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Applied and Environmental Microbiology, December 2003, p. 7409-7413, Vol. 69, No. 12
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.12.7409-7413.2003
Copyright © 2003, American
Society for
Microbiology. All Rights Reserved.
Genetic Diversity of Campylobacter jejuni Isolates from Farm Animals and the Farm Environment
F. M. Colles,1 K. Jones,2 R. M. Harding,3 and M. C. J. Maiden1*
Departments
of Zoology,1
Biological
Anthropology, University of Oxford, Oxford
OX1 3SY,3
Department of Biological
Sciences, Lancaster University, Lancaster LA1 4YQ,United Kingdom2
Received 10 April 2003/
Accepted 8 September 2003

ABSTRACT
The
genetic diversity of
Campylobacter jejuni isolates from
farm
animals and their environment was investigated by multilocus
sequence
typing (MLST). A total of 30 genotypes, defined by
allelic profiles
(assigned to sequence types [STs]), were found
in 112
C.
jejuni isolates originating in poultry, cattle, sheep,
starlings,
and slurry. All but two of these genotypes belonged
to one of nine
C. jejuni clonal complexes previously identified
in isolates
from human disease and retail food samples and one
clonal complex
previously associated with an environmental source.
There was some
evidence for the association of certain clonal
complexes with
particular farm animals: isolates belonging to
the ST-45 complex
predominated among poultry isolates but were
absent among sheep
isolates, while isolates belonging to the
ST-61 and ST-42 complexes
were predominant among sheep isolates
but were absent from the poultry
isolates. In contrast, ST-21
complex isolates were distributed among
the different isolation
sources. Comparison with MLST data from 91
human disease isolates
showed small but significant genetic
differentiation between
the farm and human isolates; however,
representatives of six
clonal complexes were found in both samples.
These data demonstrate
that MLST and the clonal complex model can be
used to identify
and compare the genotypes of
C. jejuni
isolates from farm animals
and the environment with those from retail
food and human
disease.

INTRODUCTION
Campylobacter jejuni continues to be the most common
etiological
agent of bacterial gastroenteritis in the developed world,
with
an estimated 2.5 million cases occurring per year in the United
States
(
22) and over
54,000 cases reported to the United Kingdom Public
Health Laboratory
Service during 2001 (
1).
It is also a major
problem in developing countries, particularly among
young children
(
7). In
contrast to many other food-borne bacterial pathogens,
the majority of
cases of campylobacteriosis in humans are considered
to be sporadic,
with few cases of disease traceable to point
sources. It is thought
that the consumption of inadequately
cooked contaminated meat,
particularly that from poultry, is
a major source of human infection,
but this has not been established
unequivocally
(
8). Investigations into
the epidemiology of human
infection by this bacterium have been
complicated by a number
of factors, including its high genetic and
antigenic diversity
(
2)
and its wide, perhaps ubiquitous, distribution. In addition,
serological
typing methods have lacked reproducibility among
laboratories
and discriminatory power, with a large number of isolates
reported
as untypeable
(
12,
28,
41).
C.
jejuni is an apparently harmless commensal of the gastrointestinal
tract of many domestic and wild animals, especially birds
(5). It is also readily
isolated from a range of environmental locations, including soil,
surface water, and the sand of bathing beaches, probably as a
consequence of contact with various contamination sources, including
animal feces (3,
5,
20,
26,
30). Environmental
reservoirs of C. jejuni can act as sources of infection for
humans; for example, wild birds pecking milk bottle tops have been
shown to cause outbreaks of human campylobacteriosis
(24). However, the
contribution of such sources to the overall burden of human disease and
to the colonization of farm animals has not been established. Whether
only some of the C. jejuni present in environmental reservoirs
are pathogenic to humans
(20) and whether all
animals and microenvironments harbor the same strains are important
questions that remain incompletely addressed. Population genetic
approaches that examine differentiation between populations provide a
novel approach to resolving this problem.
