Rectal Administration of Escherichia coli O157:H7: Novel Model for Colonization of Ruminants

Escherichia coli O157:H7 is an enteric pathogen of humans that causes a spectrum of illnesses, including hemorrhagic colitis and renal failure, and can be fatal (14, 24, 29). Domestic cattle are an important reservoir of this pathogen and are the source for most food-borne infections (8). Experimental and field studies of E. coli O157:H7 in beef and dairy cattle have found within-group variation between individual animals with respect to occurrence and duration of fecal culture-positive status (2, 3, 6, 10, 15, 17, 20, 21, 27, 28). This variation suggests that host factors play an important role in E. coli O157:H7 colonization of cattle. A thorough understanding of the host-bacterium interaction could lead to the development of novel interventions, and in order to investigate interactions between the ruminant host and E. coli O157:H7 researchers must recreate infections in an experimental setting. Moreover, to evaluate the efficacy of traditional types of interventions, such as vaccinations (25), probiotics (4, 5), and antimicrobials, it is necessary to be able to reproduce the state of colonization. The earliest publications of ruminant E. coli O157:H7 inoculation studies reported on the use of very young calves, either preweaned or early postweaning, receiving doses of 1010 CFU administered orally (6, 10) or through rumen cannulae (16, 30). An oral dose of 108 CFU given to four calves in one study resulted in all four being fecal culture positive for 20 to 45 days, but the calves were 1 week old at the start of the experiment (27) and lacked a developed ruminant gastrointestinal tract. When older fully ruminant animals are orally dosed, some animals have brief infections that likely reflect passage of the inocula rather than colonization. Four of eight 4-month-old calves given an oral dose of 1010 CFU became fecal culture negative after 7 days (23), and in another study, 9 of 18 yearling steers given 1010 CFU were fecal culture negative by day 9 of the study (7). In order to experimentally mimic natural, long-term colonization with E. coli O157:H7 in older calves and adults, a new method is needed. A site of E. coli O157:H7 colonization has recently been identified as the mucosal lining of the rectoanal junction (22). This finding was supported by earlier observations that E. coli O157:H7 persists in the lower gastrointestinal tract of ruminants but is not detected in the upper gastrointestinal tract a few days after oral inoculation (7, 12) and the detection of the bacterium only in the distal rectal tissue in experimentally inoculated sheep (12). As a logical follow-up to these findings, our laboratory recently demonstrated that rectoanal mucosal swab (RAMS) culture is a more sensitive detection method than traditional fecal culture (26). We further reasoned that to establish reliably colonized animals, we would have greater success using a rectoanal inoculation route than an oral route. In this study, we developed a novel rectal swab administration of E. coli O157:H7 to establish experimental colonization of ruminants with this bacterium. We compared this methodology with two other challenge strategies and various inoculation doses. The methods included: (i) oral dosing, (ii) direct contact transmission by copenning with culture-positive animals (Trojan model), and (iii) a novel rectal swab inoculation. Both sheep and cattle were used, and E. coli O157:H7 colonization status was determined by standard fecal or RAMS culture (26).