Soil solarization reduces Escherichia coli O157:H7 and total Escherichia coli on cattle feedlot pen surfaces

Feedlot pen soil is a source for transmission of Escherichia coli O157:H7, and therefore a target for preharvest strategies to reduce this pathogen in cattle. The objective of this study was to determine the ability of soil solarization to reduce E. coli O157:H7 in feedlot surface material (FSM). A feedlot pen was identified in which naturally occurring E. coli O157:H7 was prevalent and evenly distributed in the FSM. Forty plots 3 by 3 m were randomly assigned such that five plots of each of the solarization times of 0, 1, 2, 3, 4, 6, 8, and 10 weeks were examined. Temperature loggers were placed 7.5 cm below the surface of each plot, and plots to be solarized were covered with clear 6-mil polyethylene. At each sampling time, five FSM samples were collected from each of five solarized and five unsolarized plots. E. coli concentrations and E. coli O157:H7 presence by immunomagnetic separation and plating were determined for each FSM sample. Initial percentages of E. coli O157:H7-positive samples in control and solarized FSM were 84 and 80%, respectively, and did not differ (P > 0.05). E. coli O157:H7 was no longer detectable by 8 weeks of solarization, but was still detected in unsolarized FSM at 10 weeks. The average initial concentration of E. coli in FSM was 5.56 log CFU/g and did not differ between treatments (P > 0.05). There was a 2.0-log decrease of E. coli after 1 week of solarization, and a >3.0-log reduction of E. coli by week 6 of solarization (P, 0.05). E. coli levels remained unchanged in unsolarized FSM (P > 0.05). Daily peak FSM temperatures were on average 8.7°C higher for solarized FSM compared with unsolarized FSM, and reached temperatures as high as 57°C. Because soil solarization reduces E. coli O157:H7, this technique may be useful for reduction of persistence and transmission of this pathogen in cattle production, in addition to remediation of E. coli O157:H7-contaminated soil used to grow food crops.     

Comparison of two sampling methods for Escherichia coli O157:H7 detection in feedlot cattle

Two sampling methods (rectoanal swabs and rectal fecal grabs) were compared for their recovery of Escherichia coli O157:H7 from feedlot cattle. Samples were collected from 144 steers four times during the finishing period by swabbing the rectoanal mucosa with cotton swabs and immediately obtaining feces from the rectum of each individual steer. The number of steers with detectable E. coli O157:H7 increased from 2 of 144 (1.4%) cattle on arrival at the feedlot to 10 of 144 (6.9%) after 1 month, 76 of 143 (52.8%) after 7 months, and 30 of 143 (20.8%) at the last sampling time before slaughter. Wilcoxon signed-rank tests indicated that the two sampling methods gave different results for sampling times 3 and 4 (P < 0.05) but not for sampling time 2 (P = 0.16). Agreement between the two sampling methods was poor (kappa < 0.2) for three of the four sampling times and moderate (kappa = 0.6) for one sampling time, an indication that in this study rectoanal swabs usually were less sensitive than rectal fecal grabs for detection of E. coli O157:H7 in cattle. Overall, the herd of origin was not significantly associated with E. coli O157:H7 results, but the weight of the steers was. Further investigation is needed to determine the effects of potential confounding factors (e.g., size and type of swab, consistency of feces, site sampled, and swabbing technique) that might influence the sensitivity of swabs in recovering E. coli O157:H7 from the rectoanal mucosa of cattle.     

Escherichia coli O157:H7 prevalence in fecal samples of cattle from a southeastern beef cow-calf herd

The proportion of fecal samples culture-positive for Escherichia coli O157:H7 was determined for samples collected from 296 beef cows on pasture in a single Florida herd in October, November, and December 2001. The overall proportion of samples that cultured positive was 0.03. The proportion of cows that were culture-positive on at least one occasion was 0.091. No effect of pregnancy status or nutritional regimen on the proportion of culture-positive samples for E. coli O157:H7 was detected. We detected a breed effect on the shedding of E. coli O157, with Romosinuano cows having a lower (P < 0.01) proportion of samples culture-positive than Angus or Brahman cows. This difference might have resulted from the presence of confounding variables; however, it also might represent evidence of breed-to-breed genetic variation in E. coli O157 shedding. Further research is warranted to evaluate breed as a possible risk factor for shedding of this important foodborne pathogen. Further substantiated findings could indicate that breed is a cow-calf-level critical control point of E. coli O157:H7.     

