Previous Article | Next Article 
Applied and Environmental Microbiology, September 2001, p. 3810-3818, Vol. 67, No. 9
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.9.3810-3818.2001
Genotypic Analyses of Escherichia coli O157:H7 and
O157 Nonmotile Isolates Recovered from Beef Cattle and Carcasses at
Processing Plants in the Midwestern States of the United
States
Genevieve A.
Barkocy-Gallagher,*
Terrance M.
Arthur,
Gregory R.
Siragusa,
James E.
Keen,
Robert O.
Elder,
William W.
Laegreid, and
Mohammad
Koohmaraie
Roman L. Hruska U.S. Meat Animal Research
Center, Agricultural Research Service, United States Department of
Agriculture, Clay Center, Nebraska 68933-0166
Received 5 January 2001/Accepted 8 June 2001
 |
ABSTRACT |
Escherichia coli O157:H7 and O157 nonmotile
isolates (E. coli O157) previously were
recovered from feces, hides, and carcasses at four large Midwestern
beef processing plants (R. O. Elder, J. E. Keen, G. R. Siragusa, G. A. Barkocy-Gallagher, M. Koohmaraie, and W. W. Laegreid, Proc. Natl. Acad. Sci. USA 97:2999-3003,
2000). The study implied relationships between cattle infection and
carcass contamination within single-source lots as well as between
preevisceration and postprocessing carcass contamination, based on
prevalence. These relationships now have been verified based on
identification of isolates by genomic fingerprinting.
E. coli O157 isolates from all positive samples were
analyzed by pulsed-field gel electrophoresis of genomic DNA
after digestion with XbaI. Seventy-seven individual subtypes (fingerprint patterns) grouping into 47 types were
discerned among 343 isolates. Comparison of the fingerprint patterns
revealed three clusters of isolates, two of which were closely related to each other. Remarkably, isolates carrying both Shiga toxin genes and
nonmotile isolates largely fell into specific clusters. Within lots
analyzed, 68.2% of the postharvest (carcass) isolates matched
preharvest (animal) isolates. For individual carcasses, 65.3 and 66.7%
of the isolates recovered postevisceration and in the cooler,
respectively, matched those recovered preevisceration. Multiple
isolates were analyzed from some carcass samples and were found to
include strains with different genotypes. This study suggests that most
E. coli O157 carcass contamination originates from
animals within the same lot and not from cross-contamination between
lots. In addition, the data demonstrate that most carcass contamination
occurs very early during processing.
| |
INTRODUCTION |
Escherichia coli O157:H7
or O157 nonmotile (both referred to herein as E. coli
O157) are classified as enterohemorrhagic E. coli and
can cause diseases ranging in severity from nonbloody diarrhea
(46) to hemolytic-uremic syndrome and death. Several factors have been associated with E. coli O157
virulence, including production of at least one of two Shiga toxins,
intimin, and enterohemolysin (37, 41). E. coli O157:H7 has been declared an adulterant in ground beef due to
frequent association of disease with consumption of undercooked
hamburgers (26, 49). The organism commonly is present in
cattle feces, suggesting that the animal is the source of beef contamination.
Studies clarifying the direct role of animal infection in subsequent
carcass contamination, as well as the frequency of cross-contamination, have been limited (12, 13). A few reports have suggested
that hides are a significant source of bacterial carcass contamination (6, 10, 34, 44). Most reports have approached the problem by examining the potential for carcass contamination at critical processing steps by following changes in total aerobic, coliform, or
generic E. coli counts or some combination of these
(6, 17-19). These measurements can imply causality but
lack the ability to directly link sources with the introduction of
specific organisms (contamination events). Pulsed-field gel
electrophoresis (PFGE) genotyping has been used to track sources of
Listeria contamination (20), and commonly is
used by the Centers for Disease Control and Prevention and others to
track sources of E. coli O157 outbreaks (e.g., see
references 2, 5, 7, 24, and 42). However, prior to this study it had not been used to track E. coli O157 contamination of carcasses.
Previously, we reported the preharvest and postharvest prevalence of
E. coli O157 at four large, Midwestern processing
plants during July and August (13). Samples were taken
from animals (preharvest) and carcasses (postharvest) within the same
lot, but not necessarily from the same animals. A correlation was noted between the prevalence of E. coli O157 found preharvest
and postharvest. Furthermore, carcasses found contaminated in the
cooler also were found to be contaminated preevisceration. These
correlations suggested relationships (i) between isolates entering the
plant with animals and those that appear on carcasses within the same
lot and (ii) between isolates found on carcasses in the cooler
and those found on the carcass earlier in processing. However, in the
absence of specific identification of the isolates these relationships could not be confirmed. The recovered E. coli
O157 isolates have now been characterized by XbaI
PFGE genotyping. The data have been used to track and confirm the
sources of carcass contamination throughout processing and to examine
isolate relatedness along with genotypic variability.
| |
MATERIALS AND METHODS |
E. coli O157 isolates.
The recovery of
E. coli O157 isolates from cattle feces, hides, and
carcasses has been described previously (13). Within each
single-source lot, different samples were not necessarily taken from
the same animals, but each individual carcass was tracked and sampled
preevisceration, postevisceration, and postprocessing (final sampling,
in the cooler). Lots ranged in size from 35 to 85 animals, and 20% of
each lot was sampled. The characterization of one isolate per positive
sample was described in regards to biochemical and immunological
analyses, the presence of toxin and other virulence genes, and motility
(13). At the time, up to three additional E. coli O157 isolates from postharvest samples also were recovered
and stored from additional, randomly chosen, morphologically correct
colonies picked from selective plates after enrichment and
immunomagnetic separation. Some of these secondary isolates have now
been characterized by the same methods.
Genomic fingerprint analyses.
E. coli. O157
isolate fingerprints generated and analyzed in this study were based on
PFGE separation of XbaI-digested genomic DNA as
previously described (11); this is the method used by members of PulseNet
(http://www.cdc.gov/ncidod/dbmd/pulsenet/pulsenet.htm). Pulsed-field
gel certified agarose was obtained from Bio-Rad (Hercules, Calif.);
Tris-borate-EDTA running buffer and lysozyme were purchased from Sigma
(St. Louis, Mo.). XbaI and RsaI were purchased
from New England Biolabs (Beverly, Mass.), and Taq
polymerase was purchased from Promega (Madison, Wis.). Lambda
concatemers (Bio-Rad) were used as size markers. E. coli O157 strain G5244 (Bio-Rad) was used as a control and for
standardization of gels. Banding patterns were analyzed and comparisons
made using Molecular Analyst Fingerprinting software (Bio-Rad),
employing the Dice similarity coefficient in conjunction with the
unweighted pair group method using arithmetic averages (UPGMA) for
clustering. Isolates were grouped into types that likely had the
same origin based on fingerprint pattern similarities. Types were
defined strictly as isolates that grouped together and had one; one and
two; or one, two, and three band differences among their fingerprints
(approximately >95% Dice similarity) (Fig.
