and 94.4 compared with 52.6 and 84.7 for MSA-BCIG at low and high levels, respectively. A comparison of BCM O157:H7 (+) and MSA-BCIG agars using naturally contaminated beef samples was made utilizing presumptively positive enrichment broths previously identified by rapid methods. The E. coli O157:H7 cells in these broths were concentrated with Dynabeads anti-E. coli O157 before inoculating the agars. The respective percentage sensitivity and specificity values were 90.0 and 78.5 for BCM O157:H7 (+) and 70.0 and 46.4 for MSA-BCIG. Thus, under identical pre-plating conditions, BCM O157:H7 (+) medium displayed a greater sensitivity than MSA-BCIG for detecting E. coli O157:H7 in artificially inoculated beef, and both greater sensitivity and specificity upon examining naturally contaminated beef samples.     

Exposure to non-acid fresh meat decontamination washing fluids sensitizes Escherichia coli O157:H7 to organic acids

AIMS: To investigate whether Escherichia coli O157:H7 maintains acid tolerance in water meat decontamination washing fluids. METHODS AND RESULTS: A rifampicin-resistant derivative of E. coli O157:H7 strain ATCC 43895 was inoculated (10(5) cfu ml(-1)) in spray-washings from meat sprayed with cold (10 degrees C) or hot (85 degrees C) water, stored at 10 degrees C for up to 14 days, and its acid tolerance was assessed at 2 and 8 days by exposure to broth or new washings adjusted to pH 3.5 or 3.7 with lactic or acetic acid. The pathogen survived in the water washings, but it was outgrown by the natural, Pseudomonas-like flora, and it was sensitized to acid. CONCLUSIONS: The acid tolerance of E. coli O157:H7 decreases following exposure to non-acid, but otherwise stressful, conditions prevailing in water meat washings at 10 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings suggest that the more intense use of water-based technologies should be included in meat decontamination strategies because they may contribute to enhanced meat safety by inducing acid sensitization in E. coli O157:H7.     

rpoS regulation of acid, heat, and salt tolerance in Escherichia coli O157:H7.

An rpoS mutant (rpoS::pRR10) of Escherichia coli O157:H7 ATCC 43895 was generated. Stationary-phase acid, heat, and salt tolerance was significantly reduced, and starvation-induced acid tolerance did not develop in the mutant. RpoS was also important for survival of E. coli O157:H7 in dry, fermented sausage.     

Adaptation of Escherichia coli O157:H7 to pH alters membrane lipid composition, verotoxin secretion, and resistance to simulated gastric fluid acid

The influence of adaptation to pH (from pH 5.0 to 9.0) on membrane lipid composition, verotoxin concentration, and resistance to acidic conditions in simulated gastric fluid (SGF) (pH 1.5, 37 degrees C) was determined for Escherichia coli O157:H7 (HEC, ATCC 43895), an rpoS-deficient mutant of ATCC 43895 (HEC-RM, FRIK 816-3), and nonpathogenic E. coli (NPEC, ATCC 25922). Regardless of the strain, D values (in SGF) of acid-adapted cells were higher than those of non-acid-adapted cells, with HEC adapted at pH 5.0 having the greatest D value, i.e., 25.6 min. Acid adaptation increased the amounts of palmitic acid (C16:0) and decreased cis-vaccenic acid (C18:1 omega 7c) in the membrane lipids of all strains. The ratio of cis-vaccenic acid to palmitic acid increased at acidic pH, causing a decrease in membrane fluidity. HEC adapted to pH 8.3 and HEC-RM adapted to pH 7.3 exhibited the greatest verotoxin concentrations (2,470 and 1,460 ng/ml, respectively) at approximately 10(8) CFU/ml. In addition, the ratio of extracellular to intracellular verotoxin concentration decreased at acidic pH, possibly due to the decrease of membrane fluidity. These results suggest that while the rpoS gene does not influence acid resistance in acid-adapted cells it does confer decreased membrane fluidity, which may increase acid resistance and decrease verotoxin secretion.     