Monitoring Escherichia coli O157:H7 in inoculated and naturally colonized feedlot cattle and their environment

On-farm methods of monitoring Escherichia coli O157:H7 were assessed in 30 experimentally inoculated steers housed in four pens over a 12-week period and in 202,878 naturally colonized feedlot cattle housed in 1,160 pens on four commercial Alberta feedlots over a 1-year period. In the challenge study, yearling steers were experimentally inoculated with 10(10) CFU of a four-strain mixture of nalidixic acid-resistant E. coli O157:H7. After inoculation, shedding of E. coli O157:H7 was monitored weekly by collecting rectal fecal samples (FEC), oral swabs (ORL), pooled fecal pats (PAT), manila ropes (ROP) orally accessed for 4 h, feed samples, water, and water bowl interface. Collection of FEC from all animals per pen provided superior isolation (P < 0.01) of E. coli O157:H7 compared with other methods, although labor and animal restraint requirements for fecal sample collection were high. When one sample was collected per pen of animals, E. coli O157:H7 was more likely to be detected from the ROP than from the FEC, PAT, or ORL (P < 0.001). In the commercial feedlot study, samples were limited to ROP and PAT, and E. coli O157:H7 was isolated in 18.8% of PAT and 6.8% of ROP samples. However, for animals that had been resident in the feedlot pen for at least 1 month, isolation of E. coli O157:H7 from ROP was not different from that from PAT (P = 0.35). Pens of animals on feed for <30 days were six times more likely to shed E. coli O157:H7 than were animals on feed for >30 days. However, change in diet did not affect shedding of the organism (P > 0.23) provided that animals had acclimated to the feedlot for 1 month or longer. Findings from this study indicate the importance of introduction of mitigation strategies early in the feeding period to reduce transference and the degree to which E. coli O157:H7 is shed into the environment.     

Prevalence of Escherichia coli O157:H7 and performance by beef feedlot cattle given Lactobacillus direct-fed microbials

Fecal shedding of Escherichia coli O157:H7, the prevalence of Escherichia coli O157:H7 in pens and on carcasses and hides, and cattle performance as a result of daily dietary supplementation with Lactobacillus-based direct-fed microbials (DFMs) were evaluated in a feeding trial involving 180 beef steers. Steers were evaluated for shedding of E. coli O157:H7 by an immunomagnetic separation technique on arrival at the feedlot, just before treatment with the DFMs, and every 14 days thereafter until slaughter. Composite pen fecal samples were collected every 14 days (alternating weeks with animal testing), and prevalence on hides and carcasses at slaughter was also evaluated. Feedlot performance (body weight gain and feed intake) was measured for the period during which the DFMs were fed. Gain efficiency was calculated as the ratio of weight gain to feed intake. Lactobacillus acidophilus NPC 747 decreased (P < 0.01) the shedding of E. coli O157:H7 in the feces of individual cattle during the feeding period. E. coli O157:H7 was approximately twice as likely to be detected in control animal samples as in samples from animals receiving L. acidophilus NPC 747. In addition, DFM supplementation decreased (P < 0.05) the number of E. coli O157:H7-positive hide samples at harvest and the number of pens testing positive for the pathogen. Body weight gains (on a live or carcass basis) and feed intakes during the DFM supplementation period did not differ among treatments. Gain efficiencies on a live-weight basis did not differ among treatments, but carcass-based gain/feed ratios tended (P < 0.06) to be better for animals receiving the two DFM treatments than for control animals. The results of this study suggest that the feeding of a Lactobacillus-based DFM to cattle will decrease, but not eliminate, fecal shedding of E. coli O157:H7, as well as contamination on hides, without detrimental effects on performance.     