1). Isolates with two or three band
differences in their fingerprints, but not grouping with isolates that
had a one-band difference, were classified as distinct types. Subtypes
were defined as isolates with identical fingerprint patterns.
View larger version (34K):
[in this window]
[in a new window]
|
FIG. 1.
Relatedness of E. coli O157
isolates. The dendrogram was generated using the Dice coefficient and
UPGMA analysis (see Materials and Methods). The scale at the top of the
dendrogram indicates the level of similarity between isolates; types
include isolates connected at approximately >95% similarity. One
isolate of each distinct genomic pattern (subtype) was included
in the dendrogram; thus, a final branch may represent multiple isolates
as indicated at the end of each branch. The presence of
stx1 is indicated by solid circles; the presence of
stx2 is indicated by solid squares. Subtypes that
include isolates that carry stx1 and stx2
as well as isolates that carry only stx2 are indicated
by open circles and solid squares. The presence of only motile isolates
is indicated by filled triangles. The presence of nonmotile and motile
isolates is indicated by open triangles. The absence of a symbol
indicates the absence of that characteristic from all isolates within
the subtype. The numbers of isolates within each subtype that were
recovered at each sampling site are indicated. Subtype designations
follow the symbols. Abbreviations: St., subtype designations; Preevis.,
preevisceration; Postevis., postevisceration.
|
|
Genomic fingerprint stability.
Fourteen isolates were
selected for analysis of genome stability. These isolates included
pairs from types 23, 4, 6, and 1, each with fingerprints differing by
one band (Fig. 1). Also included were three isolates each from types 39 and 46; each group included two identical isolates from large subtypes
and one isolate with a one-band difference. Cells were recovered from
frozen (
70°C) stocks and heavily streaked onto Trypticase soy agar
(TSA) (Difco Laboratories, Detroit, Mich.), followed by overnight
incubation at 37°C. This was considered the day 1 culture. An
isolated colony was subsequently subcultured (passaged) daily,
alternating TSA and sorbitol MacConkey's agar supplemented with
cefixime (0.5 mg/liter) and potassium tellurite (2.5 mg/liter; Dynal,
Lake Success, N.Y.) (ctSMAC). At days 1, 5, 10, and 15 genomic
DNA fingerprints were prepared as described above. In addition, cells
from the same isolated colony subcultured from TSA to ctSMAC on day 1 were transferred to 3 ml of brilliant green bile broth (Difco) and passaged daily at 37°C. Genomic DNA fingerprints were prepared from
these broth cultures at day 3.
PCR-restriction fragment length polymorphism analysis.
The
presence of the H7 gene was detected in the nonmotile isolates by the
method of Fields et al. (15). In brief, approximately 1.8 kb of the fliC gene was amplified by PCR. Annealing
temperatures were adjusted from 60 to 45°C as necessary.
RsaI digests of the PCR products were examined by agarose
gel electrophoresis for the characteristic H7 banding pattern.
Statistical analyses.
Chi-squared analyses were performed to
compare frequencies of types per group using the general linear module
of SAS (SAS Institute, Inc., Cary, N.C.). Results were considered
significant at P 
0.5.
| |
RESULTS |
Relatedness and distribution of E. coli O157
isolates.
One randomly selected E. coli O157
isolate from each positive sample recovered during a study in beef
processing plants (13) was examined by XbaI
PFGE genomic DNA fingerprinting. A total of 77 different
patterns, or subtypes, were identified. The isolates divided into three
main clusters (see Materials and Methods and Fig. 1). Cluster A
included 76 isolates recovered from five lots during two trips to the
same plant (Fig. 1 and Table 1). Clusters B and C were derived from one branch of the dendrogram and included 191 and 74 isolates, respectively (Fig. 1). A smaller group of tightly
related strains within cluster B included 142 isolates (cluster B1
[Fig. 1]). Two isolates fell outside of the clusters but were more
closely related to clusters B and C (types 43 and 44 [Fig. 1]).
Isolates also were typed or categorized into closely related groups as
described in Materials and Methods. Forty-seven types
were identified.
Clusters A, B, and C included 3, 27, and 15 types,
respectively (Fig.
1). Thirty-seven types included a small number
of isolates and were
predominately found in cluster C and in cluster
B outside of B1 (65 isolates, one to five per type [Fig.
1]).
Types 1, 6, and 46 accounted for 155 of the 343 isolates analyzed
(45.2% [Fig.
1]).
Isolates of type 1 were recovered from samples
taken at all of the
plants (Table
1). Isolates of types 6 and
46 were recovered from
several lots sampled at individual plants
(Table
1). These data do not
indicate that the isolates are endemic
in the plants, because they were
recovered from preharvest samples
in addition to postharvest samples
(Fig.
1). Instead, they may
represent geographically predominant
strains.
The data were examined to determine if specific characteristics were
associated with closely related isolates versus isolates
distributed
throughout the
XbaI PFGE clusters. Isolates with different
Shiga toxin profiles were distributed unevenly among the clusters;
strains carrying only one
stx gene were predominantly found
among
clusters A and C. These clusters included 67% of the subtypes
(56% of the types) with isolates carrying only one
stx
gene, even
though they accounted for just 36% of the total number of
subtypes
(38% of the types). These clusters also accounted for only
19%
of the subtypes (22% of the types) with isolates carrying both
stx genes. Overall, clusters A and C accounted for 81.6% of
the
stx2 isolates, while cluster B included 85.9% of the
stx1 stx2 isolates (Fig.
1). Furthermore, cluster A included
52 of the 73
nonmotile isolates (71.2%), but at least one motile
isolate was
recovered for five of the seven subtypes in this cluster
(Fig.
1). Only one nonmotile isolate grouped into cluster B1 (Fig.
1).
The presence of the H7
fliC gene in all of the nonmotile
isolates
was confirmed by PCR-restriction fragment length polymorphism
analysis (data not
shown).
The data were further examined by chi-square analyses to determine if
various characteristics were associated specifically
with isolates
recovered from particular types of samples (see
Materials and Methods).
