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INTRODUCTION
Approximately 60 verotoxin-producing Escherichia coli (VTEC) serotypes have been associated with human illnesses (Johnson, 1996). E. coli O157:H7 is the most well studied VTEC serotype. Its clinical importance was recognized in 1982 when it was isolated from human stools during an outbreak of foodborne illness associated with the consumption of improperly cooked ground beef (Riley, 1983). Cattle are known reservoirs of VTEC (Johnson, 1996). Approximately six million cows are culled from cattle herds annually due to decreased production, health problems, or reproductive inefficiency. Because these cows enter the food chain as ground beef (James, 1998), it is important to understand their carriage of VTEC. Identification of VTEC-positive cows before slaughter may be beneficial especially if pre-harvest and post-harvest control measures are adopted to minimize beef contamination with such pathogens.
One characteristic that all VTEC serotypes share is the ability to produce two toxins known as verotoxins which are the implicated cause of human illnesses (Johnson, 1996). Verotoxins 1 (VT1) and 2 (VT2) are similar to the toxins produced by Shigella. Production of both toxins is not necessary to cause a human ilness (Lior, 1994).
In the U.S., many studies have focused on E. coli O157:H7 because most human illness outbreaks were traced to this pathogen. There are biochemical characteristics that are unique to O157:H7. These are the lack of sorbitol fermentation and the lack of b-glucuronidase activity(measured on the synthetic substrate 4-methylumbelliferyl-$-D-glucuronide [MUG]). Other VTEC do not have such identifying characteristics. Therefore, in order to detect VTEC, other methods are necessary. In the current study we used microbiological screening, cell culture, biochemcial testing, and molecular techniques to select and identify VTEC. The objective of this study, therefore, was to examine the prevalence of VTEC (in general) in culled beef cows (Angus, Hereford, or their crossbreds) at the time of shipping to slaughter.
ANIMALS AND SAMPLE ANALYSIS
Fecal samples were directly collected from the rectum of 82 culled cows (8 to 12 yr-old) representing eight Nevada ranches. The cows grazed meadow regrowth of range land forages (i.e., crested wheatgrass [Agropyron desertorum], bromegrass [Bromus inermis], and tall fescue [Festuca arundinacea]. The feces were placed in clean plastic bag and shipped (on ice) to the laboratory for analysis. Sample processing began at least 24 h (but not more than 36 h) after collection.
Samples were first screened using sorbitol-MacConkey agar (Thran et al., 2000) to select the initial bacterial isolates of interest. Initial isolates were then screened by testing for toxic effects on Vero (Africian Green Monkey kidney) cells (Konowalchuk, 1977). Isolates that were identified as verotoxin-producers were confirmed as E. coli and further characterized by using polymerase chain reaction (PCR) and serotyping (Thran et al., 2000).
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