Frequency of Escherichia coli O157:H7 in Turkish cattle

In this study, five abattoirs in Istanbul were visited between January 2000 and April 2001. During these visits, 330 cattle were selected by a systematic sampling method. Cattle were examined clinically and breed, age, and sex were recorded. Rectal swabs were taken immediately after slaughter. Immunomagnetic separation was performed, and sorbitol-negative colonies were selected on sorbitol MacConkey agar with cefixime and tellurite (CT-SMAC agar). These colonies were checked for 4-methylenebelliferyl-beta-D-glucuronide, indol, rhamnose, and urease activity and motility. Serotypes of bacteria were determined by using antisera specific for Escherichia coli O157 and H7. All cattle selected were clinically healthy. Of 88 sorbitol-negative colonies selected on CT-SMAC agar, isolates from only 14 (4.2%) cattle reacted with anti-O157, and 13 of these isolates also reacted with anti-H7. E. coli O157:H7 was isolated from all breeds, but the numbers of isolates were largest for Holstein and Swiss Brown cows. E. coli O157:H7 was most frequently isolated from 2-year-old cattle. Similarly, it was most frequently isolated from male cattle. E. coli O157:H7 was isolated from cattle slaughtered in four of the five abattoirs studied.     

Escherichia coli O157:H7 excretion by commercial feedlot cattle fed either barley- or corn-based finishing diets

Effective preharvest control measures for Escherichia coli O157:H7 in cattle may significantly reduce the incidence of human disease caused by this organism. The prevalence and magnitude of fecal E. coli O157:H7 excretion was evaluated in 15 pens (300 to 500 cattle per pen) of commercial feedlot cattle fed a barley-based finishing ration and compared with that in 15 pens of cattle fed a corn-based ration. Average E. coli O157:H7 prevalence was 2.4% in barley-fed cattle and 1.3% in the corn-fed cattle (P < 0.05), and average magnitude of fecal E. coli O157:H7 excretion was 3.3 log CFU/g in the barley-fed cattle and 3.0 log CFU/g in the corn-fed cattle (P < 0.01). Corn-fed cattle had lower average fecal pH values (5.85) than did barley-fed cattle (6.51) (P < 0.01), and the average total generic fecal E. coli concentration in this group of animals (6.24 log CFU/g) was greater than that in the barley-fed cattle (5.55 log CFU/g) (P < 0.01). Specific feed ingredients may impact the frequency and magnitude of fecal excretion of E. coli O157:H7 by cattle.     

Fecal shedding of Escherichia coli O157:H7 in North Dakota feedlot cattle in the fall and spring

Cattle are an important reservoir of Escherichia coli O157:H7, which can lead to contamination of food and water, and subsequent human disease. E. coli O157:H7 shedding in cattle has been reported as seasonal, with more animals shedding during summer and early fall than during winter. North Dakota has relatively cold weather, especially in winter and early spring, compared with many other regions of the United States. The objective was to assess fecal shedding of E. coli O157:H7 in North Dakota feedlot cattle over the fall, winter, and early spring. One hundred forty-four steers were assigned randomly to 24 pens on arrival at the feedlot. Samples of rectal feces were obtained from each steer four times (October and November 2003, and March and April 2004) during finishing. On arrival (October 2003), 2 (1.4%) of 144 cattle were shedding E. coli O157:H7. The shedding increased significantly to 10 (6.9%) of 144 after 28 days (November 2003), to 76 (53%) of 143 at the third sampling (March 2004), and dropped significantly to 30 (21%) of 143 at the fourth (last) sampling (March 2004) before slaughter. Unfortunately, we were unable to sample the cattle during winter because of the extreme weather conditions. Sampling time significantly (P < 0.0001) influenced variability in E. coli O157:H7 shedding, whereas herd (P = 0.08) did not. The prevalence of E. coli O157:H7 shedding in North Dakota steers in fall and early spring was comparable to what has been reported in other parts of the United States with relatively warmer weather. Further research into E. coli O157:H7 shedding patterns during extreme weather such as North Dakota winters is warranted in order to fully assess the seasonal effect on the risk level of this organism.     