Biofilm formation by Escherichia coli O157:H7 on stainless steel: effect of exopolysaccharide and Curli production on its resistance to chlorine

The resistance of Escherichia coli O157:H7 strains ATCC 43895-, 43895-EPS (an exopolysaccharide [EPS]-overproducing mutant), and ATCC 43895+ (a curli-producing mutant) to chlorine, a sanitizer commonly used in the food industry, was studied. Planktonic cells of strains 43895-EPS and/or ATCC 43895+ grown under conditions supporting EPS and curli production, respectively, showed the highest resistance to chlorine, indicating that EPS and curli afford protection. Planktonic cells (ca. 9 log(10) CFU/ml) of all strains, however, were killed within 10 min by treatment with 50 microg of chlorine/ml. Significantly lower numbers of strain 43895-EPS, compared to those of strain ATCC 43895-, attached to stainless steel coupons, but the growth rate of strain 43895-EPS on coupons was not significantly different from that of strain ATCC 43895-, indicating that EPS production did not affect cell growth during biofilm formation. Curli production did not affect the initial attachment of cells to coupons but did enhance biofilm production. The resistance of E. coli O157:H7 to chlorine increased significantly as cells formed biofilm on coupons; strain ATCC 43895+ was the most resistant. Population sizes of strains ATCC 43895+ and ATCC 43895- in biofilm formed at 12 degrees C were not significantly different, but cells of strain ATCC 43895+ showed significantly higher resistance than did cells of strain ATCC 43895-. These observations support the hypothesis that the production of EPS and curli increase the resistance of E. coli O157:H7 to chlorine.     

Influence of Apple Cultivars on Inactivation of Different Strains of Escherichia coli O157:H7 in Apple Cider by UV Irradiation

This study examined the effect of different apple cultivars upon the UV inactivation of Escherichia coli O157:H7 strains within unfiltered apple cider. Apple cider was prepared from eight different apple cultivars, inoculated with approximately 10⁶ to 10⁷ CFU of three strains of E. coli O157:H7 per ml (933, ATCC 43889, and ATCC 43895), and exposed to 14 mJ of UV irradiation per cm². Bacterial populations for treated and untreated samples were then enumerated by using nonselective media. E. coli O157:H7 ATCC 43889 showed the most sensitivity to this disinfection process with an average 6.63-log reduction compared to an average log reduction of 5.93 for both strains 933 and ATCC 43895. The highest log reduction seen, 7.19, occurred for strain ATCC 43889 in Rome cider. The same cider produced the lowest log reductions: 5.33 and 5.25 for strains 933 and ATCC 43895, respectively. Among the apple cultivars, an average log reduction range of 5.78 (Red Delicious) to 6.74 (Empire) was observed, with two statistically significant (α ≤ 0.05) log reduction groups represented. Within the paired cultivar-strain analysis, five of eight ciders showed statistically significant (α ≤ 0.05) differences in at least two of the E. coli strains used. Comparison of log reductions among the E. coli strains to the cider parameters of °Brix, pH, and malic acid content failed to show any statistically significant relationship (R² ≥ 0.95). However, the results of this study indicate that regardless of the apple cultivar used, a minimum 5-log reduction is achieved for all of the strains of E. coli O157:H7 tested.     

Heat adaptation alters Escherichia coli O157:H7 membrane lipid composition and verotoxin production

The influence of heat adaptation (growth at 42 and 45 degrees C) on changes in membrane lipid composition and verotoxin concentration of Escherichia coli O157:H7 (ATCC 43895), an rpoS mutant of ATCC 43895 (FRIK 816-3), a verotoxin mutant E. coli O157:H7 (B6-914), and nonpathogenic E. coli (ATCC 25922) was investigated. D values (57 degrees C) of heat-adapted cells were up to 3.9 min longer than those of control cells for all four strains. Heat adaptation increased the amounts of palmitic acid (16:0) and cis-vaccenic acid (18:1omega7c) in membrane lipids of ATCC 43895 and the rpoS mutant, whereas there was a reduction and no change in the amount of cis-vaccenic acid in nonpathogenic and verotoxin mutant E. coli, respectively. The ratio of palmitic to cis-vaccenic acids decreased in ATCC 43895 and in the rpoS mutant, whereas the ratio increased in nonpathogenic E. coli and was not different in the verotoxin mutant with elevated growth temperature. Total verotoxin concentration decreased due to a reduction in intracellular verotoxin amount in heat-adapted ATCC 43895 and rpoS mutant strains. However, extracellular verotoxin concentration increased in heat-adapted cells. The rpoS gene did not influence membrane lipid composition changes although it did affect heat resistance. Results suggest that increased membrane fluidity may have caused increased verotoxin secretion.     

