Short communication: survival of Escherichia coli O157:H7 in dairy cattle drinking water

Cattle drinking water from two dairy farms was used in a study to determine the survival characteristics of the bacterial pathogen Escherichia coli O157:H7 and wild-type E. coli. The E. coli O157:H7 inoculum consisted of a consortium of isolates obtained from dairy cattle. Fresh manure was used as the source for the wild-type E. coli. In the water source from farm 1 the pathogens were present at both 5 and 15 degrees C during the 16-d duration of the study. In the water source from farm 2, the pathogens were detected at 5 degrees C through d 8 and through d 4 at 15 degrees C. The fecal indicator, wild-type E. coli, was always present when the pathogens were present.     

Chlorine inactivation of Escherichia coli O157:H7 in water

Six human isolates of Escherichia coli O157:H7 and E. coli (ATCC 11229) were used to determine the concentrations of free chlorine and exposure times required for inactivation. Free chlorine concentrations of 0.25, 0.5, 1.0, and 2.0 ppm at 23 degrees C were evaluated, with sampling times at 0, 0.5, 1.0, and 2.0 min. Results revealed that five of six E. coli O157:H7 isolates and the E. coli control strain were highly susceptible to chlorine, with >7 log10 CFU/ml reduction of each of these strains by 0.25 ppm free chlorine within 1 min. However, comparatively, one of the seven strains was unusually tolerant to chlorine at 23 degrees C for 1 min, with a 4-, 5.5-, 5.8-, and >5.8-log CFU/ml reduction at free chlorine concentrations (ppm) of 0.25, 0.5, 1.0, and 2.0. respectively. Based on these studies most isolates of E. coli O157:H7 have no unusual tolerance to chlorine; however, one strain was exceptional in being recovered after 1-min of exposure of 10(7) CFU/ml to 2.0 ppm of free chlorine. This isolate may be a useful reference strain for future studies on chlorine tolerance of E. coli O157:H7.     

Frozen storage of Escherichia coli O157 in buffered peptone water and its detection on bovine carcasses

The adaptation of a standard Escherichia coli O157 isolation method involving immunomagnetic separation and a period of frozen storage was investigated. A series of experiments was designed to test the recovery of a bovine strain of E. coli O157 from buffered peptone water after a period of frozen storage at -80 degrees C. The effects of the addition of glycerol at 5 and 10%, freezing time, the number of freeze-thaw cycles, the method of freezing and the method of thawing, the inclusion of a resuscitation-and-incubation step, and the sensitivity of the isolation method were investigated. The most effective method of storing frozen samples for 6 months and recovering strains of E. coli O157 after storage was found to involve 6 h of incubation of sample material in buffered peptone water at 37 degrees C before frozen storage at -80 degrees C with 10% glycerol, a rapid thaw after frozen storage, and resuscitation at 27 degrees C for 1 h and incubation at 37 degrees C for 1 h to allow freeze-injured and stressed bacteria to recover with a period of growth prior to immunomagnetic separation isolation. There was no significant decrease in log counts of a bovine strain E. coli O157 over 6 months of frozen storage in buffered peptone water with 10% glycerol. With this method, it was possible to isolate E. coli O157 from naturally infected bovine carcasses after a period of frozen storage.     

Escherichia coli O157:H7 and its significance in foods

Escherichia coli O157:H7 was conclusively identified as a pathogen in 1982 following its association with two food-related outbreaks of an unusual gastrointestinal illness. The organism is now recognized as an important cause of foodborne disease, with outbreaks reported in the U.S.A., Canada, and the United Kingdom. Illness is generally quite severe, and can include three different syndromes, i.e., hemorrhagic colitis, hemolytic uremic syndrome, and thrombotic thrombocytopenic purpura. Most outbreaks have been associated with eating undercooked ground beef or, less frequently, drinking raw milk. Surveys of retail raw meats and poultry revealed E. coli O157:H7 in 1.5 to 3.5% of ground beef, pork, poultry, and lamb. Dairy cattle, especially young animals, have been identified as a reservoir. The organism is typical of most E. coli, but does possess distinguishing characteristics. For example, E. coli O157:H7 does not ferment sorbitol within 24 h, does not possess beta-glucuronidase activity, and does not grow well or at all at 44-45.5 degrees C. The organism has no unusual heat resistance; heating ground beef sufficiently to kill typical strains of salmonellae will also kill E. coli O157:H7. The mechanism of pathogenicity has not been fully elucidated, but clinical isolates produce one or more verotoxins which are believed to be important virulence factors. Little is known about the significance of pre-formed verotoxins in foods. The use of proper hygienic practices in handling foods of animal origin and proper heating of such foods before consumption are important control measures for the prevention of E. coli O157:H7 infections.     

