Prevalence of Escherichia coli O157:H7 in industrial minced beef

AIMS: The lack of baseline data on the prevalence of Escherichia coli O157:H7 in retail minced beef in France prompted this survey of industrial minced beef production. METHODS AND RESULTS: An automated enzyme-linked fluorescence immunoassay (ELFA), the VIDAS E. coli O157 method, was used to detect E. coli O157 in industrial minced beef samples. Confirmation of samples positive according to the ELFA was performed using an automated immunoconcentration (ICE) system, VIDAS ICE, which allows the selective capture and release of target organisms. The ICE was followed by culture on cefixime tellurite sorbitol MacConkey agar and a chromogenic medium, O157:H7 ID. Of the 3450 minced beef samples tested, 175 samples were positive with the ELFA method and, of these, four were confirmed by the ICE method. They were identified as sorbitol-negative, O157-positive, H7-positive, mobile, verotoxin-producing E. coli. CONCLUSIONS: The prevalence of E. coli O157:H7 in industrial French minced beef was 0.12%, consistent with many other reports. SIGNIFICANCE AND IMPACT OF THE STUDY: The low infective dose of E. coli O157:H7 presents a major threat. The main means of combating this organism are thermal destruction and good food hygiene covering activities on-farm, in the abattoir and in minced beef industries.     

Prevalence of Escherichia coli O157 and O157:H7-infecting bacteriophages in feedlot cattle feces

AIM: To estimate the distribution and prevalence of both Escherichia coli O157 and O157:H7-infecting bacteriophages within a 50,000 head commercial beef feedlot. METHODS AND RESULTS: Escherichia coli O157 was detected in approximately 27% of the individual samples, distributed across seven of the 10 pens screened. In a simple initial screen to detect O157:H7-infecting phages, none were detected in any pen or individual sample. In contrast, after a series of enrichment procedures O157:H7-infecting phages were detected in every pen and in the majority of the samples from most pens; virulent bacteriophages active against E. coli O157:H7 were detected post-enrichment from 39/60 (65%) of the feedlot samples, and 58/60 (approximately 97%) contained phage that infected E. coli B or O157:H7. CONCLUSIONS: The data we present here indicates that we may be grossly underestimating the prevalence of O157:H7-infecting phages in livestock if we simply screen samples and that enrichment screening is required to truly determine the presence of phages in these ecosystems. SIGNIFICANCE AND IMPACT OF THE STUDY: Our data suggest that O157:H7-infecting phages may play a role in the ecology and transient colonization of cattle by E. coli O157:H7. Further, this and previous data suggest that before starting in vivo pathogen eradication studies using phage or any other regime, test animals should be enrichment screened for phage to avoid erroneous results.     

Escherichia coli O157:H7 in environments of culture-positive cattle

Outbreaks of Escherichia coli O157:H7 disease associated with animal exhibits have been reported with increasing frequency. Transmission can occur through contact with contaminated haircoats, bedding, farm structures, or water. We investigated the distribution and survival of E. coli O157:H7 in the immediate environments of individually housed, experimentally inoculated cattle by systematically culturing feed, bedding, water, haircoat, and feed bunk walls for E. coli O157:H7 for 3 months. Cedar chip bedding was the most frequently culture-positive environmental sample tested (27/96 or 28.15%). Among these, 12 (44.0%) of positive bedding samples were collected when the penned animal was fecal culture negative. Survival of E. coli O157:H7 in experimentally inoculated cedar chip bedding and in grass hay feed was determined at different temperatures. Survival was longest in feed at room temperature (60 days), but bacterial counts decreased over time. The possibility that urine plays a role in the environmental survival of E. coli O157:H7 was investigated. Cedar chip bedding moistened with sterile water or bovine urine was inoculated with E. coli O157:H7. Bedding moistened with urine supported growth of E. coli O157:H7, whereas inoculated bedding moistened with only water yielded decreasing numbers of bacteria over time. The findings that environmental samples were frequently positive for E. coli O157:H7 at times when animals were culture negative and that urine provided a substrate for E. coli O157:H7 growth have implications for understanding the on-farm ecology of this pathogen and for the safety of ruminant animal exhibits, particularly petting zoos and farms where children may enter animal pens.     

