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Journal of Clinical Microbiology, December 2002, p. 4760-4762, Vol. 40, No. 12
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.12.4760-4762.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Mycobacterium avium subsp. paratuberculosis Strains from Cattle and Sheep Can Be Distinguished by a PCR Test Based on a Novel DNA Sequence Difference
Desmond M. Collins,* May De Zoete, and Sonia M. Cavaignac
AgResearch, Wallaceville Animal Research Centre, Upper Hutt, New Zealand
Received 10 June 2002/
Returned for modification 2 September 2002/
Accepted 22 September 2002

ABSTRACT
A DNA sequence differing between sheep and cattle types of
Mycobacterium avium subsp.
paratuberculosis was identified and used to develop
a PCR test. The test unequivocally distinguished all sheep types
from cattle types and was negative for a wide range of other
strains from the
Mycobacterium avium-
Mycobacterium intracellulare complex. The test will be useful for epidemiological purposes,
particularly in hosts such as deer that can be easily infected
with either type.

TEXT
Paratuberculosis, or Johne's disease, is a chronic granulomatous
enteritis that affects domestic and wild ruminants, causing
reduced food intake, weight loss, and death. The disease is
present in most countries and results in significant production
losses. The causative organism,
Mycobacterium avium subsp.
paratuberculosis (basonym
M. paratuberculosis) (
11), has also been implicated
as the etiologic agent of Crohn's disease in humans and is a
member of the
M. avium-
Mycobacterium intracellulare (MAI) complex
which includes
M. intracellulare and all subspecies of
M. avium.
Two recent discoveries have shown that the spread of
M. paratuberculosis may be more complicated than previously believed and emphasize
the need for the development of new diagnostic tools. First,
the organism has been reported to survive normal milk pasteurization
(
10), and second, it has also been isolated in the United Kingdom
from common wild nonruminant animals such as rabbits, foxes,
stoats, and crows (
2). Isolates of the organism were first classified
into cattle and sheep types in 1990 (
4) on the basis of restriction
fragment length polymorphisms (RFLPs) of the insertion sequence
IS
900, and this largely correlates with the difficulty of primary
isolation of sheep types (
4,
14). The distinction into cattle
and sheep types is epidemiologically useful, as cattle and sheep
are preferentially infected with their named types while other
ruminant species such as deer and goats appear to be equally
susceptible to either type (
4,
7,
14,
17). Recently, a two-step
method for distinguishing between cattle and sheep types of
M. paratuberculosis was developed based on polymorphic differences
in the insertion sequence IS
1311 (
13). We reasoned that the
RFLP differences between sheep and cattle types might indicate
differences in their genomic insertion sites for IS
900 that
could be used for constructing a simple PCR assay to distinguish
between the two types. This study describes the successful development
of such an assay.
The strains of the MAI complex used for this study were all characterized for the presence or absence of IS900 and IS901 and are given in Table 1. All strains of M. paratuberculosis were characterized as sheep or cattle types on the basis of their RFLPs with IS900. Strains were cultured with standard mycobacterial media (3). Purified DNA was extracted as described previously (4). When DNA from strains of each type was subjected to PCR at an annealing temperature of 50°C with primers directed outward from each end of IS900, only DNA from sheep types gave a product between 300 and 400 bp. Subsequently, it was observed that the same 342-bp product was obtained if only one PCR primer (DMC136, Fig. 1) was used. The PCR product was cloned into pBluescript KS II (Stratagene) and sequenced. Comparison of this sequence to the homologous region of a cattle type of M. paratuberculosis (National Center for Biotechnology Information database [http://www.ncbi.nlm.nih.gov/]) with BLAST revealed that the cattle type was not homologous to DMC136 at the 5' end. Further investigation with a range of PCR primers (data not shown) indicated that the two types have similar sequences at the 3' end but that only the sheep type has an IS900 site at the 5' end. The 342-bp product was thus a result of DMC136 hybridizing perfectly to the terminal part of IS900 that comprises the 5' end of the sheep sequence in Fig. 1 and partially to the last 10 nucleotides at the 3' end of this sequence. DNA from the cattle type has a similar sequence at the 3' end, but it gives no product with DMC136 because it lacks a sequence matching DMC136 at the 5' end. The site of insertion of IS900 is just downstream of the start codon for a putative gene on the complementary strand shown in lowercase letters in Fig. 1. This gene has high homology to a Mycobacterium smegmatis gene whose product is involved in phage attachment (1). The other major difference in sequence between the two types is that the sheep type has a tandem repeat of a 12-bp sequence followed by a 4-bp linker, which together contain a 14-bp palindromic sequence (Fig. 1). Such sequences are commonly found in bacterial chromosomes, often as part of a promoter region (8), a terminator (15), or a phage attachment site (12). Analysis of the sequence around the tandem