Horizontal transmission of Escherichia coli O157:H7 during cattle housing

Ruminant livestock, particularly cattle, is considered the primary reservoir of Escherichia coli O157:H7. This study examines the transmission of E. coli O157:H7 within groups of cattle during winter housing. Holstein Friesian steers were grouped in six pens of five animals. An animal inoculated with and proven to be shedding a marked strain of E. coli O157: H7 was introduced into each pen. Fecal (rectal swabs) and hide samples (900 cm2 from the right rump) were taken from the 36 animals throughout the study. Water, feed, and gate or partition samples from each pen were also examined. Within 24 h of introducing the inoculated animals into the pens, samples collected from the drinking water, pen barriers, and animal hides were positive for the pathogen. Within 48 h, the hides of 20 (66%) of 30 cohort animals from the six pens were contaminated with E. coli O157:H7. The first positive fecal samples from the noninoculated cohort animals were detected 3 days after the introduction of the inoculated steers. During the 23 days of the study, 15 of 30 cohort animals shed the marked E. coli O157:H7 strain in their feces on at least one occasion. Animal behavior in the pens was monitored during a 12-h period using closed circuit television cameras. The camera footage showed an average of 13 instances of animal grooming in each pen per hour. The study suggests that transmission of E. coli O157:H7 between animals may occur following ingestion of the pathogen at low levels and that animal hide may be an important source of transmission.     

Influence of transportation stress and animal temperament on fecal shedding of Escherichia coli O157:H7 in feedlot cattle

To test the influence of transportation stress and temperament on shedding of Escherichia coli O157:H7, cattle (n=150) were classified at various stages of production as Excitable, Intermediate or Calm based on a variety of disposition scores. Presence of E. coli O157:H7 was determined by rectal swabs from live animals and from colons collected postmortem. Percentage of cattle shedding E. coli O157:H7 at arrival at the feedlot was approximately equal among temperament groups. Before shipment to the processing facility, a higher (P=0.03) proportion of cattle from the Calm group shed E. coli O157:H7 compared to the other temperament groups. When pooled across all sampling periods, cattle from the Calm group had a greater percentage test positive for E. coli O157:H7. Neither the acute stressor of transportation nor a more excitable temperament led to increased shedding of E. coli O157:H7 in cattle.     

Escherichia coli O157:H7 shedding in vaccinated beef calves born to cows vaccinated prepartum with Escherichia coli O157:H7 SRP vaccine

Extensive research, intervention equipment, money, and media coverage have been directed at controlling Escherichia coli O157:H7 in beef cattle. However, much of the focus has been on controlling this pathogen postcolonization. This study was conducted to examine the performance, health, and shedding characteristics of beef calves that were vaccinated with an E. coli O157:H7 SRP bacterial extract. These calves had been born to cows vaccinated prepartum with the same vaccine. Cows and calves were assigned randomly to one of four treatments: (i) neither cows nor calves vaccinated with E. coli O157:H7 SRP (CON), (ii) cows vaccinated with E. coli O157:H7 SRP prepartum but calves not vaccinated (COWVAC), (iii) calves vaccinated with E. coli O157:H7 SRP but born to cows not vaccinated (CALFVAC), (iv) cows vaccinated with E. coli O157:H7 SRP prepartum and calves also vaccinated (BOTH). Calves born to vaccinated cows had significantly higher titers of anti-E. coli O157:H7 SRP antibodies (SRPAb) in circulation at branding time (P < 0.001). Upon entry to the feedlot, overall fecal E. coli O157:H7 prevalence was 23 % among calves, with 25 % in the CON treatment group, 19 % in the CALFVAC group, 32 % in the COWVAC group, and 15 % in the BOTH group (P > 0.05). Fecal shedding of E. coli O157 on arrival to the feedlot was not correlated with fecal shedding at slaughter (Spearman's rho = -0.02; P = 0.91). No significant effects of cow or calf E. coli O157:H7 SRP vaccination treatment were found on feedlot calf health or performance (P > 0.05), prevalence of lung lesions or liver abscess (P > 0.05), or morbidity, retreatment, or mortality numbers (P > 0.05). The findings of this study indicate that the timing of vaccination of calves against E. coli O157:H7 may be an important consideration for maximizing the field efficacy of this vaccine.     