Some bias was found between isolates from
various sample sites in the
XbaI PFGE genotype cluster, Shiga
toxin profile, and
motility characteristics (Table
2). Fecal
isolates were underrepresented in cluster B relative to isolates
from
other sample sites. Compared to isolates from other sample
sites,
postevisceration isolates were relatively less prevalent
in cluster C
and more prevalent in cluster A. Also, a substantially
smaller
proportion of fecal isolates than carcass isolates carried
both
stx1 and
stx2. The converse was true for isolates
carrying
only
stx2. Finally, a larger proportion of
preharvest isolates
than postharvest isolates were nonmotile.
Genomic variation among preharvest and postharvest
E. coli O157 isolates overall.
The
variation in genomic fingerprints of isolates recovered both
preharvest and postharvest was examined; fingerprints of both types of
isolates varied substantially (Table 3).
Across all lots with at least one positive sample, there was an average of one new type per 4.6 preharvest isolates and per 6.5 postharvest isolates. Thirty-two types included only preharvest or only postharvest isolates. Twenty of these types included only one isolate, and none
included more than nine isolates (Fig. 1).
In order to discern the potential for the presence of multiple isolate
types in the samples, genomic fingerprints were generated
for a
group of 153 supplemental
E. coli O157 isolates. This
group
was comprised of one to three additional isolates per sample
(depending
on availability) from 61 of the postharvest samples. The
isolates
were from samples in lots C1-3 and B2-2 or were from samples
with
an initial isolate that was (i) of a different genomic
type than
all preharvest isolates in the same lot or (ii) of a
different
genotype than another isolate(s) from a sample(s) of the same
carcass taken at a different processing point(s). One or more
of the
additional isolates from 36 samples had a genomic fingerprint
distinct from that of the initial sample isolate, although in
many
cases the change was by one band (data not shown). As many
as three
genomically distinct isolates were found within a sample.
Seven
new types were identified among the additional
E. coli
O157
isolates.
Genomic variation among preharvest and postharvest
E. coli O157 isolates within lots.
The diversity of E. coli O157 isolates within each lot
was investigated. A surprisingly high number of E. coli
O157 genomic types were recovered within each lot: as many as
six types for 11 preharvest isolates and up to eight types for 15 postharvest isolates (Table 1 and data not shown). Within lots that had
at least one positive sample, an average of one new type was recovered per 2.6 preharvest isolates and per 2.9 postharvest isolates. Larger
lots did not necessarily include more types than smaller lots. Some of
the most distantly related isolates were recovered from the same lots
(e.g., lot C1-1 [Table 1]). Twenty-one lots included at least one
isolate of a unique type within that lot, i.e., an isolate that was
unlike any others recovered within that lot. Unique isolates were
recovered proportionately from preharvest samples as well as
postharvest samples (Table 2).
In order to discern a link between carriage by live animals and carcass
contamination, the 17 lots with at least two preharvest
and two
postharvest isolates were examined by lot for a correlation
between
preharvest and postharvest isolate types (Table
4). Isolates
of the same type were
considered "a match," or "matching." Overall,
within
these 17 lots 68.2% (120 of 176) of the isolates recovered
postharvest
matched at least one preharvest isolate from the same
lot. Statistical
analyses of these data were not possible due
to the large number of
unknown variables, such as the total potential
number of isolate types
per sample.
Only one isolate was examined from each positive sample, leaving open
the possibility that a different isolate from the same
sample would
match within the lot. One to three additional isolates
(depending on
availability) were examined from 42 postharvest
samples for which the
initial isolate did not match any preharvest
isolate within the same
lot. Isolates with different genomic types
were recovered from
24 of these samples. At least one additional
isolate matched a
preharvest isolate from the same lot for eight
of the samples, slightly
increasing, to 72.7% (128 of 176), the
proportion of identified
matches between postharvest and preharvest
isolates. Seven of the
additional isolates were of unique types,
i.e., unlike any other within
the same
lot.
Genomic variation among postharvest E. coli
O157 isolates by carcass.
The fingerprints of isolates from
carcass samples taken throughout processing were compared, in order to
discern if the carcass contamination found later in processing
corresponded to that which was on the same carcass early in processing.
The initial preevisceration and postevisceration isolates examined from
32 of the 49 carcasses contaminated at both processing points (65.3%)
were of the same genomic type, i.e., matched (Table 4).
Additional preevisceration and/or postevisceration isolates from
15 of the carcasses were examined that did not have matching initial
isolates. Matches were revealed between preevisceration and
postevisceration isolates from seven more carcasses. Thus, the detected
proportion of carcasses with postevisceration isolates traceable to
preevisceration isolates increased to 79.6% (39 of 49). Again,
statistical analyses were impossible.
Upon first analysis, four of the isolates from the six final samples
positive for
E. coli O157 matched those recovered
preevisceration
from the same carcass. For the remaining two carcasses,
additional
isolates from the positive carcass samples also did not
match.
However, based on the information above, it is possible that
additional
matches were present and simply remained
undetected.
Stability of E. coli O157 genomic
fingerprints.
In order to determine the stability of
the E. coli O157 fingerprints, 14 isolates were
passaged (subcultured) as described in Materials and Methods. There was
no change in the XbaI genomic fingerprint patterns
for 12 of these isolates during passaging (data not shown).
A new
XbaI PFGE pattern appeared during passaging of isolate
60AC3. A single band of approximately 212 kb was lost after a
3-day
broth passaging or by day 5 of passaging on plates (Fig.
2A). This isolate originally was the only
one of subtype 23a.
The new pattern derived from 60AC3 after passaging
was the same
as the pattern of subtype 23b including 21 isolates. The
XbaI
PFGE pattern of isolate 48AB1, of subtype 23b, did not
change
during passaging (data not shown).
View larger version (63K):
[in this window]
[in a new window]
|
FIG. 2.
Genomic fingerprints from passaged isolates. (A)
Fingerprints of strain 60AC3 during passaging. The isolate and
passaging day are indicated for each lane. The arrow indicates the band
that was lost during passaging. (B) Fingerprints of strain 234AB1
during passaging. Size standards are lambda concatemers (Bio-Rad);
sizes are given in kilobases. The isolate and passaging day are
indicated for each lane. Dashed lines indicate bands lost from the
original fingerprint, solid lines indicate bands gained in comparison
to the original fingerprint. Bands marked between lanes 1 and 2 pertain
to changes in the day 1 pattern, bands marked outside of lanes 1 and 5 pertain to changes between the original fingerprint and the day 3, 5, and 10 fingerprints. BGB, brilliant green bile broth.
|
|
After 1 day of passaging, the fingerprint from isolate
234AB1 (subtype 6c) had a four-band difference from the original
pattern
for that isolate; slight changes in size were noted for
two bands
(Fig.