Survival of Escherichia coli O157:H7 on cattle hides

The objective of this study was to determine the time period that Escherichia coli O157:H7 survives on the hides of cattle. Extensive research has been conducted and is ongoing to identify and develop novel preharvest intervention strategies to reduce the presence of E. coli O157:H7 on live cattle and subsequent transfer to processed carcasses. If a reduction of E. coli O157:H7 levels in feces can be achieved through preharvest intervention, it is not known how long it would take for such reductions to be seen on the hide. In the study presented herein, three trials were conducted to follow E. coli O157:H7 hide prevalence over time. For each trial, 36 animals were housed in individual stanchions to minimize or prevent hide contamination events. Through prevalence determination and isolate genotyping with pulsed-field gel electrophoresis, survival of E. coli O157:H7 on the hides of live cattle was determined to be short lived, with an approximate duration of 9 days or less. The results of this study suggest that any preharvest interventions that are to be administered at the end of the finishing period will achieve maximum effect in reducing E. coli O157:H7 levels on cattle hides if given 9 days before the cattle are presented for processing. However, it should be noted that interventions reducing pathogen shedding would also contribute to decreasing hide contamination through lowering the contamination load of the processing plant lairage environment, regardless of the time of application.     

Thermal death of Escherichia coli O157:H7 in cattle feeds

AIMS: To determine if the temperatures used in feed manufacture are likely to destroy Escherichia coli O157. METHODS AND RESULTS: Two commercial feeds were ground and inoculated with E. coli O157 cells. The feeds were heated to 50, 55, 60, 65 or 70 degrees C. Heating produced quadratic survivor curves, with rapid initial decreases. The survival characteristics of E. coli O157 differed in the two feeds. The reductions observed in one feed may not have been due to heat alone. There was evidence that indigenous anti-E. coli O157 factor(s) in one feed acted with the heat and contributed to the observed rates of bacterial death. Heating at 70 degrees C for 20 or 120 s resulted in approx. 1.3 and 2.2 log reductions in E. coli O157 numbers respectively. Lesser reductions were observed at lower temperatures. CONCLUSIONS: The time/temperature combinations used in commercial pelleting processes would not effectively kill high numbers of E. coli O157. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first study to look at the survival of E. coli O157 strains after heat treatment within concentrated animal feed. The study provides information on the likely risk of E. coli O157 surviving the animal feed manufacturing process.     

Prevalence of Escherichia coli O157:H7 and Salmonella in beef steers consuming different forage diets

AIMS: To compare the prevalence of faecal shedding of Escherichia coli O157:H7 and Salmonella in growing beef cattle consuming various forages. METHODS AND RESULTS: In Experiment I, faecal samples were collected from steers grazing either endophyte-infected (E+) tall fescue or common bermudagrass (CB). Steers grazing E+ tall fescue were confined to a dry-lot pen and fed CB hay ad libitum for 10 days. In Exp. II, faecal samples were collected from steers grazing either E+ or novel endophyte-infected (NE) tall fescue and treated with one of two anthelmintics: ivermectin (I) or fenbendazole (F). In Exp. I, prevalence of E. coli O157:H7 was less in E+ tall fescue steers fed CB hay than steers grazing CB. More I-treated steers shed Salmonella than F-treated steers at 42-day postanthelmintic treatment but shedding of Salmonella was similar between anthelmintics at day 63 in Exp. II. CONCLUSIONS: Faecal shedding of pathogenic bacteria was not affected by grazing E+ tall fescue. Alterations of forage diets may influence the prevalence of E. coli O157:H7, and anthelmintic treatment could affect faecal shedding of Salmonella. SIGNIFICANCE AND IMPACT OF THE STUDY: Knowledge of factors that influence shedding of pathogenic bacteria in cattle is necessary to develop on-farm intervention strategies aimed at reducing pathogen shedding.     

Prevalence of Escherichia coli O157:H7 in Gallbladders of Beef Cattle

Gallbladders and rectal contents were collected from cattle (n = 933) at slaughter to determine whether the gallbladder harbors Escherichia coli O157:H7. Both gallbladder mucosal swabs and homogenized mucosal tissues were used for isolation. Only five gallbladders (0.54%) were positive for E. coli O157:H7. Fecal prevalence averaged 7.1%; however, none of the cattle that had E. coli O157:H7 in the gallbladder was positive for E. coli O157:H7 in feces. Therefore, the gallbladder does not appear to be a common site of colonization for E. coli O157:H7 in beef cattle.     