Polymerase chain reaction assay for detection of Escherichia coli O157: H7 and Escherichia coli O157: H-.

The DNA band patterns generated by polymerase chain reaction (PCR) using the du2 primer and template DNAs from various strains of Escherichia coli and non-E. coli bacteria were compared. Among three to five prominent bands produced, the three bands at about 1.8, 2.7, and 5.0 kb were detected in all of the E. coli O157 strains tested. Some nonpathogenic E. coli and all pathogenic E. coli except E. coli O157 showed bands at 1.8 and 5.0 kb. It seems that the band at 2.7 kb is specific to E. coli O157. Sequence analysis of the 2.7-kb PCR product revealed the presence of a DNA sequence specific to E. coli O157:H- and E. coli O157:H7. Since the DNA sequence from base 15 to base 1,008 of the PCR product seems to be specific to E. coli O157, a PCR assay was carried out with various bacterial genomic DNAs and O157-FHC1 and O157-FHC2 primers that amplified the region between base 23 and base 994 of the 2.7-kb PCR product. A single band at 970 bp was clearly detected in all of the strains of E. coli O157:H- and E. coli O157:H7 tested. However, no band was amplified from template DNAs from other bacteria, including both nonpathogenic and pathogenic E. coli except E. coli O157. All raw meats inoculated with E. coli O157:H7 at 3 x 10(0) to 3.5 x 10(2) CFU/25 g were positive both for our PCR assay after cultivation in mEC-N broth at 42 degrees C for 18 h and for the conventional cultural method.     

Survival of Escherichia coli O157:H7 and Campylobacter jejuni in bottled purified drinking water under different storage conditions

Survival of Escherichia coli O157:H7 and Campylobacter jejuni that were separately inoculated into bottled purified drinking water was investigated during storage at 22, 4, and -18 °C for 5, 7, and 2 days, respectively. Two inoculation levels were used, 1 and 10 CFU/ml (10(2) and 10(3) CFU/100 ml). In samples inoculated with 10(2) CFU/100 ml, C. jejuni was not detectable (>2-log reduction) after storage under the conditions specified above. E. coli O157:H7 was detected on nonselective and selective media at log reductions of 1.08 to 1.25 after storage at 22 °C, 1.19 to 1.56 after storage at 4 °C, and 1.54 to 1.98 after storage at -18 °C. When the higher inoculation level of 10(3) CFU/100 ml was used, C. jejuni was able to survive at 22 and 4 °C, with 2.25- and 2.17-log reductions, respectively, observed on nonselective media. At these higher inoculation levels, E. coli O157:H7 was detectable at 22, 4, and -18 °C, with log reductions of 0.76, 0.97, and 1.21, respectively, achieved on nonselective media. Additionally, E. coli O157:H7 showed significant differences in culturability (P<0.05) on the nonselective and selective culture media under the different storage conditions, with storage at -18 °C for 2 days being the treatment most inhibiting. The percentage of sublethal injury of E. coli O157:H7 ranged from ～33 to 75%, indicating that microbial examination of bottled water must be done carefully, otherwise false-negative results or underestimation of bacterial numbers could pose a health risk when low levels of pathogens are present.     

The epidemiology,detection and control of Escherichia coli O157

Numbers of cases of human infections caused by Escherichia coli O157 have increased over the last decade in many countries. As well as the typical symptoms of gastrointestinal illness, the organism can cause more life-threatening diseases such as haemorrhagic colitis and haemolytic uraemic syndrome (HUS). Although the incidence remains relatively low compared with the other foodborne pathogens such as Campylobacter and Salmonella, the mortality rate associated with infection is much higher. Cattle are thought to be the main environmental source of this organism, and most cases have been associated with consumption of beef and beef products. However, other food vehicles have been identified, such as apple cider and raw or unpasteurised milk and milk products. Cross-contamination has been shown to be an important factor in outbreaks, which, together with the fact that the infectious dose is low (as few as 10 cells), means that robust hygienic procedures are essential at all stages of the food chain to reduce risk of infection. Person-to-person spread is a common source of illness, and several laboratory-acquired cases have also occurred. Efficient detection, isolation and confirmation techniques are required to establish the reservoirs of this organism in the environment and its spread into, and within, the food chain. This article reviews the epidemiology of E coli O157 and discusses detection and preventative methods, both developed and developing. © 1999 Society of Chemical Industry     

