In contrast to results from initial studies in the United States suggesting that feeding distillers grain to cattle increases fecal shedding ofEscherichia coli 0157: H7(E. coli 0157)bacteria, a comprehensive project at Lethbridge Research Centre is showing that there is no direct link between the two.
University of Lethbridge PhD candidate and research affiliate Jennyka Hallewell conducted two studies under the supervision of doctors Tim McAllister, Kim Stanford and Jim Thomas. The commercial cattle study involved 6,817 yearling steers to evaluate the total number ofE. coli 0157bacteria as well as the survival time ofE. coli 0157in feces samples. It was followed by a challenge study on 32 steers.
In the commercial cattle study, the yearling steers were divided equally into three pens fed barley-based rations either with or without dried distillers grain with solubles (DDGS). One pen received the control diet without DDGS, the second group received 22.5 per cent corn DDGS (CDDGS), and the third group received 22.5 per cent wheat DDGS (WDDGS). Fresh fecal samples were collected from the pen floors on a monthly basis for 11 months for a total of 558 samples.
There were no significant differences inE. coli 0157numbers among the three diets. Overall, about 30 per cent of the samples from each pen tested positive forE. coli 0157.
The second part of the study was to test the persistence ofE. coli 0157in feces from cattle on the above-mentioned diets. The samples were inoculated with a five-strain mixture of naldixic acid-resistantE. coli 0157.(Naldixic acid-resistant strains are selected-for in the lab to allow researchers to track the bacteria.) Portions of fecal samples were tested weekly untilE. coli 0157could no longer be detected in the feces for three consecutive weeks.
The number ofE. coli 0157in samples from all three diet types declined steadily and uniformly, with no significant differences in the persistence ofE. coli 0157in the feces among diets.
In the challenge study, the steers were inoculated with a four-strain mixture of naldixic acid-resistantE. coli 0157.They received one of four barley-based finishing rations: without DDGS, 40 per cent CDDGS, 40 per cent WDDGS, or 20 per cent CDDGS and 20 per cent WDDGS. Fecal grab samples from individual steers were taken for 70 days to obtain a total of 544 samples.
Although there were no significant differences in overall rates of shedding among the diets, the numbers ofE. coli 0157did differ on certain days when the CDDGS group had lower numbers ofE. coli 0157compared to the other groups.
Hallewell says the design of this study with a large number of animals and samples taken in the commercial cattle trial along with the challenge study may account for the difference in findings from the earlier studies, many of which had a low prevalence ofE. coli 0157shedding in the sampling group and, or fewer sampling periods. As well, a laboratory study last year by Yang et al found thatE. coli 0157concentrations vary with the source of corn or wheat DDGS, suggesting that factors other than diet alone may be important.
Diet composition affects totalE. colipopulation insofar as it alters the microflora and pH level in the digestive tract.E. coliprefer a slightly-acidic (low pH) environment.
Cattle fed high-starch, grain-based diets typically have a more acidic environment in the digestive tract than cattle fed forage-based diets and some studies have shown that totalE. colicounts are higher in feces from cattle on grain-based diets than from those on forage-based diets. The type of grain in the ration may make a difference, with some studies showingE. coli 0157counts higher in feces from cattle on barley-based rations than from those on corn-based rations.
DDGS contains very little starch because most of it is removed from the grain in the ethanol production process. Therefore, replacing some of the grain with DDGS could lead to a decrease in acid production, thus raising the pH level in the digestive tract relative to that in cattle on grain-based rations without DDGS. Hallewell s commercial study did find that fecal pH was significantly different among diets, however, fecal pH had no effect on the prevalence ofE. coli 0157shedding.
The form of distillers grain wet versus dry in the ration could also alter the pH level in the digestive tract, however, studies investigating the effect of wet distillers grain on fecal shedding ofE. colihave shown conflicting results as well.
A 2009 study showed that steers on backgrounding diets including 13.9 per cent wet distillers grain shed twice as manyE. coli 0157as those on the corn grain-based control diet. In the finishing phase, the presence ofE. coli 0157in the feces and on the hides was much higher for steers fed 40 per cent wet distillers grain than for those not receiving wet distillers grain. Yet a year later, another study concluded that feeding 20 per cent wet corn distillers grain had no effect on fecal prevalence ofE. coli 0157.
OTHER CONTRIBUTING FACTORS
Hallewell points out that only about 10 to 25 per cent of cattle harbourE. coli 0157and, typically, they are intermittent shedders, with some animals testing positive for the pathogen one day and negative the next day.
Some individual animals seem to have unique shedding patterns with periods of high shedding and longer periods of reduced shedding. These animals, called supershedders, commonly haveE. coli 0157concentrations of more than 10,000 colony-forming units per gram (cfu/g) and as high as 100,000 cfu/g, whereas, the concentration ofE. coli 0157in feces from the intermittent shedders generally ranges from 10 to 100 cfu/g.
The age of the animal can affect the prevalence ofE. coli 0157.Calves and heifers have been found to shed moreE. coli 0157than adult animals and they are more likely to shedE. coli 0157just after weaning.
There is a seasonal effect as well, with the warmer months of May through September being the peak shedding time.
Animal housing has been implicated in the spread ofE. coli 0157within a herd or pen. Bacteria spread from animal to animal by direct contact as well as shared feed and water sources, therefore, the prevalence ofE. coli 0157shedding can be higher among animals kept in close quarters. The presence of other persistent animal reservoirs, such as wild ruminants, birds, flies and dogs, that can transmitE. coli 0157can be a factor, as well.
At the feedlot, stress from trans port, weaning, animal turnover, high stocking density, and mixing of animals can affect the numbers and spread ofE. coli 0157.Levels tend to be highest shortly after entry to the feedlot and usually decline thereafter. However, that trend can be influenced by environmental factors, such as mud and even the hours of sunshine, she adds.
Evolution ofE. colibacteria may also contribute to an increase in prevalence. Based on evidence thatE. colimay develop resistance to volatile fatty acids, which are fermentation byproducts that are present at higher concentrations when cattle are fed grain, it follows thatE. coli 0157could also develop an increasing tolerance to digestive acids. Acidtolerant bacteria would have higher survival rates than other bacteria in the digestive tract.
The presence of all of these factors presents challenges in assessing diet as an individual factor as no single factor alone is responsible for shedding ofE. coli 0157,Hallewell says.
It s not clear as to whetherE. coli 0157was a newly emerging strain when it was first identified as a food-borne pathogen in 1982, or whether it had always been present in cattle and improved detection methods led to its discovery at that time. Though E. coli 0157is now the most commonly isolated pathogenicE. coli that causes disease in people in North America, new pathotypes are continuously emerging and some of those have been implicated in disease in people as well. One thing for certain is that new strains of potentially pathogenicE. coliwill continue to emerge.
The beef industry has been working to address theE. coli 0157issue through research projects such as those mentioned above to determine the effect of diet on fecal shedding and by implementing practical measures on farms and feedlots to prevent runoff from manure from directly entering waterbodies. Packing plants have invested millions of dollars to implement a number of safety measures to preventE. coli 0157bacteria from tag on the hide or from the digestive organs from contaminating meat. Processes such as steam pasteurization, steam vacuum cleaning and lactic acid washes alongside adherence to strict hygiene protocols are standard in federal packing plants.
Taking steps to determine the factors that define the ecology ofE. coli 0157and identify methods to reduce its presence on the farm will continue to ensure healthful, safe beef products for consumers, Hallewell concludes.