Prevalence of Escherichia coli O157:H7 in white-tailed deer from Louisiana

Escherichia coli O157:H7 (EC O157) is an important zoonosis. White-tailed deer (Odocoileus virginianus) have been implicated in transmission of this bacterium to humans and have been suggested as reservoirs that might affect carriage in cattle populations. Our study objectives were to estimate prevalence of EC O157 in feces of hunter-harvested deer and to describe fecal shedding patterns in a captive herd sampled over 1 yr. Prevalence of EC O157 in hunter-harvested deer was 0.3% (n = 338). In August 2001, EC O157 was detected in one of 55 deer (1.8%) from the captive herd. Prevalence over the 1-yr period was 0.4% (n = 226). Escherichia coli O157:H7 was rarely isolated from hunter-harvested deer during the winter. We could not describe a seasonal shedding pattern based on one positive sample in the captive herd. These data do not support a prominent role of deer as a reservoir for EC O157 for cattle or humans.     

Prevalence, antibiotic susceptibility, and diversity of Escherichia coli O157:H7 isolates from a longitudinal study of beef cattle feedlots

Prevalence, antibiotic susceptibility, and genetic diversity were determined for Escherichia coli O157:H7 isolated over 11 months from four beef cattle feedlots in southwest Kansas. From the fecal pat (17,050) and environmental (7,134) samples collected, 57 isolates of E. coli O157:H7 were identified by use of bacterial culture and latex agglutination (C/LA). PCR showed that 26 isolates were eaeA gene positive. Escherichia coli O157:H7 was identified in at least one of the four feedlots in 14 of the 16 collections by C/LA and in 9 of 16 collections by PCR, but consecutive positive collections at a single feedlot were rare. Overall prevalence in fecal pat samples was low (0.26% by C/LA, and 0.08% by PCR). No detectable differences in prevalence or antibiotic resistance were found between isolates collected from home pens and those from hospital pens, where antibiotic use is high. Resistant isolates were found for six of the eight antibiotics that could be used to treat E. coli infections in food animals, but few isolates were multidrug resistant. The high diversity of isolates as measured by random amplification of polymorphic DNA and other characteristics indicates that the majority of isolates were unique and did not persist at a feedlot, but probably originated from incoming cattle. The most surprising finding was the low frequency of virulence markers among E. coli isolates identified initially by C/LA as E. coli O157:H7. These results demonstrate that better ways of screening and confirming E. coli O157:H7 isolates are required for accurate determination of prevalence.     

Clonal turnover of enterohemorrhagic Escherichia coli O157:H7 in experimentally infected cattle

A total of 401 enterohemorrhagic Escherichia coli (EHEC) O157:H7 isolates from two experimentally infected calves were analyzed using molecular biological methods. Genetic differences detected by pulsed-field gel electrophoresis were observed between the inoculated and recovered strains as early as 1 day post inoculation. The loss of the inoculated clone was observed in one calf. Replication and dissemination of the EHEC O157:H7 strains that mutated in cattle may result in the diversification of this organism among cattle populations.     

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.     

Effect of cattle diet on Escherichia coli O157:H7 acid resistance.

The duration of shedding of Escherichia coli O157 isolates by hay-fed and grain-fed steers experimentally inoculated with E. coli O157:H7 was compared, as well as the acid resistance of the bacteria. The hay-fed animals shed E. coli O157 longer than the grain-fed animals, and irrespective of diet, these bacteria were equally acid resistant. Feeding cattle hay may increase human infections with E. coli O157:H7.     

Association of Escherichia coli O157:H7 with houseflies on a cattle farm

The ecology of Escherichia coli O157:H7 is not well understood. The aims of this study were to determine the prevalence of and characterize E. coli O157:H7 associated with houseflies (HF). Musca domestica L. HF (n = 3,440) were collected from two sites on a cattle farm over a 4-month period and processed individually for E. coli O157:H7 isolation and quantification. The prevalence of E. coli O157:H7 was 2.9 and 1.4% in HF collected from feed bunks and a cattle feed storage shed, respectively. E. coli O157:H7 counts ranged from 3.0 x 10(1) to 1.5 x 10(5) CFU among the positive HF. PCR analysis of the E. coli O157:H7 isolates revealed that 90.4, 99.2, 99.2, and 100% of them (n = 125) possessed the stx1, stx2, eaeA, and fliC genes, respectively. Large populations of HF on cattle farms may play a role in the dissemination of E. coli O157:H7 among animals and to the surrounding environment.     