UV inactivation, liquid-holding recovery, and photoreactivation of Escherichia coli O157 and other pathogenic Escherichia coli strains in water

Drinking water, water used in food production and for irrigation, water for fish farming, waste water, surface water, and recreational water have been recently recognized as a vector for the transmission of pathogenic Escherichia coli, especially serotype O157:H7. We investigated the UV (253.7 nm) inactivation behavior and the capability of dark repair (liquid-holding recovery) and photoreactivation of seven pathogenic (including three enterohemorrhagic E. coli) strains and one nonpathogenic strain of E. coli (ATCC 11229) with respect to the use of UV light for water disinfection purposes. Because most bacteria and yeast are known to be able to repair UV damage in their nucleic acids, repair mechanisms have to be considered to ensure safe water disinfection. We found a wide divergence in the UV susceptibility within the strains tested. A 6-log reduction of bacteria that fulfills the requirement for safe water disinfection was reached for the very most susceptible strain O157:H7 (CCUG 29199) at a UV fluence of 12 J/m2, whereas for the most resistant strain, O25:K98:NM, a UV fluence of about 125 J/m2 was needed. Except for one strain (O50:H7) liquid-holding recovery did not play an important role in recovery after UV irradiation. By contrast, all strains, particularly strains O25:K98:NM, O78:K80:H12, and O157:H7 (CCUG 29193), demonstrated photorepair ability. For a 6-log reduction of these strains, a UV fluence (253.7 nm) up to 300 J/m2 is required. The results reveal that the minimum fluence of 400 J/m2 demanded in the Austrian standard for water disinfection is sufficient to inactivate pathogenic E. coli. A fluence of 160 J/m2 (recommendation in Norway) or 250 J/m2 (recommendation in Switzerland) cannot be regarded as safe in that respect.     

Inactivation of escherichia coli O157:H7 in cattle drinking water by sodium caprylate

Escherichia coli O157:H7 is an important foodborne pathogen. Cattle serve as one of the major reservoirs of E. coli O157:H7, excreting the pathogen in feces. Environmental persistence of E. coli O157:H7 is critical in its epidemiology on farms, and the pathogen has been isolated from cattle water troughs. Thus, there is a need for an effective method for killing E. coli O157:H7 in cattle drinking water. In this study, the efficacy of sodium caprylate for killing E. coli O157:H7 in cattle drinking water was investigated. A four-strain mixture of E. coli O157:H7 was inoculated (6.0 log CFU/ml) into 100-ml samples of well water containing 0, 75, 100, or 120 mM sodium caprylate. Water samples containing 1% (wt/vol) bovine feces or feed also were included. The samples were incubated at 21 or 8 degrees C for 21 days. Water samples were analyzed for viable E. coli O157:H7 on days 0, 1, 3, 5, and 7 and weekly thereafter. Triplicate samples of each treatment and control were included, and the study was repeated twice. The magnitude of E. coli O157:H7 inactivation in water significantly increased (P < 0.01) with increases in caprylate concentration and storage temperature. At 120 mM, sodium caprylate completely inactivated E. coli O157:H7 in all the samples after 1 to 20 days, depending on the treatments. The presence of feces or feed also had a significant effect (P < 0.01) on the antibacterial property of caprylate; the presence of feces decreased the antibacterial effect, whereas addition of feed enhanced the effect. These results indicate that sodium caprylate is effective in killing E. coli O157:H7 in cattle drinking water, but detailed cattle palatability studies of water containing caprylate are necessary.     

A study of U.S. orchards to identify potential sources of Escherichia coli O157:H7