Efficacy of Ozone Against Escherichia coli O157:H7 on Apples

Apples were inoculated with Escherichia coli O157:H7 and treated with ozone. Sanitization treatments were more effective when ozone was bubbled during apple washing than by dipping apples in pre‐ozonated water. The corresponding decreases in counts of E. coli O157:H7 during 3‐min treatments were 3.7 and 2.6 log₁₀ CFU on apple surface, respectively, compared to < 1 log₁₀ CFU decrease in the stem‐calyx region in both delivery methods. Optimum conditions for decontamination of whole apples with ozone included a pretreatment with a wetting agent, followed by bubbling ozone for 3 min in the wash water, which decreased the count of E. coli O157:H7 by 3.3 log₁₀CFU/g.     

Seasonal prevalence of Escherichia coli O157:H7 in beef cattle feces

Cattle are an asymptomatic reservoir of Escherichia coli O157:H7, but the bacterial colonization and shedding patterns are poorly understood. The prevalence and shedding of this human pathogen have been reported to be seasonal with rates typically increasing during warm months. The objectives of this study were (i) to assess the prevalence of E. coli O157:H7 in feces of feedlot cattle in Kansas during summer, fall, and winter months, and (ii) to characterize E. coli O157:H7 by screening for virulence factors. Of 891 fecal samples collected, 82 (9.2%) were positive for E. coli O157:H7. No significant differences in prevalence were detected among summer, fall, and winter months. The highest monthly prevalence (18.1%) was detected in February. All tested isolates were positive for stx2 (Shiga toxin 2) and eaeA (intimin) genes; 14 isolates (12.8%) also carried stx1. Our results indicate the prevalence of E. coli O157:H7 in beef cattle feces is not necessarily season dependent.     

Incidence and toxin production ability of Escherichia coli O157:H7 isolated from cattle trucks

Twelve cattle trucks were analyzed for the presence of Escherichia coli O157:H7. Three of them had been washed prior to arrival, and the others had not. Seventy-five percent of the trailers were positive for the presence of this foodborne pathogen. A total of 54 cultures were isolated and identified as E. coli O157:H7, all from the trucks that had not been cleaned. Most of the cultures (96.4%) produced Shiga-like toxin (verotoxin). No E. coli O157:H7 was detected in cattle trucks that were cleaned before arrival at the cattle pens. The incidence of E. coli O157:H7 in transport trailers increases the potential risk of contamination of cattle and transmission from farms to feedlots and to packing plants. This contamination increases the potential of contamination of meat during harvest and the risk of foodborne illnesses.     

Molecular characterization of Escherichia coli O157:H7 hide contamination routes: feedlot to harvest