2B). After further passaging in broth or on
plates, a seven-band
difference altogether occurred in the
isolate 234AB1 fingerprints.
One of the two previously altered bands
was lost, as were two
large bands. In addition, new bands appeared.
This resulted in
an apparent net loss of approximately 227 kb of DNA.
The new pattern
was unlike any others observed in this study and
therefore constituted
a new, closely related
type.
| |
DISCUSSION |
E. coli O157 genomic variability.
Many genomic types and subtypes of E. coli O157
have been identified by XbaI PFGE fingerprinting in this and
other studies (3, 7, 14, 21, 23, 31, 43). The nature and
significance of these genomic differences remain unclear.
Bacteriophage have been implicated as a causative agent, and the
genomic variations have been suggested to be related to the
direct or indirect ability to cause disease (30). A study
using multilocus enzyme electrophoresis analysis of housekeeping genes
detected little difference among E. coli O157 strains
(51), which argues that the diversity in PFGE patterns is
due in large part to nonevolutionary events such as horizontal DNA
transfer. However, inversions, translocations, and point mutations
could have caused some of the genotypic variation (9, 47).
For example, the fingerprint pattern changes that resulted from
repeated subculturing of isolate 234AB1 were suggestive of inversion or
translocation events, as well as a loss of large amounts of DNA.
Changes in genome size may not be uncommon in Enterobacteriaceae (8, 50) and can be the
result of duplications, deletions, and horizontal DNA transfer events
associated with elements such as conjugative transposons, insertion
elements, and lysogenic bacteriophage (9, 47). Although
duplications may not be unlikely in E. coli
(22), numerous studies have suggested that horizontal DNA
transfer by a variety of mechanisms occurs in and across many bacterial
species in vivo (16, 25, 27, 35, 38-40, 48). Rode et al.
(45) found that a sepsis-associated strain of
E. coli and a uropathogenic strain of E. coli had distinct deletion and insertion events associated with
novel DNA in comparison to E. coli K-12 rather than
genomic rearrangements, which suggests horizontal DNA transfer
events had occurred. DNA transfer by the E. coli O157
Shiga toxin phages in vivo has been reported (1), and even
the E. coli K-12 genome contains evidence of
substantial horizontal or lateral DNA transfer (8, 32).
This study is the first to report that motility as well as carriage of
stx genes corresponded to specific genotypic clusters.
The
only similar observation was made by Karch et al. (
28),
who reported a group of distinct patterns for sorbitol-fermenting
E. coli O157:H
isolates. For
the most part, the nonmotile isolates recovered
in this study did have
XbaI PFGE genotypes identical to those
of some motile,
H7
+ isolates. These nonmotile isolates may have
undetected mutations
or may simply prefer different conditions to
stimulate expression
of the H7 antigen and motility, such as passaging
in semisolid
media. Reports differ on the correlation of
E. coli O157 Shiga
toxin profiles with subtypes (
33,
43). The data reported here
demonstrated a strong association
between Shiga toxin profile
and
XbaI PFGE subtype, although
two subtypes included both
stx2 and
stx1 stx2
isolates (subtypes 1d and 4a [Fig.
1]). The absence
of
stx1 in the
stx2 isolates was confirmed by colony
blotting
and an additional PCR procedure (data not shown). Murase et
al.
(
36) noted by a different PFGE technique that loss of
an ~70-kb
band corresponded to loss of either
stx gene. It
is possible that
the presence or absence of a band this size was not
clearly distinguished
in these analyses. Alternatively, other phage may
be present in
the strains lacking
stx1, such that
their size and position masked
the absence of an
stx1 phage.
Two or three main
E. coli O157 relatedness clusters
have been found in this and other studies by various genomic
analyses
(
30,
33,
51). It has been suggested that
genomic variation
is related to the ability of the organism to
cause disease (
30).
It is possible that each cluster
consists of one or a few core
genotypes that are primarily altered by
independent horizontal
DNA transfer events resulting in the multitude
of subtypes and
types. The detection of a few predominant genotypes and
many less
populous genotypes in this and other studies is in keeping
with
this hypothesis (
31,
43). The derivation of several
genotypes
from a single genotype during in vivo passaging of
E. coli O157
has been observed, although the causes of
the alterations were
not determined (
4,
29). The
derivation of two new genotypes
from a single
E. coli
O157 isolate was also observed during in
vitro passaging in this
study.
Up to four types or subtypes of
E. coli O157 were found
per lot in cattle feces (data not shown). This observation is similar
to the results of previous studies (
31,
43). Rice et al.
also
(
43) reported no relationship between the number of
subtypes
and the number of samples per farm. The number of preharvest
E. coli O157
XbaI PFGE types recovered per
lot increased to as many
as six when hide isolates were added to the
analysis. Therefore,
the cattle may have actually carried or been
exposed to more types
of
E. coli O157 than those
recovered from feces. Exposure of hides
to
E. coli O157
in the feces of wild animals, potential difficulties
recovering all
possible types from different sources (i.e., feces
or hides), or the
possible inability of all
E. coli O157 types
to survive
under various conditions could account for the extra
variation.
Tracking of E. coli O157 carcass
contamination.
Chapman et al. (12) studied
E. coli O157 contamination of carcasses at a South
Yorkshire abattoir and found that 30% of the carcasses (seven
carcasses) from cattle with feces positive for E. coli
O157 were contaminated, and 8% of the adjacent carcasses (two
carcasses) also were contaminated. Direct contamination and cross-contamination were implicated by phage typing and plasmid profiles of the strains. In addition, Byrne et al. (10)
showed that spreading E. coli O157:H7-inoculated feces
onto hides resulted in contamination of the carcass and workers' hands
and knives. Because of the relatively low number of E. coli O157-positive hides detected, the data from this study were
insufficient to provide evidence that either hides or feces were more
likely to be the direct source of E. coli O157 on the
carcasses. The data did clearly demonstrate a strong relationship
between preharvest and postharvest isolates within a lot, corroborating
the previous observation of an overall positive correlation between
preharvest and postharvest prevalence by lot (13). The
isolate in vitro-passaging data suggest that more of the carcass
isolates could have originated from preharvest isolates within the same
lot, but the matches were not identified because of genomic
alterations between individual sample isolations that were
sufficient to change the designated XbaI PFGE type.