Multilocus sequence
typing (MLST) is a molecular isolate characterization technique that
exploits recent advances in high-throughput molecular technology to
generate accurate isolate characterization data efficiently and
inexpensively. Data are highly reproducible among laboratories and can
be shared electronically via the Internet
(10). It has sufficient
resolution to accommodate the high levels of diversity present in many
bacterial pathogen populations but can also rationalize this diversity
into groups of isolates with related genotypes. MLST has been
successfully employed to characterize a variety of bacteria, including
C. jejuni (11,
35), but to date studies
of C. jejuni have focused on isolates obtained from human
disease and retail food products, with relatively few environmental
isolates having been investigated.
In the present work, MLST was
used to characterize 112 isolates from a variety of farm and
environmental sources with the aim of establishing the genetic
diversity of C. jejuni in the farm setting and comparing this
diversity with that found among collections of human disease isolates.
There was small but significant differentiation between the two
populations, although many of the genotypes found among isolates
derived from human disease were present in the environmental samples.
The results indicated the potential for combining MLST data with
population genetic analyses in elucidating the relative importance of
the various possible sources of human infection by C.
jejuni.

MATERIALS AND
METHODS
Bacterial isolates.
A total of 112
C. jejuni
isolates obtained from poultry, cattle,
sheep, and environmental
sources from northwestern England during
the 1990s were investigated.
These comprised 16 isolates from
the whole intestine of broiler birds
from a poultry processing
plant supplied by 35 farms located within a
150-mile radius
(
38); 12
isolates from fresh feces of turkey chicks kept on
a dairy farm
(
39); 14 isolates from
the small intestines of
adult beef cattle at an abattoir receiving
cattle and sheep
from northwestern England, northern Wales, and
southwestern
Scotland; 9 isolates from fresh feces of newborn calves
(
32);
9 isolates from the
small intestines of lambs at the same abattoir
as the beef cattle
(
33); 14 isolates from
intestinal contents
of sheep at slaughter; 10 isolates from fresh feces
of sheep
grazing on salt marsh; 9 isolates from fresh feces of sheep
grazing
on upland fell
(
21); 10 isolates from
slurry storage tanks
(
34);
and 9 isolates from
starling feces collected on the same dairy
farm as the isolates from
turkey chicks
(
31).
Microbiological
isolate characterization.
The isolates had been presumptively
identified as thermophilic Campylobacter species on the basis
of microscopy and catalase and oxidase reactions. Isolates from the
sheep and poultry were biotyped by using the modified scheme of Bolton
et al. (4). Some isolates
from sheep grazing on salt marsh and fell pastures were biotyped by
using the MAST ID Camp Identification System (Mast Diagnostics, Bootle,
United Kingdom). Where necessary, identification of some of the
isolates to species level was confirmed by nucleotide sequencing of an
812-bp segment of 23S RNA, using the 43a and 69ar primers designed by
van Camp et al.
(36).
DNA
preparation.
The isolates,
which had been stored at -80°C in a cryopreservative
medium, were revived on Columbia agar (CM 0331; Oxoid Ltd.,
Basingstoke, United Kingdom) with 5% horse blood in a
microaerophilic atmosphere produced by using gas-generating sachets
(CN0025A; Oxoid Ltd.; and 96125; bioMerieux, Basingstoke, United
Kingdom) at 42°C for 48 h. A suspension of
approximately 106 cells per ml was prepared in sample
buffer, and chromosomal DNA was extracted by using IsoQuick nucleic
acid extraction kits (ISC Bioexpress, Kaysville, Utah) and the rapid
DNA extraction protocol according to the manufacturer's
instructions.
MLST.
The previously published protocol for
C. jejuni MLST was used
(11). Briefly, fragments
of seven housekeeping genes (aspartase A, aspA; glutamine
synthetase, glnA; citrate synthase, gltA; serine
hydroxymethyl transferase, glyA; phosphoglucomutase,
pgm; transketolase, tkt; and ATP synthase subunit,
uncA) were amplified by PCR, and the nucleotide sequence of
the amplicons was determined with the published oligonucleotide primers
and reaction conditions. Nucleotide sequence extension reaction
products were separated and detected on an ABI Prism 3700 or an ABI
Prism 377 automated DNA analyzer. Nucleotide sequences were determined
at least once on each DNA strand and were assembled with the STADEN
software package (29).