The association of unpasteurized apple cider with Escherichia coli O157:H7 foodborne illness has led to increased interest in potential reservoirs of this pathogen in the orchard. Fourteen U.S. orchards were surveyed in autumn 1999 to determine the incidence and prevalence of E. coli O157:H7, E. coli, total aerobic microflora, and yeasts and molds. Fruit samples (n = 63) (eight apple and two pear varieties) and soil, water, and fecal samples were collected. Samples were plated on (i) tryptic soy agar for total mesophilic aerobic count, (ii) E. coli and coliform Petrifilm for total coliforms and E. coli, and (iii) yeast and mold Petrifilm. Samples positive for coliforms and E. coli were enriched and tested for E. coli O157:H7. Fruit was also tested for internalization of microflora by aseptically removing the core, stem, and calyx areas, and the individual sections were assessed for the categories of microflora listed above. E. coli was detected in soil and water and in 6% of fruit samples (three pear samples and one apple sample), generally collected from areas previously designated as high risk in this study. However, no E. coli O157:H7 was found. Coliforms were found in 74% of fruit samples and were internalized in the cores of 40% of fruit tested. Yeasts and molds were internalized in 96.7% of samples and aerobic bacteria in 89.6%. E. coli was not found to be internalized. Total aerobic counts and total coliforms were higher in dropped and damaged fruit (P < 0.05). Findings suggest that dropped or damaged fruit should not be included in fruit designated for the production of unpasteurized juice or for the fresh or fresh-cut market. In addition, orchards should be located away from potential sources of contamination, such as pastures.     

Escherichia coli O157 and non-O157 Shiga toxin-producing Escherichia coli in fecal samples of finished pigs at slaughter in Switzerland

Fecal samples from 630 slaughtered finisher pigs were examined by PCR to assess the shedding of Escherichia coli O157 (rfbE) and Shiga toxin-producing E. coli (STEC, stx). The proportion of positive samples was 7.5% for rfbE and 22% for stx. By colony hybridization, 31 E. coli O157 and 45 STEC strains were isolated, and these strains were further characterized by phenotypic and genotypic traits. Among E. coli O157 strains, 30 were sorbitol positive, 30 had an H type other than H7, and none harbored stx genes. Intimin (eae), enterohemolysin (ehxA), EAST1 (astA), and porcine A/E-associated protein (paa) were present in 10, 3, 26, and 6% of strains. Among them, one eae-gamma1-positive O157:H7 strain testing positive for ehxA and astA and two eae-alpha1-positive O157:H45 strains were classified as enteropathogenic E. coli (EPEC). The O157:H45 EPEC harbored the EAF plasmid and the bfpA gene, factors characteristic for typical EPEC. The isolated STEC strains (43 sorbitol positive) belonged to 11 O:H serotypes, including three previously reported in human STEC causing hemolytic uremic syndrome (O9:H-, O26:H-, and O103:H2). All but one strain harbored stx2e. The eae and ehxA genes, which are strongly correlated with human disease, were present in only one O103:H2 strain positive for stx1 and paa, whereas the astA gene was found more frequently (14 strains). High prevalence of STEC was found among finisher pigs, but according to the virulence factors the majority of these strains seem to be of low virulence.     

Hot water treatments to inactivate Escherichia coli O157:H7 and Salmonella in mung bean seeds

The majority of the seed sprout-related outbreaks have been associated with Escherichia coli O157:H7 and Salmonella. Therefore, an effective method is needed to inactivate these organisms on the seeds before they are sprouted. This study was conducted to assess the effectiveness of various hot water treatments to inactivate E. coli O157:H7 and Salmonella populations on mung beans seeds intended for sprout production and to determine the effect of these treatments on seed germination after the seeds were dipped in chilled water for 30 s. Mung bean seed inoculated with four-strain cocktails of E. coli O157:H7 and Salmonella were soaked into hot water at 80 and 90 degrees C with shaking for various periods and then dipped in chilled water for 30 s. The treated seeds were then assessed for the efficacy of the treatment for reducing populations of the pathogens and the effects of the treatment on germination. After inoculation and air drying, 6.08 +/- 0.34 log CFU/g E. coli O157:H7 and 5.34 +/- 0.29 log CFU/g Salmonella were detected on the seeds. After hot water treatment at 90 degrees C for 90 s followed by dipping in chilled water for 30 s, no viable pathogens were found and no survivors were found in the enrichment medium and during the sprouting process. The germination yield of the seed was not affected significantly. Therefore, hot water treatment followed by dipping in chilled water for 30 s could be an effective seed decontamination method for mung bean seeds intended for sprout production.     

Antimicrobial activity of nutmeg against Escherichia coli O157

We examined the difference between Escherichia coli O157 and non-pathogenic E. coli in their tolerance to spices. Various spices (5 g each) were homogenized at 25 degrees C for 10 min with 5 ml of 70% ethyl alcohol, and the supernatant solutions obtained by centrifugation were used as spice extracts. When the E. coli strains were incubated with each spice extract at concentrations of 0.01% and 0.1%, a noteworthy difference was observed between the O157 and non-pathogenic strains in their tolerance to nutmeg. The populations of the non-pathogenic strains could not be reduced, but those of the O157 strains were remarkably reduced. Antibacterial activity by the nutmeg extract was also found against the enteropathogenic E. coli O111, but not against enterotoxigenic (O6 and O148) and enteroinvasive (O29 and O124) E. coli. When we examined the antibacterial effect of volatile oils in nutmeg on the O157 and non-pathogenic E. coli strains, all O157 strains tested were found to be more sensitive to beta-pinene than non-pathogenic E. coli strains.     