Expanding the typing limitations may have revealed additional
valid matches but likely also would have misidentified matches not
reflective of the actual source of contamination. In addition, the
ratios of types to numbers of isolates suggest that if additional
preharvest isolates had been recovered, more potentially matching types
might have been found.
The presence of more types of
E. coli O157 in
postharvest samples than in preharvest samples suggests that additional
types
of
E. coli O157 were present preharvest and were
not identified
in this study. Since a proportionate number of unique
isolates
were recovered preharvest as well as postharvest, and a
proportionate
number of types were unique to preharvest
isolates and postharvest
isolates, the data do not suggest
cross-contamination. Cross-contamination
of the carcasses
presumably would be due to animals within lots
entering the plant
earlier in the day. Cross-contamination previously
was suggested as the
source of
E. coli O157 on carcasses within
lots that
did not include positive preharvest samples (
13).
When the
XbaI PFGE genotypes were examined with regard to matches
between postharvest and preharvest isolates across lots, the data
were
found to be inconclusive. In some cases preharvest (animal)
isolates
matched postharvest (carcass) isolates from an earlier
lot, and in
several instances preharvest isolates from different
lots were of the
same type (data not shown). Therefore, cross-contamination
between lots
could not be discerned. However, for each trip to
a plant, there was no
evidence of cross-contamination worsening
over the course of the
sampling period. The numbers of matching
preharvest and postharvest
isolates did not decrease for lots
processed later in the day, and
carcasses from later lots were
not more frequently contaminated than
carcasses from lots processed
earlier in the day. The latter
observations need to be interpreted
with caution, though, as the study
was not designed to compare
the data in this manner. Overall, while
cross-contamination probably
occurred to a limited extent, some of the
prevalence data may
have been reflective of a higher difficulty in
recovering
E. coli O157 from preharvest sources.
Competing microflora in fecal and
hide samples necessitates the use of
more-stringent enrichment
conditions for these samples compared to
carcass samples (
13).
For many postharvest samples, more than one genomic type of
E. coli O157 was recovered. These data could suggest
that individual
carcass contamination originated from multiple sources
or that
contamination sources (feces and hide) may harbor multiple
isolates
that can be transferred to the carcass in a single
contamination
event. Work identifying
E. coli O157 of
multiple genomic types
in individual cattle feces supports the
latter interpretation
(
31). Only one isolate was examined
per preharvest sample in
this study, so additional types may have been
present in hide
or fecal samples and remained
undetected.
Further studies are needed to detail the potential for
carcass-to-carcass cross-contamination. We did not sample adjacent
carcasses, so the potential for direct cross-contamination was
not
discerned. The clear majority of carcass contamination with
E. coli O157 occurred prior to any direct contact
between carcasses
(
13), although cross-contamination
during very early processing
steps via equipment and workers could have
occurred (
10). Regarding
potential cross-contamination
later in processing, most of the
carcasses found to be contaminated
later in processing had been
contaminated earlier in processing (49 of
59 [reference
13]).
In addition, the PFGE patterns of
most isolates from the later
processing samples matched those of
isolates recovered from the
same carcass earlier in processing.
Therefore,
E. coli O157 contamination
found later in
processing was due largely to contamination that
occurred early in
processing and not to carcass-to-carcass
contamination.
Several additional and particularly interesting observations were made
during this study. First, postevisceration isolates
were
underrepresented in cluster C. Second, predominant preharvest
and
postharvest
E. coli O157 types within a lot often were
not
the same (data not shown). For example in lot A1-1, one type
included
12 of 15 preharvest isolates and only 2 of 13 postharvest
isolates,
and a second type included only 2 preharvest isolates but
included
11 postharvest isolates. (The third type included one
preharvest
isolate.) Third, fecal isolates were less likely than
isolates
from other samples to group into cluster B. Fecal isolates
also
were less likely to carry both
stx genes, being more
likely to
carry only
stx2. Finally, compared to carcass
isolates, fecal
and hide isolates were slightly more likely
to be nonmotile. Further
experiments are necessary to
examine these phenomena. The data
could be evidence
that some
E. coli O157 genomic types are
more
successful at passing through processing steps or are more easily
transferred to the carcass or could reflect the fact that different
types are more easily recovered from the different sample sites
based
on recovery and/or enrichment methods. If these hypotheses
were true,
one would expect that a limited number of preevisceration
types or
preharvest types would be found postevisceration or postharvest,
respectively. However, of the preevisceration isolates overall,
67.6%
were of the same type as postevisceration isolates, and
within the 17 lots used to analyze preharvest/postharvest isolate
relatedness, 72.1%
of the preharvest isolates were of types also
found
postharvest.
In summary, this study tracked carcass contamination by
E. coli O157 in processing plants within the United States. A strong
association between
E. coli O157 carried by live
animals and on
carcasses within the same lot was first demonstrated by
prevalence
data (
13).
XbaI PFGE genotyping data
have further implied that
E. coli O157 found on
carcasses is primarily the result of transfer
within a lot rather than
cross-contamination between lots, although
some cross-contamination may
occur (
13). Furthermore, the tracking
data based on
XbaI PFGE genotyping confirm that the majority of
E. coli O157 found on the carcass is the result of
preevisceration
contamination, despite a dramatic reduction throughout
processing
in the number of carcasses contaminated (
13).
Taken together,
these data indicate the need to apply additional
in-plant intervention
strategies aimed at preventing direct
contamination of the carcasses
early in processing. In addition, a
substantial level of genomic
variation was observed among the
E. coli O157 isolates recovered
during this study. The
significance of this divergence remains
to be seen, although it has
been implied to affect, either directly
or indirectly, the ability of
the organism to cause disease in
humans (
30). Work is in
progress to investigate the relationships
between these genomic
differences and various aspects of the organism's
ability to
eventually cause disease, such as relative virulence
or survival during
processing, storage, and
cooking.
| |
ACKNOWLEDGMENTS |
We thank Kim Kucera, Sandy Fryda-Bradley, and Ron Mlejnek for
excellent technical assistance and Carol Grummert for excellent secretarial assistance.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: USDA, ARS, Roman
L. Hruska U.S. Meat Animal Research Center, P.O. Box 166, Spur 18D, Clay Center, NE 68933-0166. Phone: (402) 762-4228. Fax: (402) 762-4149. E-mail: gallagher{at}emailmarc.usda.gov.
Present address: USDA, ARS, Russell Research Center, Athens, GA
30604-5677.
Present address: USDA, ARS, South Plains Agricultural
Research Center, College Station, TX 77845.
| |
REFERENCES |
| 1.
|
Acheson, D. W. K.,
J. Reidl,
X. Zhang,
G. T. Keusch,
J. J. Mekalanos, and M. K. Waldor.
1998.