Prevalence of Escherichia coli O157 in cattle feeds in Midwestern feedlots

Comparisons of enrichment methods (with or without antibiotics and with or without a preenrichment step) using gram-negative (GN) broth or tryptic soy broth (TSB) were conducted with feeds inoculated with Escherichia coli O157:H7. TSB was more sensitive than GN broth, and TSB with a preenrichment step followed by TSB with antibiotics was more sensitive than plain TSB enrichment, in detecting E. coli O157 in inoculated feeds. Feed samples were collected from feed bunks from 54 feedlots to determine the prevalence of E. coli O157 in cattle feeds. TSB preenrichment followed by TSB with antibiotics and the standard GN broth enrichment were used for each feed sample. All samples underwent immunomagnetic separation and were plated onto sorbitol MacConkey agar with cefixime and potassium tellurite. Identification of E. coli O157 was based on indole production, positive latex agglutination for O157 antigen, API 20E test strip results, PCR for the eaeA gene, and the presence of at least one Shiga toxin. E. coli O157 was detected in 52 of 504 feed samples (10.3%) by using GN broth enrichment and in 46 of 504 feed samples (9.1%) by using TSB followed by TSB supplemented with cefixime and vancomycin. E. coli O157 was detected in 75 of 504 feed bunk samples (14.9%) by one or both methods. There was no correlation between E. coli O157 prevalence and generic coliform counts in feeds. The prevalence of E. coli O157 in cattle feed warrants further studies to increase our knowledge of the on-farm ecology of E. coli O157 in order to develop strategies to prevent food-borne disease in humans.     

Dose-Response Envelope for Escherichia coli O157:H7

Escherichia coli O157:H7 is an emerging food and waterborne pathogen in the U.S. and internationally. The objective of this work was to develop a dose-response model for illness by this organism that bounds the uncertainty in the dose-response relationship. No human clinical trial data are available for E. coli O157:H7, but such data are available for two surrogate pathogens: enteropathogenic E. coli (EPEC) and Shigella dysenteriae. E. coli O157:H7 outbreak data provide an initial estimate of the most likely value of the dose-response relationship within the bounds of an envelope defined by beta-Poisson dose-response models fit to the EPEC and S. dysenteriae data. The most likely value of the median effective dose for E. coli O157:H7 is estimated to be approximately 190[emsp4 ]000 colony forming units (cfu). At a dose level of 100[emsp4 ]cfu, the median response predicted by the model is six percent.     

Prevalence of Escherichia coli O157:H7 in Organically and Naturally Raised Beef Cattle