蔬菜中大肠杆菌O157的分离与鉴定

对武汉市售蔬菜(50份)进行大肠杆菌O157的检验,经过新生霉素-EC增菌液增菌、免疫磁珠富集、选择性平板培养和血清学鉴定,从一份生菜中筛出1株O157阳性菌株EC9.23。PCR鉴定该菌毒力基因,stx1、stx2、rfbO157基因均为阳性,hly、eae、fliCH7基因均为阴性。说明从武汉市售蔬菜中可检出携带志贺毒素基因的O157菌株。     

Stationary-phase acid resistance and injury of recent bovine Escherichia coli O157 and non-O157 biotype I Escherichia coli isolatest

Stationary-phase acid resistance and the induction of acid resistance were assessed for recent bovine carcass isolates of Escherichia coli, including 39 serotype O157 strains and 20 non-O157 strains. When grown to stationary phase in the absence of glucose and without prior acid exposure, there was a range of responses to a pH challenge of 6 h at pH 2.5. However, populations of 53 of the 59 E. coli isolates examined were reduced by less than 2.00 log CFU/ml, and populations of 24 of these isolates were reduced by less than 1.00 log CFU/ml. In contrast, there was little variation in population reductions when the E. coli were grown with glucose and preadapted to acidic conditions. With few exceptions, acid adaptation improved survival to the acid challenge, with 57 of the 59 isolates exhibiting a log reduction of less than 0.50. Differences in acid resistance or the ability to adapt to acidic conditions between E. coli O157:H7 and non-O157 commensal E. coli were not observed. However, we did find that the E. coli O157 were disposed to greater acid injury after the low pH challenge than the non-O157 E. coli, both for cells that were and were not adapted to acidic conditions before the challenge. The enhancement of low pH survival after acid adaptation that was seen among these recent natural isolates of E. coli O157 further supports the idea that the previous environment of this pathogen should be a consideration when designing microbial safety strategies for foods preserved by low pH and acid.     

Development of a medium for differentiation between Escherichia coli and Escherichia coli O157:H7.

A new medium (Escherichia coli O157:H7 medium: EOH) was developed for differentiation between E. coli and E. coli O157:H7. The EOH medium was compared with sorbitol MacConkey agar (SMAC), which is the most popular medium to enumerate E. coli O157:H7. Several combinations of 35 dyes were evaluated to develop the new medium. Indigo carmine (0.03) g/liter) and phenol red (0.036 g/liter) were found as the best combination for differentiation between E. coli O157:H7 and E. coli and added to the basal agar medium (SMAC medium excluding neutral red and crystal violet) for EOH medium. On the dark blue EOH medium, E. coli produced a yellow color with clear zone, whereas E. coli O157:H7 produced a red color without clear zone. For differentiation between E. coli and E. coli O157:H7, EOH has much better potential than SMAC. Furthermore. the red color produced by normal E. coli in SMAC may mask the light gray color produced by E. coli O157: H7, whereas the yellow color with clear zone did not mask the red color without clear zone in the EOH medium. The recovery numbers of E. coli O157:H7 from inoculated ground beef, pork, and turkey were not significantly different between SMAC and EOH media (P > 0.05). The recovery rates of heat- and cold-injured E. coli O157:H7 also were not significantly different (P > 0.05).     

Selection of recently isolated colicinogenic Escherichia coli strains inhibitory to Escherichia coli O157:H7