IMPROVEMENT OF SELECTIVITY FOR ISOLATION OF ESCHERICHIA COLI O157:H7 FROM GENERIC ESCHERICHIA COLI

A modified selective medium was developed to increase selectivity for isolation of Escherichia coli O157 from generic E. coli based on the knowledge that E. coli O157:H7 has more resistance against HCl condition than E. coli. As a preliminary experiment, four strains of E. coli O157:H7 (ATCC 35150, 43889, 43890, and 43894) were tested to determine the maximum concentration of 6N HCl (from 0 to 250 μL) added to 50 mL of MacConkey agar medium (MAC). The maximum level was 125 μL/50 mL (6N HCl/MAC), which E. coli O157:H7 strains could tolerate against the HCl concentration. After determination, comparative growth of 15 isolates of E. coli O157 and generic E. coli were evaluated on modified selective medium (HCl-MAC; with the addition of 125 μL/50 mL) and conventional MAC, respectively. All tested strains of E. coli O157 were grown on both media, whereas 9 out of 15 generic E. coli (60% of tested strains) were strongly inhibited on HCl-MAC. For selective isolation of E. coli O157 from generic E. coli, HCl-MAC has an effective potential for an implemental use. This information can extend as a baseline for use of HCl to conventional medium for successful isolation E. coli O157 from generic E. coli.     

Survival or Growth of Escherichia coli O157:H7 in a Model System of Fresh Meat Decontamination Runoff Waste Fluids and Its Resistance to Subsequent Lactic Acid Stress

A potential may exist for survival of and resistance development by Escherichia coli O157:H7 in environmental niches of meat plants applying carcass decontamination interventions. This study evaluated (i) survival or growth of acid-adapted and nonadapted E. coli O157:H7 strain ATCC 43895 in acetic acid (pH 3.6 ± 0.1) or in water (pH 7.2 ± 0.2) fresh beef decontamination runoff fluids (washings) stored at 4, 10, 15, or 25°C and (ii) resistance of cells recovered from the washings after 2 or 7 days of storage to a subsequent lactic acid (pH 3.5) stress. Corresponding cultures in sterile saline or in heat-sterilized water washings were used as controls. In acetic acid washings, acid-adapted cultures survived better than nonadapted cultures, with survival being greatest at 4°C and lowest at 25°C. The pathogen survived without growth in water washings at 4 and 10°C, while it grew by 0.8 to 2.7 log cycles at 15 and 25°C, and more in the absence of natural flora. E. coli O157:H7 cells habituated without growth in water washings at 4 or 10°C were the most sensitive to pH 3.5, while cells grown in water washings at 15 or 25°C were relatively the most resistant, irrespective of previous acid adaptation. Resistance to pH 3.5 of E. coli O157:H7 cells habituated in acetic acid washings for 7 days increased in the order 15°C > 10°C > 4°C, while at 25°C cells died off. These results indicate that growth inhibition by storage at low temperatures may be more important than competition by natural flora in inducing acid sensitization of E. coli O157:H7 in fresh meat environments. At ambient temperatures in meat plants, E. coli O157:H7 may grow to restore acid resistance, unless acid interventions are applied to inhibit growth and minimize survival of the pathogen. Acid-habituated E. coli O157:H7 at 10 to 15°C may maintain a higher acid resistance than when acid habituated at 4°C. These responses should be evaluated with fresh meat and may be useful for the optimization of decontamination programs and postdecontamination conditions of meat handling.     

Influence of the plant defense response to Escherichia coli O157:H7 cell surface structures on survival of that enteric pathogen on plant surfaces

Consumption of fresh and fresh-cut fruits and vegetables contaminated with Escherichia coli O157:H7 has resulted in hundreds of cases of illness and, in some instances, death. In this study, the influence of cell surface structures of E. coli O157:H7, such as flagella, curli fimbriae, lipopolysaccharides, or exopolysaccharides, on plant defense responses and on survival or colonization on the plant was investigated. The population of the E. coli O157:H7 ATCC 43895 wild-type strain was significantly lower on wild-type Arabidopsis plants than that of the 43895 flagellum-deficient mutant. The population of the E. coli O157:H7 43895 flagellum mutant was greater on both wild-type and npr1-1 mutant (nonexpressor of pathogenesis-related [PR] genes) plants and resulted in less PR gene induction, estimated based on a weak β-glucuronidase (GUS) signal, than did the 43895 wild-type strain. These results suggest that the flagella, among the other pathogen-associated molecular patterns (PAMPs), made a substantial contribution to the induction of plant defense response and contributed to the decreased numbers of the E. coli O157:H7 ATCC 43895 wild-type strain on the wild-type Arabidopsis plant. A curli-deficient E. coli O157:H7 86-24 strain survived better on wild-type Arabidopsis plants than the curli-producing wild-type 86-24 strain did. The curli-deficient E. coli O157:H7 86-24 strain exhibited a GUS signal at a level substantially lower than that of the curli-producing wild-type strain. Curli were recognized by plant defense systems, consequently affecting bacterial survival. The cell surface structures of E. coli O157:H7 have a significant impact on the induction of differential plant defense responses, thereby impacting persistence or survival of the pathogen on plants.     