This study was conducted to identify the origin of Escherichia coli O157:H7 contamination on steer hides at the time of harvest. Samples were collected from the feedlot, transport trailers, and packing plant holding pens and from the colons and hides of feedlot steers. A total of 50 hide samples were positive for E. coli O157:H7 in two geographical locations: the Midwest (25 positive hides) and Southwest (25 positive hides). Hide samples were screened, and the presence of E. coli O157: H7 was confirmed. E. coli O157:H7 isolates were fingerprinted by pulsed-field gel electrophoresis and subjected to multiplex PCR procedures for amplification of E. coli O157:H7 genes stx1, stx2, eaeA, fliC, rfbEO157, and hlyA. Feedlot water trough, pen floor, feed bunk, loading chute, truck trailer side wall and floor, packing plant holding pen floor and side rail, and packing plant cattle drinking water samples were positive for E. coli O157:H7. Pulsed-field gel electrophoresis banding patterns were analyzed after classifying isolates according to the marker genes present and according to packing plant. In this study, hide samples positive for E. coli O157:H7 were traced to other E. coli O157:H7-positive hide, colon, feedlot pen floor fecal, packing plant holding pen drinking water, and transport trailer side wall samples. Links were found between packing plant side rails, feedlot loading chutes, and feedlot pens and between truck trailer, different feedlots, and colons of multiple cattle. This study is the first in which genotypic matches have been made between E. coli O157:H7 isolates obtained from transport trailer side walls and those from cattle hide samples within the packing plant.     

Experimental use of a gas sensor-based instrument for differentiation of Escherichia coli O157:H7 from non-O157:H7 Escherichia coli field isolates

The rapid and economical detection of human pathogens in animal and food production systems would enhance food safety efforts. An instrument based on gas sensors coupled with an artificial neural network (ANN) was developed for the detection of and differentiation between laboratory isolates of Escherichia coli O157:H7 and non-O157:H7 E. coli. The purpose of this study was to use field isolates of E. coli to further evaluate the sensor system. This gas sensor-based, computer-controlled detection system was used to monitor gas emissions from 12 isolates of E. coli O157:H7 and 8 non-O157:H7 E. coli isolates. A standard concentration of each isolate was grown in 10 ml of nutrient broth at 37 degrees C for 16 h, and gas sampling was carried out every 5 min. Readings were continuously plotted to generate gas signatures. A back-propagation ANN algorithm was used to interpret the gas patterns. By analysis of the response of the ANN, the sensitivity and specificity of the instrument were calculated. Detectable differences between the gas signatures of the E. coli O157:H7 isolates and the non-O157:H7 isolates were observed. The instruments degree of sensitivity was high for E. coli O157:H7 isolates, but a lower degree of accuracy was observed for non-O157:H7 isolates because of increased strain variation. The sensitivity of the detection system was improved by the normalization of the data generated from the gas sensors. Because of its ability to detect differences in gas patterns, this instrument has a broad range of potential food safety applications.     

Effect of a vaccine product containing type III secreted proteins on the probability of Escherichia coli O157:H7 fecal shedding and mucosal colonization in feedlot cattle

Preharvest intervention strategies to reduce Escherichia coli O157:H7 in cattle have been sought as a means to reduce human foodborne illness. A blinded clinical trial was conducted to test the effect of a vaccine product on the probability that feedlot steers, under conditions of natural exposure, shed E. coli O157:H7 in feces, are colonized by this organism in the terminal rectum, or develop a humoral response to the respective antigens. Steers (n = 288) were assigned randomly to 36 pens (eight head per pen), and pens were randomized to vaccination treatment in a balanced fashion within six dietary treatments of an unrelated nutrition study. Treatments included vaccination or placebo (three doses at 3-week intervals). Fecal samples for culture (n = 1,410) were collected from the rectum of each steer on pretreatment day 0 and posttreatment days 14, 28, 42, and 56. Terminal rectum mucosal (TRM) cells were aseptically collected for culture at harvest (day 57 posttreatment) by scraping the mucosa 3.0 to 5.5 cm proximal to the rectoanal junction. E. coli O157:H7 was isolated and identified with selective enrichment, immunomagnetic separation, and PCR confirmation. Vaccinated cattle were 98.3% less likely to be colonized by E. coli O157:H7 in TRM cells (odds ratio = 0.014, P < 0.0001). Diet was also associated with the probability of cattle being colonized (P = 0.04). Vaccinated cattle demonstrated significant humoral responses to Tir and O157 lipopolysaccharide. These results provide evidence that this vaccine product reduces E. coli O157:H7 colonization of the terminal rectum of feedlot beef cattle under conditions of natural exposure, a first step in its evaluation as an effective intervention for food and environmental safety.     

