Original Article Use of Multiple-Locus Variable-Number Tandem Repeat Analysis to Evaluate Escherichia coli O157 Subtype Distribution and Transmission Dynamics Following Natural Exposure on a Closed Beef Feedlot Facility Michele L. Williams, 1 David L. Pearl, 2 Katherine E. Bishop, 2 and Jeffrey T. LeJeune 1 Abstract To better understand the epizootiology of Escherichia coli O157:H7 among cattle, all E. coli O157 isolates re- covered on a research feedlot during a single feeding period were characterized by multiple-locus variable- number tandem repeat analysis (MLVA). Three distinct MLVA subtypes (A, B, C), accounting for 24%, 15%, and 64% of total isolates, respectively, were identified. Subtypes A and B were isolated at the initiation of sampling, but their prevalence waned and subtype C, first isolated on the third sampling date, became the predominant subtype on the feedlot. Supershedding events, however, occurred with equal frequency for all three MLVA- types. Using a multilevel logistic regression model, we investigated whether the odds of shedding subtype C relative to subtypes A or B were associated with time, diet, or the presence of a penmate shedding high numbers of subtype C. Only time and exposure to an animal shedding MLVA-type C at 10 3 colony-forming units or greater in the pen at the time of sampling were significantly associated with increased shedding of subtype C. High-level shedding of those E. coli O157 subtypes better suited for survival in the environment and/or in the host appear to play a significant role in the development of predominant E. coli O157 subtypes. Supershedding events alone are neither required nor sufficient to drive the epidemiology of specific E. coli O157 subtypes. Additional factors are necessary to direct successful on-farm transmission of E. coli O157. Introduction E ven with increased awareness and research to control Escherichia coli O157:H7 in our food supply, it is still re- sponsible for an estimated 63,000 cases of foodborne illness a year in the United States (Scallan et al., 2011). Preharvest in- terventions that decrease shedding of E. coli O157 by cattle are predicted to decrease carcass contamination and will posi- tively impact public health ( Jordan et al., 1999; Elder et al., 2000; LeJeune and Wetzel, 2007; Callaway et al., 2009). One preharvest control measure is the manipulation of the intestinal environment by dietary modification. Previous studies by our research group have assessed the effects of moisture content of corn-based diets and microbial feed ad- ditives on shedding of E. coli O157:H7 (Cernicchiaro et al., 2010; Cernicchiaro et al., 2011). A statistically significant increase in the prevalence of E. coli O157 by fecal- immunomagnetic separation (IMS) detection in steers fed dry whole-shelled corn compared to high moisture corn was found. Similar models based on rectoanal mucosal swab (RAMS)-IMS testing indicated that the effect of corn type on the prevalence of E. coli O157 varied with the type of feed additive used. The only consistent variable of detecting pos- itive E. coli O157, regardless of testing protocol, was exposure to another animal in the same pen experiencing a super- shedding event at ‡ 10 4 colony forming units (CFU) of E. coli O157:H7 per gram of feces as based on the working definition of Chase-Topping et al. (2008). Our research group is interested in defining the role of diet on the colonization and shedding of E. coli O157 subtypes and the role of supershedding events on the transmission dy- namics and longitudinal shedding of E. coli O157 from indi- vidual steers within the feedlot. We hypothesized that the study diets modified the colonic environment and preferen- tially supported different E. coli O157 subtypes with different capacities for colonizing the bovine host. If E. coli O157 numbers are above the detection threshold of 100 CFU/g of feces (LeJeune et al., 2006), analysis of fecal grab samples will 1 Food Animal Health Research Program, Ohio Agricultural Research and Development Center, Ohio State University, Wooster, Ohio. 2 Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, Canada. FOODBORNE PATHOGENS AND DISEASE Volume 0, Number 0, 2013 ª Mary Ann Liebert, Inc. DOI: 10.1089/fpd.2013.1484 1