Examination of recovery in vitro and in vivo of nonculturable Escherichia coli O157:H7

Escherichia coli O157:H7 (strains ATCC 43895 and FO46) became nonculturable in sterile, distilled, deionized water or after exposure to chlorine. Recovery of nonculturable E. coli O157:H7 was examined by in vitro and in vivo methods. The decline in culturability of starved E. coli O157:H7 was measured by plate count on rich medium. Recovery in vitro of nonculturable cells was conducted with media amended with catalase or sodium pyruvate; however, there was no apparent increase over culturable cell counts on amended versus nonamended media. Although nonculturable E. coli O157:H7 did not recover under in vitro conditions, a mouse model was used to determine if in vivo conditions would provide sufficient conditions for recovery of nonculturable E. coli O157:H7. In separate studies, mice were orally challenged with starvation-induced nonculturable cells (FO46) or chlorine-induced nonculturable cells (43895 and FO46). Passage through the mouse gastrointestinal tract had no effect on recovery of nonculturable (starvation or chlorine induced) E. coli O157:H7 (43895 or FO46), based on analysis of fecal samples. Mouse kidneys were assayed for the presence of Shiga toxin using the Vero cell assay. Differences in cytotoxicity towards Vero cells from kidney samples of mice receiving nonculturable cells and control mice were not significant, suggesting a loss of virulence.     

The prevalence and spread of Escherichia coli O157:H7 at a commercial beef abattoir

AIMS: To investigate the prevalence and virulence characteristics of Escherichia coli O157:H7 after a number of beef process operations at a commercial Irish abattoir. METHODS AND RESULTS: Two 12-month studies were carried out. The first study (study 1) examined the prevalence of E. coli O157:H7 at up to six sites on carcasses at eight stages of the dressing, washing, chilling and boning process. The second study (study 2) examined the prevalence of E. coli O157:H7 in bovine faeces and rumen contents post-slaughter and on dressed, washed carcasses. Isolates from both studies were phage-typed and the presence of genes encoding verocytotoxin, enterohaemolysin and intimin production was determined. E. coli O157:H7 was isolated from four of 36 carcasses in study 1. E. coli O157:H7 was detected during hide removal and was detected at multiple carcass sites and multiple process stages, including boning. On two carcasses, contamination was first detected at the bung following its freeing and tying. All isolates from study 1 were phage type (PT) 2, eaeAO157 and ehlyA positive, but were verocytotoxin 1 (VT1) and verocytotoxin 2 (VT2) negative. In study 2, E. coli O157:H7 was isolated from 2.4% of faecal, 0.8% of rumen and 3.2% of carcass samples. In some cases, isolates recovered from the faeces of a particular animal, the resulting carcass and adjacent carcasses on the line had the same phage typing and virulence characteristic profile patterns. All isolates from study 2 were eaeAO157 and ehlyA positive and only one isolate was VT1 and VT2 negative. Most isolates were PT 32. A higher frequency of positive isolations was noted from samples taken during spring and late summer. CONCLUSION: These studies show that in a typical Irish beef abattoir, carcass contamination with E. coli O157:H7 can occur during hide removal and bung tying and this contamination can remain on the carcass during subsequent processing. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides data that is necessary for the understanding of how E. coli O157:H7 contamination of beef occurs.     

