Feeding crude glycerin decreases fecal shedding of E. coli O157:H7 in growing cattle

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Feeding Crude Glycerin Decreases Fecal Shedding of E. coli O157:H7 in Growing Cattle C. Aperce and J.S. Drouillard

Introduction
Crude glycerin is a byproduct of ethanol production and is used as a carbohydrate source for cattle feed.Glycerin levels in previous studies have ranged from 0 to 20% of diet dry matter, and concentrations of 8% or less generally improve feedlot performance.At even low levels of glycerin, however, the activity of cellulolytic bacteria is depressed, ultimately leading to poorer fiber digestion.This observation suggests that glycerin may affect a specific population of bacteria in the gut.Crude glycerin can account for 8 to 10% of the weight of dried distillers grains with solubles, because it is one of the primary end-products when yeast ferments sugars to produce ethanol.Addition of 25% dried distillers grains with solubles to a feedlot diet increased the prevalence of Escherichia coli O157:H7 in feces of cattle.These observations led us to question whether glycerin might be the component of distillers grains responsible for the increases in prevalence of E. coli O157:H7 that often are observed in cattle fed distillers grains.To address this question, we added glycerin to diets of growing cattle and subsequently evaluated fecal shedding of E. coli O157:H7.

Experimental Procedures
We added three levels of crude glycerin, 0, 4, or 8%, to growing diets containing dryrolled corn, corn silage, alfalfa hay, and corn steep liquor (Table 1).We formulated all diets so they would be isonitrogenous.
Each treatment was represented by 16 pens, each containing 7 to 8 heifers.We obtained fecal samples by fecal grab at the chute once a week for 6 weeks.Fecal samples were kept on ice until analysis.We weighted approximately 1 gram of feces and placed it in 9 mL Gram Negative broth (Difco, Inc., Corpus Christi, TX) with cefixime (0.05 mg/L), cefsulodin (10 mg/L), and vancomycin (8mg/L; GNccv) for a 6-hour incubation at 104ºF.We added 1 mL of GNccv to a sterile tube containing 20 µL of E. coli O157specific beads and subjected it to immunomagnetic separation.We then resuspended the resulting E. coli O157 beads in 100 µL of phosphate buffer and plated them onto a selective agar for E. coli O157:H7 for an overnight incubation at 98ºF.After incubation, we picked up to 6 non-sorbitol fermenting colonies and tested them for indole production.We further analyzed indole-positive colonies using an O157 antigen agglutination kit.We considered colonies positive for agglutination and indole production as E. coli O157:H7.

Results and Discussion
The crude glycerin used in this experiment contained 81.5% glycerol.After statistical analysis, we concluded that no interaction occurred between sampling date and the levels of crude glycerin (P > 0.2).Sampling date, however, did have an effect (P < 0.01), as shown in Figure 1.For the first 2 weeks, percentages of samples that tested positive for E. coli O157:H7 were 1.3 and 0.8%, respectively.The prevalence then increased to a peak of 8.8% during the fourth week, then stabilized around 4.3 to 5.8% during the final 2 weeks of the experiment.We also observed an effect of glycerin inclusion levels (P < 0.01; Figure 2).Fecal incidence rates of E. coli O157:H7 were 5.8, 4.3, and 2.4% for heifers fed 0, 4, and 8% glycerin, respectively.The prevalence we observed in heifers fed 4% glycerin tended to differ from that of cattle fed 8% glycerin (P = 0.06), but was not different from that of cattle fed the diet with 0% glycerin.Glycerin previously has been shown to inhibit the activity of cellulolytic bacteria in the rumen.Consequently, changes in fecal prevalence of E. coli O157:H7 observed in this study might be explained by alterations in gastrointestinal flora, with higher levels of glycerin producing a less favorable environment for the proliferation of pathogenic E. coli O157:H7.

Implications
Our goal in this study was to determine if feeding glycerin would affect shedding of E. coli O157:H7 in cattle feces.Our results demonstrated that increasing levels of crude glycerin decreased the prevalence of E. coli O157:H7, and that this could be a useful pre-harvest strategy for controlling the shedding of pathogenic E. coli in cattle.

Table 1 .
Composition of experimental diets (dry basis)