Allele numbers and sequence types (ST) were assigned by using the
Campylobacter MLST database
(http://campylobacter.mlst.net/).
Data
from human disease isolates.
Data from 91 human disease isolates
of C. jejuni, isolated from cases of gastroenteritis from
stool cultures in northwestern England during 1999, were obtained from
the PubMLST isolate database
(http://campylobacter.mlst.net)
by using the "Search databaseadvanced queries"
feature with the following criteria: id, <1316; country, United
Kingdom; year, 1999; source, human stool. These data included sequence
types for 40 previously published
(10) and 51 unpublished
isolates.
Data analysis.
The STs were assigned to clonal
complexes as described previously
(10). Central genotypes
were defined by using UPGMA cluster analysis and the BURST algorithm
(13), both implemented
with the computer program START
(18), and split
decomposition analysis was implemented with the program SPLITSTREE
(17). Isolates were
defined as belonging to a clonal complex if they shared at least four
alleles with the central ST. The ratio of nonsynonymous to synonymous
substitutions (dN/dS), and
numbers of fixed differences and shared polymorphisms were calculated
by using the DNASP software package version 3.53
(25). The pairwise
FST and test of significance calculations were
performed by using the Arlequin software package, version 2.000
(27). For some of the
analyses the gene sequences for each of the MLST loci were concatenated
to give a single continuous nucleotide sequence of 3,309 bp for each
isolate.

RESULTS
Diversity
of MLST allele sequences.
It
was possible to determine the nucleotide sequences at each
of the MLST
loci from all of the farm and environmental isolates
with previously
published methods and reagents. The number of
unique sequences at each
locus varied from 5 for the
aspA locus
to 12 for the
gltA locus, with the percentage of variable sites
in this data
set ranging from 2.7% for the
aspA locus to
14.5%
at the
uncA locus. The apparent high diversity of
the
uncA locus
was caused by a single allele, allele 17, which
occurred 18
times in the data set. Exclusion of this allele gave
diversity
for this locus of 0.8%. The ratio of nonsynonymous to
synonymous
substitutions
(
dN/dS) observed ranged from
0.000 to 0.143 (Table
1).
All except two of the MLST allele sequences found in this isolate
collection
had been described
previously.
Diversity of sequence
types.
The 112 isolates
contained a total of 30 different STs (Table
2).
The most common ST was ST-45, which was represented by 26 isolates,
with
several STs occurring only once in the data set (Table
2). The
four most
predominant STs (ST-45, ST-42, ST-61, and ST-262)
represented
approximately half of the isolates (61 out of 112
isolates, or
54%). Virtually all of the isolates (110 out of
112 isolates, or
98%) were assigned to 1 of 10 previously described
clonal
complexes, with the remaining two isolates unassigned
as they did not
share four alleles in common with any of the
previously recognized
central genotypes. The clonal complexes
were represented by between 2
isolates (ST-52 complex) and 30
isolates (ST-45 complex). The number of
STs within each clonal
complex ranged from one (ST-52, ST-177, and
ST-206 complexes)
to eight (ST-21 complex). With the exception of the
ST-21 complex,
the previously assigned central genotype was the most
predominant
ST present in each of the clonal complexes identified in
this
sample. A total of 12 STs were novel to this
study.
View this table:
[in this window]
[in a new window]
|
TABLE 2. Distribution
of STs among 112 C. jejuni isolates from farm animals and
their environment and their resolution into clonal complexes
|
Distribution of genotypes among
isolation sources.
The most
common clonal complexes among the farm isolates were
ST-45 complex (30
isolates), ST-21 complex (26 isolates), ST-61
complex (14 isolates),
and ST-42 complex (11 isolates) (Table
2).