In vivo transduction with Shiga toxin 1-encoding phage.
Infect. Immun.
66:4496-4498[Abstract/Free Full Text].
|
| 2.
|
Ackman, D.,
S. Marks,
P. Mack,
M. Caldwell,
T. Root, and G. Birkhead.
1997.
Swimming-associated haemorrhagic colitis due to Escherichia coli O157:H7 infection: evidence of prolonged contamination of a fresh water lake.
Epidemiol. Infect.
119:1-8[CrossRef][Medline].
|
| 3.
|
Akiba, M.,
T. Masuda,
T. Sameshima,
K. Katsuda, and M. Nakazawa.
1999.
Molecular typing of Escherichia coli O157:H7 (H-) isolates from cattle in Japan.
Epidemiol. Infect.
122:337-341[CrossRef][Medline].
|
| 4.
|
Akiba, M.,
T. Sameshima, and M. Nakazawa.
2000.
Clonal turnover of enterohemorrhagic Escherichia coli O157:H7 in experimentally infected cattle.
FEMS Microbiol. Lett.
184:79-83[CrossRef][Medline].
|
| 5.
|
Banatvala, N.,
A. R. Magnano,
M. L. Carter,
T. J. Barrett,
W. F. Bibb,
L. L. Vasile,
P. Mshar,
M. A. Lambert-Fair,
J. H. Green,
N. H. Bean, and R. V. Tauxe.
1996.
Meat grinders and molecular epidemiology: two supermarket outbreaks of Escherichia coli O157:H7 infection.
J. Infect. Dis.
173:480-483[Medline].
|
| 6.
|
Bell, R. G.
1997.
Distribution and sources of microbial contamination on beef carcasses.
J. Appl. Microbiol.
82:292-300[CrossRef][Medline].
|
| 7.
|
Bender, J. B.,
C. W. Hedberg,
J. M. Besser,
D. J. Boxrud,
K. L. MacDonald, and M. T. Osterholm.
1997.
Surveillance for Escherichia coli O157:H7 infections in Minnesota by molecular subtyping.
N. Engl. J. Med.
337:388-394[Abstract/Free Full Text].
|
| 8.
|
Bergthorsson, U., and H. Ochman.
1998.
Distribution of chromosome length variation in natural isolates of Escherichia coli.
Mol. Biol. Evol.
15:6-16[Abstract].
|
| 9.
|
Brunder, W., and H. Karch.
2000.
Genome plasticity in Enterobacteriaceae.
Int. J. Med. Microbiol.
290:153-165[Medline].
|
| 10.
|
Byrne, C. M.,
D. J. Bolton,
J. J. Sheridan,
D. A. McDowell, and I. S. Blair.
2000.
The effects of preslaughter washing on the reduction of Escherichia coli O157:H7 transfer from cattle hides to carcasses during slaughter.
Lett. Appl. Microbiol.
30:142-145[CrossRef][Medline].
|
| 11.
|
Centers for Disease Control and Prevention.
1998.
Standardized molecular subtyping of foodborne bacterial pathogens by pulsed-field gel electrophoresis. CDC training manual.
Foodborne and Diarrheal Diseases Branch, Centers for Disease Control and Prevention, Atlanta, Ga.
|
| 12.
|
Chapman, P. A.,
C. A. Siddons,
D. J. Wright,
P. Norman,
J. Fox, and E. Crick.
1993.
Cattle as a possible source of verocytotoxin-producing Escherichia coli O157 infections in man.
Epidemiol. Infect.
111:439-447[Medline].
|
| 13.
|
Elder, R. O.,
J. E. Keen,
G. R. Siragusa,
G. A. Barkocy-Gallagher,
M. Koohmaraie, and W. W. Laegreid.
2000.
Correlation of enterohemorrhagic Escherichia coli prevalence in feces, hides, and carcasses of beef cattle during processing.
Proc. Natl. Acad. Sci. USA
97:2999-3003[Abstract/Free Full Text].
|
| 14.
|
Faith, N. G.,
J. A. Shere,
R. Brosch,
K. W. Arnold,
S. E. Ansay,
M.-S. Lee,
J. B. Luchansky, and C. W. Kaspar.
1996.
Prevalence and clonal nature of Escherichia coli O157:H7 on dairy farms in Wisconsin.
Appl. Environ. Microbiol.
62:1519-1525[Abstract].
|
| 15.
|
Fields, P. I.,
K. Blom,
H. J. Hughes,
L. O. Helsel,
P. Feng, and B. Swaminathan.
1997.
Molecular characterization of the gene encoding H antigen in Escherichia coli and development of a PCR-restriction fragment length polymorphism test for identification of E. coli O157:H7 and O157:NM.
J. Clin. Microbiol.
35:1066-1070[Abstract].
|
| 16.
|
Garrigues-Jeanjean, N.,
A. Wittmer,
M. F. Ouriet, and Y. Duval-Iflah.
1999.
Transfer of the shuttle vector pRRI207 between Escherichia coli and Bacteroides spp. in vitro and in vivo in the digestive tract of axenic mice and in gnotoxenic mice inoculated with a human microflora.
FEMS Microbiol. Ecol.
29:33-43.
|
| 17.
|
Gill, C. O., and J. C. McGinnis.
1999.
Improvement of the hygienic performance of the hindquarters skinning operations at a beef packing plant.
Int. J. Food Microbiol.
51:123-132[CrossRef][Medline].
|
| 18.
|
Gill, C. O.,
J. C. McGinnis, and M. Badoni.
1996.
Assessment of the hygienic characteristics of a beef carcass dressing process.
J. Food Prot.
59:136-140.
|
| 19.
|
Gill, C. O.,
J. C. McGinnis, and M. Badoni.
1996.
Use of total or Escherichia coli counts to assess the hygienic characteristics of a beef carcass dressing process.
Int. J. Food Microbiol.
31:181-196[CrossRef][Medline].
|
| 20.
|
Giovannacci, I.,
C. Ragimbeau,
S. Queguiner,
G. Salvat,
J.-L. Vendeuvre,
V. Carlier, and G. Ermel.
1999.
Listeria monocytogenes in pork slaughtering and cutting plants. Use of RAPD, PFGE and PCR-REA for tracing and molecular epidemiology.
Int. J. Food Microbiol.
53:127-140[CrossRef][Medline].
|
| 21.
|
Grif, K.,
H. Karch,
C. Schneider,
F. D. Daschner,
L. Beutin,
T. Cheasty,
H. Smith,
B. Rowe,
M. P. Dierich, and F. Allerberger.
1998.