We determined the prevalence of Escherichia coli O157:H7 in organically and naturally raised beef cattle at slaughter and compared antibiotic susceptibility profiles of the isolates to those of isolates from conventionally raised beef cattle. The prevalences of E. coli O157:H7 were 14.8 and 14.2% for organically and naturally raised cattle, respectively. No major difference in antibiotic susceptibility patterns among the isolates was observed.Many cattle producers have adopted production methods termed niche marketing to meet consumer demand for safe and healthy beef. The two main niches for beef cattle producers are organic and natural production (3). Organic beef cattle production, regulated by the U.S. Department of Agriculture, requires feeding with certified organic feed (16) and raising cattle without the use of antibiotics, hormones, and other veterinary products (3). Guidelines for producers to label the product as “natural” differ among natural beef programs, and such programs are administered and regulated by the company or organization that owns the brand name rather than the U.S. Department of Agriculture (11). Natural production guidelines often include a complete restriction on the use of antibiotics and growth-promoting hormones, but unlike guidelines for organic production, they allow feed from nonorganic sources (11). Escherichia coli O157:H7 is a major food-borne pathogen that causes outbreaks of hemorrhagic enteritis, which often leads to hemolytic uremic syndrome in children and the elderly (10). Cattle are major reservoirs of E. coli O157:H7, which colonizes the hindgut, specifically the rectoanal mucosal region. Cattle feces are the major source of food and water contamination (10). The impact of organic production methods on the prevalence of food-borne pathogens, including E. coli O157:H7 and Campylobacter spp. in dairy cattle (7, 14) and Campylobacter and Salmonella spp. in chickens (6, 19), has been studied previously. However, there is no published study on the prevalence of E. coli O157:H7 in organically and naturally raised beef cattle. Additionally, nothing is known regarding the effects of organic and natural production methods on the antibiotic susceptibilities of E. coli O157:H7 in beef cattle. Our objectives were to determine the prevalence of E. coli O157:H7 in the feces of organically and naturally raised beef cattle at slaughter and compare the antibiotic susceptibilities of isolates from organically, naturally, and conventionally raised beef cattle.Cattle included in this study were from three types of production systems, organic, natural, and conventional. Organically raised beef cattle were from farms that were certified by the National Organic Program (17). The naturally raised beef cattle were from farms that were certified by the All Natural Source Verified Beef Program (17). The collection of samples from these cattle occurred in an abattoir. Samples from conventionally raised cattle from two feedlots were collected in a different abattoir so that the antibiotic susceptibilities of their isolates could be compared with those of isolates from organically and naturally raised cattle. Fecal samples were obtained by cutting open the rectum and spooning out the contents. The mucosa of the rectum was then rinsed with water until free of visible fecal material and swabbed with a sterile foam-tipped applicator (4). The isolation and identification of E. coli O157 and PCR detection of major virulence genes (eae, stx₁, stx₂, hlyA, and fliC) were carried out as described by Reinstein et al. (13). A subset of 60 isolates, 20 (10 from fecal samples and 10 from rectoanal mucosal swabs [RAMS]) from each production system, was randomly chosen to determine the antibiotic susceptibility patterns by the broth microdilution method (9). The antibiotics (all from Sigma-Aldrich) tested were amikacin, amoxicillin (amoxicilline), ampicillin, apramycin, bacitracin, cefoxitin, ceftazidime, ceftriaxone, cephalothin (cefalotin), chloramphenicol, chlortetracycline, ciprofloxacin, enrofloxacin, erythromycin, florfenicol, gentamicin, kanamycin, lincomycin, monensin, nalidixic acid, neomycin, norfloxacin, novobiocin, oxytetracycline, penicillin, rifampin (rifampicin), spectinomycin, streptomycin, tetracycline, tilmicosin, trimethoprim, tylosin, and vancomycin. The MIC was defined as the lowest concentration of an antibiotic that prevented visible growth of the organism. Each concentration of the antibiotic compound was duplicated in the microtiter plate, and the MIC determination was repeated with a different inoculum preparation. Logistic regression was performed using the PROC GENMOD procedure in the SAS system (SAS Institute, Cary, NC) to compare the prevalences of E. coli O157:H7 (with binomial distribution of outcomes) in fecal samples, RAMS samples, and fecal or RAMS samples (overall animal level prevalence). The MICs of antibiotics for E. coli O157:H7 isolates were analyzed using a nonparametric survival test in the PROC LIFETEST program of SAS to determine the effects of the production system (natural, organic, or conventional). Data were right censored when necessary (when the organism was resistant to the highest concentration evaluated). The Wilcoxon test was utilized to determine the effect of the production system on MICs.Samples from a total of 553, 506, and 322 organically, naturally, and conventionally raised cattle, respectively, were collected. In organically raised cattle, the prevalence of E. coli O157:H7 in fecal samples ranged from 0 to 24.4% across sampling days, with an average of 9.3%, and the prevalence in RAMS ranged from 0 to 30.9%, with an average