The remaining clonal
complexes were represented by nine or fewer
isolates, with the ST-52
complex, represented by two isolates,
being the smallest. The number of
clonal complexes identified
in each isolation source varied from two
for turkey chicks,
broiler chicks, and slurry to six for adult beef
cattle at slaughter
and sheep at slaughter (Table
3). The ST-21 complex was the
most widely distributed clonal complex,
present in 8 of the
10 different isolation sources. The ST-52 and
ST-177 complexes
were the least widely distributed and were each
identified in
one isolation source. The ST-45 complex was predominant
among
turkey and broiler chick isolation sources and was absent from
ovine
isolation sources. Conversely, ST-42 and ST-61 complexes were
predominant
among ovine and bovine sources but were absent from avian
sources.
Two clonal complexes were identified in both turkey and
broiler
chicks, although only the ST-45 complex was common to both
isolation
sources in this study. Adult cattle and calves shared the
ST-21,
ST-45, and ST-48 complexes, but each had further complexes.
Sheep
at slaughter, sheep grazing on salt marsh, and sheep grazing
on
fell (hill) land shared three clonal complexes, ST-21 complex,
ST-45
complex, and ST-61 complex. Adult sheep and lambs shared
the ST-61 and
ST-48 complexes, but only the ST-61 complex was
found in all ovine
groups. Isolates belonging to the ST-21 complex,
ST-45 complex, and
ST-257 complex were present in starlings,
which were the only source
from which the ST-177 complex was
recovered. Both of the unassigned STs
(ST-690 and ST-688) were
from bovine isolation
sources.
Comparison of genotypes with
human disease isolates.
The
gene flow analysis (pairwise
FST) between the 112
isolates
in this study and the 91 isolates from human disease gave
values
(0.015 to 0.094) that ranged from significantly different to
zero
for all of the alleles, except for
uncA (Table
1) and for the
concatenated
sequences. A total of 183 polymorphisms were shared
between
the two data sets, and there were no fixed nucleotide
differences
between them.

DISCUSSION
The
contributions of the various possible sources of infection
to the total
disease burden of human campylobacteriosis have
not been definitively
established. In addition to the problems
inherent in analyzing a
sporadic zoonotic infection, this has
been to a large extent due to the
lack of a reliable and portable
scheme for isolate characterization
that enables comparisons
among different studies. The
Campylobacter MLST scheme was devised
to address this problem
and has been shown to be effective for
the analysis of isolates from
human disease and retail food
(
11).
The analyses
described here demonstrate that this scheme can
be applied to isolates
from the farm environment without modification,
allowing direct and
unambiguous comparison among
C. jejuni isolates
from a variety
of sources (
10,
11).
The nucleotide
diversity present in the farm-derived isolates was similar to that
described for human disease and retail food isolates
(10) in terms of the
number of alleles, nucleotide variability, and
dN/dS ratio at each locus
(Table 1). The largest
degree of variability was seen at the uncA locus with the
inclusion of allele 17, which may have originated in a different
Campylobacter species, such as Campylobacter coli
(11). All but two of the
alleles, 16 of the 30 STs, and all of the clonal complexes identified
in the farm and environment isolates had been previously described
among the human disease and retail food isolates
(10,
11), with the majority of
alleles and STs identified in this study being commonly found among
isolates on the Campylobacter PubMLST database.
The data
were consistent with the idea that particular genotypes, indicated by
clonal complex, are associated with given host sources, as suggested
previously by using MLST, serotyping, and pulsed-field gel
electrophoresis (10,
14,
28,
40). Isolates belonging
to the ST-45 clonal complex were dominant among turkey and broiler
chick samples and were absent from sheep and lamb samples. In contrast,
ST-61 and ST-42 complexes were dominant among sheep isolates but were
absent from poultry isolates: similar trends have been reported for
retail food sources (10).
The ST-21 complex appeared to have a wide distribution; this clonal
complex probably corresponds to the large stable cluster of isolates
capable of colonizing a wide range of hosts identified with other
techniques (14,
28).
