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Infection and Immunity, May 2002, p. 2704-2707, Vol. 70, No. 5
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.5.2704-2707.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Intimin Facilitates Colonization by Escherichia coli O157:H7 in Adult Ruminants
Nancy A. Cornick,1* Sheridan L. Booher,2 and Harley W. Moon2
Department of Veterinary Microbiology and Preventative Medicine,1
Department of Veterinary Pathology, College of Veterinary Medicine, Iowa State University, Ames, Iowa 500112
Received 15 November 2001/
Returned for modification 3 January 2002/
Accepted 13 February 2002

ABSTRACT
We compared the magnitude and duration of fecal shedding of
wild-type
Escherichia coli O157:H7 to that of an isogenic intimin
mutant in young adult cattle and sheep. In both ruminant species,
wild-type
E. coli O157:H7 was shed in greater numbers and for
a longer duration than was the intimin mutant.

TEXT
Cattle are a major reservoir of
Escherichia coli O157:H7, and
estimates of the prevalence in the United States cattle population
range from 2 to 28% (
9-
11,
13,
21,
22).
E. coli O157:H7 can
persist for more than 2 months in clinically normal adult cattle
and sheep (
3-
5,
14,
16). It is not known if the organism actually
colonizes the alimentary tracts of adult ruminants or if persistence
is the result of continuous reintroduction from the environment
(
17). The bacterial outer membrane protein intimin mediates
intimate attachment of
E. coli O157:H7 to intestinal epithelial
cells in vitro and is thought to facilitate intestinal colonization
by the bacterium in humans (
19,
20). Although attaching and
effacing (AE) lesions are extensive in the intestines of neonatal
calves with clinical
E. coli O157:H7 infections (
6), they either
do not occur during the carrier state in adult cattle or affect
such small areas of the intestine as to be below the level of
detection (
3,
5). However, AE lesions are occasionally detected
in older calves with asymptomatic
E. coli O157:H7 infections
(
7). The latter observation, plus the ubiquity of intimin among
E. coli O157:H7 strains, suggests that intimin provides some
ecological advantage to the organism in its natural environment,
which would putatively include the adult ruminant gastrointestinal
tract. We hypothesized that intimin facilitates colonization
by
E. coli O157:H7 in mature ruminants.
A preliminary report of this work was presented at the Verocytotoxigenic E. coli in Europe Pathogenicity and Virulence meeting, Liege, Belgium, 1999.
The strains of E. coli O157:H7 used in this study are listed in Table 1. Strain 86-24
eae10 contains an in-frame deletion in the eae gene for intimin, and strain 86-24
eae10(pEB310) is the deletion mutant complemented with eae on a plasmid (18). Spontaneous mutants resistant to nalidixic acid and streptomycin were selected from the wild-type strain 86-24 and the intimin mutant.
Trial 1.
Sheep (6 to 12 months old) were obtained from the Iowa State
University herd, housed 2/room in BL-2 isolation facilities,
and acclimated to a maintenance diet of concentrate and alfalfa
and grass hay for at least 2 weeks as previously described (
4).
Groups of 8 sheep were given 10
10 CFU of either wild-type
E. coli O157:H7, the intimin mutant, or the complemented intimin
mutant. Individual fecal samples were collected from each sheep
on 2, 3, 4, 14, 15, 16, 58, 59, and 60 days postinoculation
(p.i.). Samples (5 g) were immediately processed in a Stomacher,
serially diluted in phosphate-buffered saline, and plated in
triplicate onto MacConkey agar with antibiotics (Table
1). The
sensitivity of the direct plating was