Escherichia coli strains were screened for their ability to inhibit E. coli O157:H7. An initial evaluation of 18 strains carrying previously characterized colicins determined that only colicin E7 inhibited all of the E. coli O157:H7 strains tested. A total of 540 strains that had recently been isolated from humans and nine different animal species (cats, cattle, chickens, deer, dogs, ducks, horses, pigs, and sheep) were tested by a flip-plating technique. Approximately 38% of these strains were found to inhibit noncolicinogenic E. coli K12 strains. The percentage of potentially colicinogenic E. coli per animal species ranged from 14% for horse isolates to 64% for sheep strains. Those isolates that inhibited E. coli K12 were screened against E. coli O157:H7, and 42 strains were found to be capable of inhibiting all 22 pathogenic strains tested. None of these 42 strains produced bacteriophages, and only 24 isolates inhibited serotype O157:H7 in liquid culture. The inhibitory activity of these strains was completely eliminated by treatment with proteinase K. When mixtures of these 24 colicinogenic strains were grown in anaerobic continuous culture, the four-strain E. coli O157:H7 population was reduced at a rate of 0.25 log10 cells per ml per h, which was fivefold faster than the washout rate. Two strains originally isolated from cat feces (F16) and human feces (H30) were identified by repetitive sequences polymerase chain reaction as the predominant isolates in continuous cultures. The results of this work indicate that animal species other than cattle can be sources of anti-O157 colicinogenic strains, and these results also lead to the identification of at least two isolates that could potentially be used in preharvest control strategies.     

Escherichia coli O157 prevalence and enumeration of aerobic bacteria, Enterobacteriaceae, and Escherichia coli O157 at various steps in commercial beef processing plants

The effectiveness of current antimicrobial interventions used in reducing the prevalence or load of Escherichia coli O157 and indicator organisms on cattle hides and carcasses at two commercial beef processing plants was evaluated. Sponge sampling of beef cattle was performed at five locations from the initial entry of the animals to the slaughter floor to the exit of carcasses from the "hotbox" cooler. For each sample, E. coli O157 prevalence was determined and total aerobic bacteria, Enterobacteriaceae, and E. coli O157 were enumerated. E. coli O157 was found on 76% of animal hides coming into the plants, but no carcasses leaving the cooler were identified as contaminated with E. coli O157. A positive relationship was seen between the incidence of E. coli O157 in hide samples and that in preevisceration samples. Aerobic plate counts and Enterobacteriaceae counts averaged 7.8 and 6.2 log CFU/100 cm2, respectively, on hides, and 1.4 and 0.4 log CFU/100 cm2, respectively, on chilled carcasses. Aerobic plate counts and Enterobacteriaceae counts on preevisceration carcasses were significantly related to the respective levels on the corresponding hides; the carcasses of animals whose hides carried higher numbers of bacteria were more likely to carry higher numbers of bacteria. Implementation of the sampling protocol described here would allow processors to evaluate the efficacy of on-line antimicrobial interventions and allow industrywide benchmarking of hygienic practices.     

Association of Escherichia coli O157:H7 with preharvest leaf lettuce upon exposure to contaminated irrigation water.

Recent foodborne outbreaks have linked infection by enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 to the consumption of contaminated lettuce. Contamination via food handler error and on-the-farm contamination are thought to be responsible for several outbreaks. Though recent studies have examined the application of EHEC to store-bought lettuce, little is known about the attachment of EHEC to growing plants. We investigated the association of lettuce seedlings with EHEC O157:H7 strains implicated in lettuce or fruit outbreaks using hydroponic and soil model systems. EHEC strains that express the green fluorescent protein were observed by stereomicroscopy and confocal laser scanning microscopy to determine adherence patterns on growing lettuce seedlings. Bacteria adhered preferentially to plant roots in both model systems and to seed coats in the hydroponic system. Two of five nonpathogenic E. coli strains showed decreased adherence to seedling roots in the hydroponic system. EHEC was associated with plants in as few as 3 days in soil, and contamination levels were dose-dependent. EHEC levels associated with young plants inoculated with a low dose suggested that the bacteria had multiplied. These data suggest that preharvest crop contamination via contaminated irrigation water can occur through plant roots.     

Comparative die-off of Escherichia coli O157:H7 and fecal indicator bacteria in pond water

In situ and in vitro experiments were performed to assess the effects of solar radiation and predation by indigenous microflora on the relative die-off rates of a toxigenic strain of Escherichia coli O157:H7, commensal E. coli, and fecal enterococci in surface waters from ponds in agricultural watersheds. The objective of these experiments was to discern a mechanism of persistence of E. coli O157:H7 in surface waters compared to fecal indicator bacteria. Results of these experiments indicated that E. coli and fecal enterococci were affected by both insolation and apparent predation; whereas E. coli O157:H7 appeared to be resistant to both of these environmental stressors. The number of days to reach 99% die-off (T(99)-values) for E. coli O157:H7 was significantly greater than that for the indicator bacteria. The capacity to prolong die-off may be connected to the apparent persistence of E. coli O157:H7 in surface waters.