Several other
observations concerning the distribution of clonal complexes among
isolation source, while based on smaller numbers of isolates, warrant
further investigation. Wild birds have been implicated in spreading
infection in the farm environment on numerous occasions, although the
extent of their contribution is as yet unknown
(6,
9,
19,
20,
23,
37). The results from
this investigation suggest that a range of C. jejuni genotypes
may be prevalent in wild birds. Starlings were found to carry C.
jejuni belonging to STs from complexes associated with poultry and
environment sources as well as the ST-21 complex; however, none of the
complexes potentially associated with cattle or sheep was present. This
could be a reflection of host adaptation, behavioral patterns of the
birds, or small sample size. A larger number of clonal complexes were
represented in animals at slaughter than in the other animal groups,
perhaps reflecting different sampling sites or contamination within the
slaughterhouse. Differences in clonal complex distribution between both
adult cattle and calves and adult sheep and lambs could reflect the
different conditions and farming practices in which the animal age
groups are kept or could reflect host immunological maturity
(20,
21,
32,
33).
Of the 30 STs,
8 accounted for 73% (82 out of 112) of the isolates, implying
that these genotypes may be particularly stable. Possible explanations
of such stability remain to be determined but could reflect stabilizing
selection imposed by, for example, niche adaptation. Members of the
ST-21 complex in particular might be well adapted for long-term
survival given their apparently ubiquitous distribution. This idea was
further supported by the observation that they were one of only two
genotypes isolated from slurry to which a potentially large number of
different genotypes were added on a regular basis
(34). With the exception
of ST-21 complex, the most common genotype in each of the major clonal
complexes was the central ST, which is usually the most common ST among
the clonal complexes in the Campylobacter PubMLST database. In
contrast to isolate data available in the database, ST-262 was the most
common ST-21 complex genotype in this study, found among slurry, sheep,
and cattle isolates, while ST-53 isolates, the second most common type
in the complex, were present among calves, sheep, chicks, and
starlings. There were 12 (10% of the total number of isolates)
STs unique to this investigation, which all differed from the central
genotype of the clonal complex at one or two loci. These could be
specific host-adapted STs but are more likely to represent recent
variants from the persistent central genotypes.
A number of
studies have indicated that isolates from human disease and farm
animals are very similar
(12,
14-16,
19). In the largest MLST
study of human disease published to date, six clonal complexes
accounted for more than 60% of human disease isolates, namely,
the ST-21, ST-45, ST-206, ST-61, ST-48, and ST-257 complexes
(10). The same clonal
complexes were also predominant among retail meat isolates in the
Campylobacter PubMLST database, although the frequency of each
varies according to isolation source. These clonal complexes were all
present among the farm and environmental isolates. Further, while the
two isolate collections shared many polymorphisms, there were no fixed
differences between them. These observations are consistent with the
farm populations of C. jejuni being a source for food
contamination and human infection. The slightly different clonal
complex composition of the human disease and farm isolate collections
was reflected by the nonzero values of pairwise FST
values for most loci and for the concatenated sequences. This is
consistent with other populations of C. jejuni contributing to
human disease. The high degree of discrimination possible with such
analyses presents the prospect of employing MLST data to improve
estimates of the contribution of different Campylobacter
populations to the human disease burden.

ACKNOWLEDGMENTS
This work was supported by
contract number OZ0604 from the United
Kingdom Department of
Environment, Food, and Rural Affairs in
collaboration with Eric Bolton,
David Wareing, Andrew Fox, and
Roisin Ure of the Public Health
Laboratory Service, North West
group.
This work made use of the
Campylobacter PubMLST database, located at
http://campylobacter.mlst.net/,
developed by Man-Suen Chan and Keith Jolley, University of
Oxford.

FOOTNOTES
* Corresponding
author. Mailing address: Peter Medawar Building for Pathogen Research,
Department of Zoology, University of Oxford, South Parks Rd., Oxford
OX1 3SY, United Kingdom. Phone and fax: 44 (1865) 271284. E-mail:
martin.maiden{at}zoo.ox.ac.uk.


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Applied and Environmental Microbiology, December 2003, p. 7409-7413, Vol. 69, No. 12
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.12.7409-7413.2003
Copyright © 2003, American
Society for
Microbiology. All Rights Reserved.
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