of 8.7% (Fig. ​(Fig.1).1). In naturally raised cattle, the prevalence of E. coli O157:H7 in fecal samples ranged from 0 to 20.3%, with an average of 7.2%, and the prevalence in RAMS ranged from 0 to 23.8%, with an average of 8.9% (Fig. ​(Fig.1).1). In both organically and naturally raised cattle, the prevalence (total) detected by both sampling methods together was greater (P < 0.05) than the prevalence detected by either method alone (Fig. ​(Fig.1).1). Samples (either feces or RAMS) from 36 (11.2%) of 322 conventionally raised feedlot cattle were culture positive for E. coli O157:H7. The fecal prevalence of E. coli O157:H7 was 6.5%, and the prevalence determined by the RAMS sampling method was 7.1%. Most isolates (66.7% from organically raised beef cattle and 77.8% from naturally raised beef cattle) were positive for eae, stx₂, hlyA, and fliC but negative for stx₁. The stx₂ gene was present in 100 and 95% of isolates from organically and naturally raised cattle, respectively. The prevalences of E. coli O157:H7 that we observed in organically and naturally raised beef cattle were similar to the previously reported prevalence in conventionally raised cattle (1). Our study did not include a statistical comparison of the prevalence data because of a number of differences, particularly in diet, among the organic, natural, and conventional production systems. Organically and naturally raised cattle are either required to graze a pasture or fed a forage-based diet. Although conflicting data exist (1), studies have shown that cattle fed a forage diet have both higher levels and longer durations of fecal shedding of E. coli O157:H7 than cattle fed a grain diet (18).Open in a separate windowFIG. 1.Prevalences of E. coli O157:H7 in organically and naturally raised beef cattle at slaughter. For each production system, bars not labeled with the same letter represent significantly different levels at P of <0.05.None of the tested isolates from the three production systems were susceptible to bacitracin, lincomycin, monensin, novobiocin, tilmicosin, tylosin, and vancomycin (MICs > 50 μg/ml). The MICs of 12 antibiotics (amikacin, apramycin, cefoxitin, ceftriaxone, gentamicin, kanamycin, nalidixic acid, neomycin, penicillin, rifampin, streptomycin, and tetracycline) for isolates collected from different production systems were significantly different (P < 0.05). MICs of gentamicin and neomycin for E. coli O157:H7 isolates from conventionally raised cattle were higher (P < 0.05) than those for isolates from naturally and/or organically raised cattle (Table ​(Table1).1). However, MICs of amikacin, apramycin, cefoxitin, ceftriaxone, kanamycin, nalidixic acid, penicillin, rifampin, and tetracycline for isolates from conventionally fed cattle were lower (P < 0.05) than those for isolates from naturally and/or organically raised cattle (Table ​(Table1).1). Among the 60 isolates tested for antibiotic susceptibilities, 6 isolates (10%) were susceptible to all antibiotics included in the study, excluding the seven antibiotics to which all isolates were resistant. Forty-two isolates (70%) were resistant to one antibiotic (MIC, >50 μg or >50 IU/ml), nine isolates (15%) were resistant to two antibiotics, and two isolates (3%) were resistant to five antibiotics. One isolate from the organically raised cattle group was resistant to 10 (amoxicillin, ampicillin, cefoxitin, cephalothin, chloramphenicol, florfenicol, oxytetracycline, penicillin, streptomycin, and tetracycline) of the 26 antibiotics that were inhibitory to other isolates. We have presented the data as the median MICs for each production system. In some instances, the median values were the same but the actual MIC data differed between production systems. This effect occurred because the data were right censored if isolates were not susceptible at 50 μg or 50 IU/ml. If more isolates from a particular production system than from another are censored, it may lead to statistical differences. This pattern justifies the use of survival analysis for this type of data. There were differences between MICs of many antibiotics (cefoxitin, ceftriaxone, gentamicin, nalidixic acid, neomycin, penicillin, rifampin, and tetracycline) for isolates from organically raised cattle and conventionally raised cattle. Similarly, there were differences between MICs of many antibiotics (amikacin, apramycin, ceftriaxone, kanamycin, nalidixic acid, and rifampin) for isolates from naturally raised cattle and conventionally raised cattle. For many of these antibiotics, MICs for isolates from organically or naturally raised cattle were greater than those for isolates from conventionally raised cattle. Resistance genes can be transferred among the enteric pathogen populations in food animals and humans (8), and it is possible that resistance genes from other bacteria in the gastrointestinal system of cattle may be acquired by E. coli O157:H7. For cattle, heavy metals like copper and zinc, which are also antimicrobial, are included in diets at concentrations in excess of the nutritional requirements, often replacing conventional antibiotics, to achieve growth promotion (5). Feeding with metals also results in the emergence of bacterial populations resistant to metals (5), which in some instances may lead to resistance to antibiotics. Mechanisms of resistance to copper at concentrations above those usually tolerated by normal cellular processes have been found on plasmids linked to resistance to antibiotics in some bacteria (5). Therefore, it is possible that isolates from organically or naturally raised cattle that are not exposed to antibiotics still may become resistant to antibiotics.