Thermal tolerance of acid-adapted and unadapted Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes in cantaloupe juice and watermelon juice

AIMS: A study was performed to determine D values of acid-adapted and unadapted cells of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes in cantaloupe juice and watermelon juice. METHODS AND RESULTS: Salmonella enterica serotype Poona, S. enterica serotype Saphra, two strains of E. coli O157:H7, and two strains of L. monocytogenes were grown in tryptic soy broth (TSB) and TSB supplemented with 1% glucose for 24 h at 37 degrees C. Decimal reduction times (D values) of cells suspended in unpasteurized cantaloupe juice and watermelon juice were determined. Acid-adapted cells of Salmonella and E. coli O157:H7, but not L. monocytogenes, had increased thermal tolerance compared with cells that were not acid-adapted. There was no correlation between soluble solids content of the two types of juice and thermal resistance. CONCLUSIONS: Growth of Salmonella and E. coli O157:H7 in cantaloupe juice, watermelon juice, or other acidic milieu, either in preharvest or postharvest environments, may result in cross protection to heat. The pasteurization conditions necessary to achieve elimination of pathogens from these juices would consequently have to be more severe if cells are habituated to acidic environments. SIGNIFICANCE AND IMPACT OF THE STUDY: Insights from this study provide guidance to developing pasteurization processes to eliminate Salmonella, E. coli O157:H7, and L. monocytogenes in cantaloupe juice and watermelon juice.     

Survival of Escherichia coli O157:H7 ATCC 43895 in a model apple juice medium with different concentrations of proline and caffeic acid

The effects of proline and caffeic acid on the survival of Shiga toxin-producing Escherichia coli (STEC) O157:H7 strain ATCC 43895 in a model apple juice medium were studied. It is hypothesized that the inhibitory effect of caffeic acid may explain why almost all outbreaks of STEC O157:H7 infections linked to apple juice or cider have occurred in October or November.     

Biofilm Formation by Escherichia coli O157:H7 on Stainless Steel: Effect of Exopolysaccharide and Curli Production on Its Resistance to Chlorine

The resistance of Escherichia coli O157:H7 strains ATCC 43895-, 43895-EPS (an exopolysaccharide [EPS]-overproducing mutant), and ATCC 43895+ (a curli-producing mutant) to chlorine, a sanitizer commonly used in the food industry, was studied. Planktonic cells of strains 43895-EPS and/or ATCC 43895+ grown under conditions supporting EPS and curli production, respectively, showed the highest resistance to chlorine, indicating that EPS and curli afford protection. Planktonic cells (ca. 9 log₁₀ CFU/ml) of all strains, however, were killed within 10 min by treatment with 50 μg of chlorine/ml. Significantly lower numbers of strain 43895-EPS, compared to those of strain ATCC 43895-, attached to stainless steel coupons, but the growth rate of strain 43895-EPS on coupons was not significantly different from that of strain ATCC 43895-, indicating that EPS production did not affect cell growth during biofilm formation. Curli production did not affect the initial attachment of cells to coupons but did enhance biofilm production. The resistance of E. coli O157:H7 to chlorine increased significantly as cells formed biofilm on coupons; strain ATCC 43895+ was the most resistant. Population sizes of strains ATCC 43895+ and ATCC 43895- in biofilm formed at 12°C were not significantly different, but cells of strain ATCC 43895+ showed significantly higher resistance than did cells of strain ATCC 43895-. These observations support the hypothesis that the production of EPS and curli increase the resistance of E. coli O157:H7 to chlorine.     

Occurrence of Escherichia coli O157:H7 in faeces, skin and carcasses from sheep and goats in Ethiopia

Aims:  To determine the occurrence and proportion of Escherichia coli O157:H7 in faeces, skin swabs and carcasses before and after washing, from sheep and goats in Ethiopia.
Method and Results:  Individual samples were enriched in modified tryptic soy broth with novobiocin, concentrated using immunomagnetic separation (IMS) and plated onto cefixime-tellurite containing sorbitol MacConkey agar. Presumptive colonies were confirmed by biochemical tests and subjected to latex agglutination tests. A PCR was performed on isolates for the detection of stx ₁, stx ₂ and eae genes. Escherichia coli O157:H7 was isolated from faeces (4·7%), skin swabs (8·7%) and carcasses before washing (8·1%) and after washing (8·7%) and on water samples (4·2%). The proportion of carcasses contaminated with E. coli O157:H7 was strongly associated with those recovered from faecal and skin samples. Both stx ₁ and stx ₂ genes were identified from one E. coli O157:H7 isolate from a goat carcass.
Conclusions:  Even though the numbers of samples examined in this study were limited to one abattoir, sheep and goats can be potential sources of E. coli O157:H7 for human infection in the country. Control measures to reduce the public health risks arising from E.   coli O157:H7 in reservoir animals need to be addressed at abattoir levels by reducing skin and faecal sources and carcass contaminations at different stages of slaughter operations.
Significance and Impact of the Study:  Escherichia coli O157:H7 was detected from carcasses before and after washing during slaughtering operations, and one O157 isolate was positive for verotoxins.