Development of a dose-response relationship for Escherichia coli O157:H7

E. coli O157:H7 is an emerging food and waterborne pathogen. The development of acceptable guidelines for exposure to this organism based on quantitative microbial risk assessment requires a dose response curve. In this study, a prior animal study was used to develop a dose response relationship. The data was adequately fit by the beta-Poisson dose response relationship. This relationship was validated with reference to two outbreaks of this organism. It was found that the low dose extrapolation of the animal data using the beta-Poisson relationship provided estimates of risk concordant with those noted in the outbreaks. The fitted dose response relationship in conjunction with population estimates of the prevalence of E. coli O157:H7 illness indicates that the overall exposure is quite low in the US.     

Comparative effect of direct-fed microbials on fecal shedding of Escherichia coli O157:H7 and Salmonella in naturally infected feedlot cattle

The effect of direct-fed microbials (DFM) on fecal shedding of Escherichia coli O157:H7 and Salmonella in naturally infected feedlot cattle was evaluated in a clinical trial involving 138 feedlot steers. Following standard laboratory methods, fecal samples collected from steers were evaluated for change in the detectable levels of E. coli O157:H7 and Salmonella shed in feces after DFM treatment. Sampling of steers was carried out every 3 weeks for 84 days. A significant reduction (32%) in fecal shedding of E. coli O157:H7 (P < 0.001), but not Salmonella (P = 0.24), was observed among the treatment steers compared with the control group during finishing. The probability of recovery of E. coli O157:H7 from the feces of treated and control steers was 34.0 and 66.0%, respectively. Steers placed on DFM supplement were almost three times less likely to shed E. coli O157:H7 (odds ratio, 0.36; 95% confidence interval, 0.25 to 0.53; P < 0.001) in their feces as opposed to their control counterparts. The probability of recovery of Salmonella from the feces of the control (14.0%) and the treated (11.3%) steers was similar. However, the DFM significantly reduced probability of new infections with Salmonella among DFM-treated cattle compared with controls (nontreated ones). It appears that DFM as applied in our study are capable of significantly reducing fecal shedding of E. coli O157:H7 in naturally infected cattle but not Salmonella. The factors responsible for the observed difference in the effects of DFM on E. coli O157:H7 and Salmonella warrants further investigation.     

Antibiotic resistance and hypermutability of Escherichia coli O157 from feedlot cattle treated with growth-promoting agents

In a longitudinal study (165 days), we investigated the effect of growth-promoting agents (monensin and trenbolone acetate-estradiol) and an antibiotic (oxytetracycline) on the incidence in feedlot steers of Escherichia coli O157, including antibiotic-resistant and hypermutable isolates. Eighty steers in 16 pens were treated with eight combinations of promoters, and each treatment was duplicated. Fecal samples were collected at nine different sampling times for detection of E. coli O157. Overall, 50 E. coli O157 isolates were detected in treated animals, and none were found in untreated animals. Compared with untreated controls, there was a significant association between the utilization of growth-promoting agents or antibiotics and the shedding of E. coli O157 at day 137 (P = 0.03), when a prevalence peak was observed and 50% of the isolates were detected. Multiplex PCR assays were conducted for some virulence genes. PCR results indicated that all except one isolate possessed at least the Shiga toxin gene stx2. MICs for 12 antibiotics were determined, and eight oxytetracycline-resistant E. coli O157 strains were identified. Antibiotic-resistant strains were considered a distinct subpopulation of E. coli O157 by pulsed-field gel electrophoresis typing. Seven of these antibiotic-resistant strains were isolated early in the study (on or before day 25), and among them two were also hypermutable as determined by rifampin mutation frequencies. The proportion of hypermutable strains among E. coli O157 isolates remained relatively constant throughout the study period. These results indicate that the use of growth-promoting agents and antibiotics in beef production may increase the risk of environmental contamination by E. coli O157.