Comparative study of five different techniques for epidemiological typing of Escherichia coli O157.
Diagn. Microbiol. Infect. Dis.
32:165-176[CrossRef][Medline].
|
| 22.
|
Haack, K. R., and J. R. Roth.
1995.
Recombination between chromosomal IS200 elements supports frequent duplication formation in Salmonella typhimurium.
Genetics
141:1245-1252[Abstract].
|
| 23.
|
Heuvelink, A. E.,
F. L. A. M. Van Den Biggelaar,
E. De Boer,
R. G. Herbes,
W. J. G. Melchers,
J. H. J. Huis in 'T Veld, and L. A. H. Monnens.
1998.
Isolation and characterization of verocytotoxin-producing Escherichia coli O157 strains from Dutch cattle and sheep.
J. Clin. Microbiol.
36:878-882[Abstract/Free Full Text].
|
| 24.
|
Hilborn, E. D.,
J. H. Mermin,
P. A. Mshar,
J. L. Hadler,
A. Voetsch,
C. Wojtkunski,
M. Swartz,
R. Mshar,
M. A. Lambert-Fair,
J. A. Farrar,
M. K. Glynn, and L. Slutsker.
1999.
A multistate outbreak of Escherichia coli O157:H7 infections associated with consumption of mesclun lettuce.
Arch. Intern. Med.
159:1758-1764[Abstract/Free Full Text].
|
| 25.
|
Jain, R.,
M. C. Rivera, and J. A. Lake.
1999.
Horizontal gene transfer among genomes: the complexity hypothesis.
Evolution
96:3801-3806.
|
| 26.
|
Johnson, R. P.,
R. C. Clark,
J. B. Wilson,
S. C. Read,
K. Rahn,
S. A. Renwick,
K. A. Sandhu,
D. Alves,
M. A. Karmali,
H. Lior,
S. A. McEwen,
J. S. Spika, and C. L. Gyles.
1996.
Growing concerns and recent outbreaks involving non-O157:H7 serotypes of verotoxigenic Escherichia coli.
J. Food Prot.
59:1112-1122.
|
| 27.
|
Karaolis, D. K. R.,
S. Somara,
D. R. Maneval, Jr.,
J. A. Johnson, and J. B. Kaper.
1999.
A bacteriophage encoding a pathogenicity island, a type-IV pilus and a phage receptor in cholera bacteria.
Nature
399:375-379[CrossRef][Medline].
|
| 28.
|
Karch, H.,
H. Böhm,
H. Schmidt,
F. Gunzer,
S. Aleksic, and J. Heesemann.
1993.
Clonal structure and pathogenicity of Shiga-like toxin-producing, sorbitol-fermenting Escherichia coli O157:H.
J. Clin. Microbiol.
31:1200-1205[Abstract/Free Full Text].
|
| 29.
|
Karch, H.,
H. Rüssmann,
H. Schmidt,
A. Schwarzkopf, and J. Heesemann.
1995.
Long-term shedding and clonal turnover of enterohemorrhagic Escherichia coli O157 in diarrheal diseases.
J. Clin. Microbiol.
33:1602-1605[Abstract].
|
| 30.
|
Kim, J.,
J. Nietfeldt, and A. K. Benson.
1999.
Octamer-based genome scanning distinguishes a subpopulation of Escherichia coli O157:H7 strains in cattle.
Proc. Natl. Acad. Sci. USA
96:13288-13293[Abstract/Free Full Text].
|
| 31.
|
Laegreid, W. W.,
R. O. Elder, and J. E. Keen.
1999.
Prevalence of Escherichia coli O157:H7 in range beef calves at weaning.
Epidemiol. Infect.
123:291-298[CrossRef][Medline].
|
| 32.
|
Lawrence, J. G., and H. Ochman.
1998.
Molecular archaeology of the Escherichia coli genome.
Proc. Natl. Acad. Sci. USA
95:9413-9417[Abstract/Free Full Text].
|
| 33.
|
Lee, M.-S.,
C. W. Kaspar,
R. Brosch,
J. Shere, and J. B. Luchansky.
1996.
Genomic analysis using pulsed-field gel electrophoresis of Escherichia coli O157:H7 isolated from dairy calves during the United States national dairy heifer evaluation project (1991-1992).
Vet. Microbiol.
48:223-230[CrossRef][Medline].
|
| 34.
|
McEvoy, J. M.,
A. M. Doherty,
M. Finnerty,
J. J. Sheridan,
L. McGuire,
I. S. Blair,
D. A. McDowell, and D. Harrington.
2000.
The relationship between hide cleanliness and bacterial numbers on beef carcasses at a commercial abattoir.
Lett. Appl. Microbiol.
30:390-395[CrossRef][Medline].
|
| 35.
|
Muñoz, R.,
E. García, and R. López.
1998.
Evidence for horizontal transfer from Streptococcus to Escherichia coli of the kfiD gene encoding the K5-specific UDP-glucose dehydrogenase.
J. Mol. Evol.
46:432-436[CrossRef][Medline].
|
| 36.
|
Murase, T.,
S. Yamai, and H. Watanabe.
1999.
Changes in pulsed-field gel electrophoresis patterns in clinical isolates of enterohemorrhagic Escherichia coli O157:H7 associated with loss of Shiga toxin genes.
Curr. Microbiol.
38:48-50[CrossRef][Medline].
|
| 37.
|
Nataro, J. P., and J. B. Kaper.
1998.
Diarrheagenic Escherichia coli.
Clin. Microbiol. Rev.
11:142-201[Abstract/Free Full Text].
|
| 38.
|
Netherwood, T.,
R. Bowden,
P. Harrison,
A. G. O'Donnell,
D. S. Parker, and H. J. Gilbert.
1999.
Gene transfer in the gastrointestinal tract.
Appl. Environ. Microbiol.
65:5139-5141[Abstract/Free Full Text].
|
| 39.
|
Nikolich, M. P.,
G. Hong,
N. B. Shoemaker, and A. A. Salyers.
1994.
Evidence for natural horizontal transfer of tetQ between bacteria that normally colonize humans and bacteria that normally colonize livestock.
Appl. Environ. Microbiol.
60:3255-3260[Abstract/Free Full Text].
|
| 40.
|
Ochman, H.,
J. G. Lawrence, and E. A. Groisman.
2000.
Lateral gene transfer and the nature of bacterial innovation.