TABLE 1.

MICs of antimicrobials for E. coli O157:H7 isolates from conventionally, naturally, and organically raised beef cattle

Effect of the thyroid on faecal shedding of E. coli O157:H7 and Escherichia coli in naturally infected yearling beef cattle

AIMS: To determine if thyroid function affects faecal shedding of Escherichia coli O157:H7. METHODS AND RESULTS: Eight yearling cattle (n = 4 per treatment group), previously identified as shedding E. coli O157:H7, received either 0 or 10 mg 6-N-propyl-2-thiouracil (PTU) kg(-1) BW day(-1) for 14 days to reduce serum concentrations of the thyroid hormones, T(3) and T(4). Animals were monitored daily for changes in faecal shedding of E. coli O157:H7 and E. coli (EC) for the 14-day treatment period and an additional 7 days post-treatment. Body weight was measured weekly and serum concentrations of T(3) and T(4) were determined every 3 days. No differences in faecal shedding of E. coli O157:H7 were observed during the 14-day treatment period. However, compared with control animals, a greater percentage of PTU-treated cattle ejected E. coli O157:H7 on day 16 (100 vs 25%) and 18 (75 vs 0%) of the post-treatment period. Serum T(3) was lower in PTU-treated cattle during the 14-day treatment period and greater on day 18 of the post-treatment period. CONCLUSION: Cattle with chemically altered thyroid hormones had similar shedding patterns of faecal E. coli O157:H7 and EC during the 14-day treatment period. However, faecal shedding of E. coli O157:H7 tended to be greater, and serum concentrations of T(3), were greater for PTU-treated cattle immediately following the termination of PTU treatment. SIGNIFICANCE AND IMPACT OF THE STUDY: Short-term chemical inhibition of thyroid hormones had minimal effects on faecal shedding of E. coli O157:H7 in naturally infected cattle. However, a hyperthyroid state as observed postdosing might play a role in the seasonal shedding of E. coli O157:H7 in cattle.     

Dipstick immunoassay to detect enterohemorrhagic Escherichia coli O157:H7 in retail ground beef.

A sensitive and easy-to-perform dipstick immunoassay to detect Escherichia coli O157:H7 in retail ground beef was developed by using a sandwich-type assay (with a polyclonal antibody to E. coli O157 as the capture antibody and a monoclonal antibody to E. coli O157:H7 as the detection antibody) on a hydrophobic polyvinylidine difluoride-based membrane. E. coli O157:H7 in ground beef could be detected within 16 h, including incubation for 12 h in enrichment broth and the immunoassay, which takes 4 h. Pure culture cell suspensions of 10(5) or 10(6) E. coli O157:H7 organisms per ml produced intense color reactions in the immunoassay, whereas faint but detectable reactions occurred with 10(3) CFU/ml. The sensitivity of the combined enrichment-immunoassay procedure as determined by using ground beef inoculated with E. coli O157:H7 was 0.1 to 1.3 cells per g, with a false-positive rate of 2.0%. A survey of retail ground beef using this procedure revealed that 1 of 76 samples was contaminated by E. coli O157:H7.     

Dipstick immunoassay to detect enterohemorrhagic Escherichia coli O157:H7 in retail ground beef.

A sensitive and easy-to-perform dipstick immunoassay to detect Escherichia coli O157:H7 in retail ground beef was developed by using a sandwich-type assay (with a polyclonal antibody to E. coli O157 as the capture antibody and a monoclonal antibody to E. coli O157:H7 as the detection antibody) on a hydrophobic polyvinylidine difluoride-based membrane. E. coli O157:H7 in ground beef could be detected within 16 h, including incubation for 12 h in enrichment broth and the immunoassay, which takes 4 h. Pure culture cell suspensions of 10(5) or 10(6) E. coli O157:H7 organisms per ml produced intense color reactions in the immunoassay, whereas faint but detectable reactions occurred with 10(3) CFU/ml. The sensitivity of the combined enrichment-immunoassay procedure as determined by using ground beef inoculated with E. coli O157:H7 was 0.1 to 1.3 cells per g, with a false-positive rate of 2.0%. A survey of retail ground beef using this procedure revealed that 1 of 76 samples was contaminated by E. coli O157:H7.