repeat site showed that it is inserted at the stop codon of a hypothetical gene in the cattle type that encodes a putative protein of 97 amino acids with no good homology to other proteins. This hypothetical gene is interrupted by insertion of IS900 in the sheep type, but a truncated version of it might be produced, as there are other putative start codons. The tandem repeat is just 19 bp before an open reading frame encoding a putative protein of 134 amino acids with good homology to two hypothetical proteins of Mycobacterium tuberculosis and a hypothetical protein encoded by a small Yersinia pestis virulence plasmid. The palindromic sequence might therefore be involved in controlling gene expression of this second gene, either by limiting read-through from the truncated first gene to the second gene or by acting as part of an independent promoter region for the second gene. These sequence differences between sheep and cattle types may therefore be important in determining the host preference of the two types.
A PCR assay was developed with a GeneAmp PCR System 9600 (Applied
Biosystems) and the three primers DMC529, DMC531, and DMC533
(Fig.
1) under the following conditions: 1 cycle at 95°C
for 3 min; 25 cycles at 60°C for 30 s, 72°C for 30 s,
and 94°C for 30 s; and 1 cycle at 72°C for 7 min. DNA
from all strains of the cattle type gave the expected product
of 310 bp, while DNA from all strains of the sheep type gave
the expected product of 162 bp (Fig.
2). A PCR product was not
observed for any of the wide range of strains of the MAI complex
(Table
1) that did not contain IS
900 and were not
M. paratuberculosis.
The PCR was also negative for two isolates of
Mycobacterium bovis and an isolate of
M. tuberculosis.
The MAI complex covers a relatively broad group of genetically
related mycobacteria that, with the exception of
M. paratuberculosis,
are found in many environmental niches and are occasional mammalian
pathogens. Because of the potential of these organisms to confuse
the diagnosis of paratuberculosis, strains of the MAI complex
tested in this study were weighted toward those that had been
isolated from humans or from a range of different animal hosts
and that might be expected to be most closely related to
M. paratuberculosis (
3). It is highly encouraging that none of
these strains was positive in the PCR assay. In the case of
M. paratuberculosis, the inclusion of 10 strains from five other
countries enabled a cross section of strains with different
IS
900 RFLP types to be tested. This included sheep strains from
Canada and Iceland, previously described as having an "intermediate"
IS
900 RFLP type (
4,
7), which we now regard as variant sheep
types because their RFLP patterns are more similar to those
of sheep types than to those of cattle types (
7,
14). In New
Zealand, paratuberculosis is an emerging disease of farmed deer
(
6), and since this host can be infected with either the sheep
or the cattle type (
7), it was important to establish whether
the PCR test could be used for epidemiological studies. For
this reason, nine strains of the cattle type and three strains
of the sheep type tested were deer isolates. For all
M. paratuberculosis strains in Table
1, the PCR results were consistent with the
RFLP division into sheep and cattle types. These results demonstrate
the utility of the test particularly for those hosts such as
deer that can be infected with either type. At present, PCR
testing for paratuberculosis where both cattle and sheep types
are potentially present involves a PCR assay based on IS
900 to confirm the presence of
M. paratuberculosis followed by a
PCR test based on IS
1311 whose product is then subjected to
restriction endonuclease analysis (
17). The final step is necessary
because, while the presence of IS
1311 is not specific for
M. paratuberculosis, some copies of IS
1311 in
M. paratuberculosis have polymorphisms that are specific for the cattle and sheep
types (
13). The results of the present study indicate that this
three-step process might be replaceable by a single PCR assay
which appears specific both for
M. paratuberculosis and for
distinguishing between sheep and cattle types. Since the PCR
is based on a single-copy sequence, it may not be as sensitive
as PCR assays based on IS
900 (
5,
9), which has 14 to 18 copies
in the genome. Confirmation of these conclusions will require
investigation of a wider group of strains of the MAI complex
and other mycobacterial species. Nevertheless, the assay described
here has the potential to make a useful contribution to epidemiological
studies of paratuberculosis and possibly Crohn's disease.

ACKNOWLEDGMENTS
We thank G. W. de Lisle for culturing and helpful advice and
D. Dawson, H. F. A. K. Huchzermeyer, B. Brooks, and F. Saxegaard
for providing mycobacterial strains.
We thank the New Zealand Foundation for Research Science and Technology for financial support.

FOOTNOTES
* Corresponding author. Mailing address: AgResearch, Wallaceville Animal Research Centre, P.O. Box 40063, Upper Hutt, New Zealand. Phone: 64-4-922-1310. Fax: 64-4-922-1413. E-mail:
desmond.collins{at}agresearch.co.nz.


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Journal of Clinical Microbiology, December 2002, p. 4760-4762, Vol. 40, No. 12
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.12.4760-4762.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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