Nature
405:299-304[CrossRef][Medline].
|
| 41.
|
Ostroff, S. M.,
P. I. Tarr,
M. A. Neill,
J. H. Lewis,
N. Hargrett-Bean, and J. M. Kobayashi.
1989.
Toxin genotypes and plasmid profiles as determinants of systemic sequelae in Escherichia coli O157:H7 infections.
J. Infect. Dis.
160:994-998[Medline].
|
| 42.
|
Preston, M. A.,
W. Johnson,
R. Khakhria, and A. Borczyk.
2000.
Epidemiologic subtyping of Escherichia coli serogroup O157 strains isolated in Ontario by phage typing and pulsed-field gel electrophoresis.
J. Clin. Microbiol.
38:2366-2368[Abstract/Free Full Text].
|
| 43.
|
Rice, D. H.,
K. M. McMenamin,
L. C. Pritchett,
D. D. Hancock, and T. E. Besser.
1999.
Genetic subtyping of Escherichia coli O157 isolates from 41 Pacific Northwest USA cattle farms.
Epidemiol. Infect.
122:479-484[CrossRef][Medline].
|
| 44.
|
Ridell, J., and H. Korkeala.
1993.
Special treatment during slaughtering in Finland of cattle carrying an excessive load of dung; meat hygienic aspects.
Meat Sci.
35:223-228[CrossRef].
|
| 45.
|
Rode, C. K.,
L. J. Melkerson-Watson,
A. T. Johnson, and C. A. Bloch.
1999.
Type-specific contributions to chromosome size differences in Escherichia coli.
Infect. Immun.
19:230-236.
|
| 46.
|
Rodrigue, D. C.,
E. E. Mast,
K. D. Greene,
J. P. Davis,
M. A. Hutchinson,
J. G. Wells,
T. J. Barrett, and P. M. Griffin.
1995.
A university outbreak of Escherichia coli O157:H7 infections associated with roast beef and an unusually benign clinical course.
J. Infect. Dis.
172:1122-1125[Medline].
|
| 47.
|
Sanderson, K. E., and S.-L. Liu.
1998.
Chromosomal rearrangements in enteric bacteria.
Electrophoresis
19:569-572[CrossRef][Medline].
|
| 48.
|
Shoemaker, N. B.,
G.-R. Wang, and A. A. Salyers.
1992.
Evidence for natural transfer of a tetracycline resistance gene between bacteria from the human colon and bacteria from the bovine rumen.
Appl. Environ. Microbiol.
58:1313-1320[Abstract/Free Full Text].
|
| 49.
|
Takeda, Y.
1997.
Enterohaemorrhagic Escherichia coli.
World Health Stat. Q.
50:74-80[Medline].
|
| 50.
|
Thong, K. L.,
S. D. Puthucheary, and T. Pang.
1997.
Genome size variation among recent human isolates of Salmonella typhi.
Res. Microbiol.
148:229-235[Medline].
|
| 51.
|
Whittam, T. S.,
M. L. Wolfe,
I. K. Wachsmuth,
F. Ørskov,
I. Ørskov, and R. A. Wilson.
1993.
Clonal relationships among Escherichia coli strains that cause hemorrhagic colitis and infantile diarrhea.
Infect. Immun.
61:1619-1629[Abstract/Free Full Text].
|
Applied and Environmental Microbiology, September 2001, p. 3810-3818, Vol. 67, No. 9
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.9.3810-3818.2001
This article has been cited by other articles:
-
Stephens, T. P., McAllister, T. A., Stanford, K.
(2009). Perineal swabs reveal effect of super shedders on the transmission of Escherichia coli O157:H7 in commercial feedlots. J ANIM SCI
87: 4151-4160
[Abstract]
[Full Text]
-
Brichta-Harhay, D. M., Guerini, M. N., Arthur, T. M., Bosilevac, J. M., Kalchayanand, N., Shackelford, S. D., Wheeler, T. L., Koohmaraie, M.
(2008). Salmonella and Escherichia coli O157:H7 Contamination on Hides and Carcasses of Cull Cattle Presented for Slaughter in the United States: an Evaluation of Prevalence and Bacterial Loads by Immunomagnetic Separation and Direct Plating Methods. Appl. Environ. Microbiol.
74: 6289-6297
[Abstract]
[Full Text]
-
Standley, T., Paterson, J., Skinner, K., Rainey, B., Roberts, A., Geary, T., Smith, G., White, R.
(2008). The Use of an Experimental Vaccine in Gestating Beef Cows to Reduce the Shedding of Escherichia coli O157:H7 in the Newborn Calf. Professional Animal Scientist
24: 260-263
[Abstract]
-
Shea, K. M., Committee on Environmental Health, , Committee on Infectious Diseases,
(2004). Nontherapeutic Use of Antimicrobial Agents in Animal Agriculture: Implications for Pediatrics. Pediatrics
114: 862-868
[Abstract]
[Full Text]
-
Dodd, C. C., Sanderson, M. W., Sargeant, J. M., Nagaraja, T. G., Oberst, R. D., Smith, R. A., Griffin, D. D.
(2003). Prevalence of Escherichia coli O157 in Cattle Feeds in Midwestern Feedlots. Appl. Environ. Microbiol.
69: 5243-5247
[Abstract]
[Full Text]
-
Aslam, M., Nattress, F., Greer, G., Yost, C., Gill, C., McMullen, L.
(2003). Origin of Contamination and Genetic Diversity of Escherichia coli in Beef Cattle. Appl. Environ. Microbiol.
69: 2794-2799
[Abstract]
[Full Text]
-
Davis, M. A., Hancock, D. D., Besser, T. E., Call, D. R.
(2003). Evaluation of Pulsed-Field Gel Electrophoresis as a Tool for Determining the Degree of Genetic Relatedness between Strains of Escherichia coli O157:H7. J. Clin. Microbiol.
41: 1843-1849
[Abstract]
[Full Text]
-
Renter, D. G., Sargeant, J. M., Oberst, R. D., Samadpour, M.
(2003). Diversity, Frequency, and Persistence of Escherichia coli O157 Strains from Range Cattle Environments. Appl. Environ. Microbiol.
69: 542-547
[Abstract]
[Full Text]
-
Arthur, T. M., Barkocy-Gallagher, G. A., Rivera-Betancourt, M., Koohmaraie, M.
(2002). Prevalence and Characterization of Non-O157 Shiga Toxin-Producing Escherichia coli on Carcasses in Commercial Beef Cattle Processing Plants. Appl. Environ. Microbiol.
68: 4847-4852
[Abstract]
[Full Text]
Top
Abstract
Introduction