50 CFU/g. A 10-g fecal
sample was also added to enrichment broth (Tryptic soy broth
with 0.15% bile salts), incubated overnight, and plated onto
selective medium (
4,
5). Colonies isolated on selective medium
were confirmed as
E. coli O157 with a latex agglutination kit.
Bacterial counts (CFU/gram) were converted to log
10 and averaged
over days 2, 3, 4 (initial period); 14, 15, 16 (2 weeks); 28,
29, 30 (1 month, trial 2 only); and 58, 59, 60 (2 months). The
differences in the magnitude of fecal shedding (CFU/gram) between
strains inoculated individually into sheep were compared with
repeated measures analysis of variance (ANOVA).
The average magnitude of fecal shedding of E. coli O157:H7 by sheep inoculated with either the wild type, the intimin mutant, or the complemented mutant is shown in Fig 1. All of the sheep shed the organism during the initial period (2 to 4 days p.i.). The magnitude of shedding by sheep inoculated with the wild-type strain was significantly greater (P < 0.001) than that of the sheep inoculated with either the intimin mutant or the complemented mutant. The wild type was recovered from 8 out of 8 sheep, the mutant from 2 out of 7 sheep, and the complemented mutant from 6 out of 8 sheep at 2 weeks p.i. At this time, the magnitude of shedding was significantly greater by sheep inoculated with either the wild-type strain (P < 0.001) or the complemented mutant (P < 0.05) compared to that by sheep inoculated with the intimin mutant (Fig. 1). At 2 months p.i. the intimin mutant was not recovered from any sheep. In contrast, the wild-type strain was isolated from 3 out of 8 sheep, and the complemented mutant was recovered from 1 out of 8 sheep.
Trial 2.
To further define the role of intimin and to compare the magnitude
and duration of fecal shedding by sheep with that of cattle,
experiments involving dual inoculation with the wild-type strain
and the intimin mutant were conducted. Yearling cattle (
n =
8) were obtained from the Iowa State University beef herd, housed
1/pen in BL-2 isolation facilities, and acclimated to a maintenance
diet of grain and alfalfa and grass hay for at least 2 weeks.
Sheep (
n = 8) were obtained and acclimated as described for
trial 1. Each animal was orally inoculated with both the wild-type
parent
E. coli O157:H7 and the intimin mutant at a dose of 10
10 CFU/strain/animal. Fecal samples were collected and plated as
described for trial 1. Fecal samples cultured in enrichment
broth overnight were also concentrated with anti-O157 immunomagnetic
beads and were plated onto selective media. Differences in the
magnitude of shedding between strains were evaluated with the
paired
t test. Differences in shedding between cattle and sheep
were measured with repeated measures ANOVA.
The magnitude and duration of shedding of the dually inoculated animals is shown in Fig. 2. All of the animals shed both strains during the initial period p.i. (days 2 to 4). The mean fecal count of E. coli O157:H7 detected in sheep was 3.2 x 105 CFU/g for the wild-type strain and 3.2 x 104 CFU/g for the intimin mutant. The mean shedding of the wild-type strain by cattle during the initial p.i. period was 4.0 x 104 CFU/g and was 1.6 x 103 CFU/g for the intimin mutant. The difference in shedding between the wild-type strain and the intimin mutant during the initial period was significant for both sheep (P < 0.05) and cattle (P < 0.01).
The differences in shedding between the wild-type strain and
the intimin mutant were again significant in both sheep and
cattle (
P < 0.01) at 2 weeks p.i. and at 1 month (
P <
0.05) p.i. At 2 months p.i. 2 out of 7 sheep and 4 out of 8
cattle continued to shed the wild-type strain. The intimin mutant
was recovered from none of the sheep and only one of the cattle.
There were no consistent significant differences between cattle
and sheep in the magnitude or duration of shedding of either
strain.
The differences in both the magnitude and duration of colonization between the intimin mutant and the wild-type strain (Fig. 1 and 2) indicate that intimin is active in the alimentary tract of adult ruminants and facilitates colonization by E. coli O157:H7 in this setting. The partial restoration of persistence at 2 weeks p.i. by the complemented mutant carrying eae on a plasmid (Fig. 1) provides additional evidence for the role of intimin in the persistence of the organism in mature ruminants. Unless intimin is acting by some unknown mechanism (for which we have no evidence), it seems likely that AE lesions probably occur in the intestinal tracts of asymptomatic cattle and sheep carrying E. coli O157:H7. AE lesions are formed on bovine mucosal explants of Peyer's patch (20), colonic and rectal tissue (1) collected from mature cattle inoculated with E. coli O157:H7. The lack of reports of detectable AE lesions in mature ruminants combined with the data presented here suggests that some such lesions probably occur but are difficult to detect, since histological methods require approximately 106 CFU/cm of tissue in order for patches of colonized bacteria to be detected by microscopy (2, 7). The data presented here provides rationale in support of intimin as a potential antigen in vaccines or in alternative anti-intimin strategies to reduce the colonization of ruminants by E. coli O157:H7 (8, 15). These data also provide evidence that experimentally infected adult sheep are an appropriate model for E. coli O157:H7 colonization of adult cattle.

ACKNOWLEDGMENTS
We thank Pedro Navarro, Megan Black, Amy Helgerson, and Carisa
Ralph for technical assistance and Ilze Matise, Dianna Jordan,
and Nathan Eslick for help with the animal work. We also thank
Richard Evans for statistical advise and Alison O'Brien for
bacterial strains.
This work was supported in part by the U.S. Department of Agriculture National Research Initiative Competitive Grants Program 00-02515, National Institutes of Health grant AI-41328, and the Frank K. Ramsey endowment.

FOOTNOTES
* Corresponding author. Mailing address: 2130 Veterinary Medicine Building, Iowa State University, Ames, IA 50011. Phone: (515) 294-6499. Fax: (515) 294-8500. E-mail:
ncornick{at}iastate.edu.

Editor: J. D. Clements

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Infection and Immunity, May 2002, p. 2704-2707, Vol. 70, No. 5
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.5.2704-2707.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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