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Infection and Immunity, March 2000, p. 1535-1541, Vol. 68, No. 3
Departments of Veterinary
Science1 and Microbiology and
Immunology,2 The University of Melbourne, and
National Escherichia coli Reference Laboratory,
Victorian Infectious Diseases Laboratory,3
Parkville, Victoria 3052, Australia
Received 26 July 1999/Returned for modification 21 September
1999/Accepted 12 December 1999
The E3 strain of E. coli was isolated in an outbreak of
respiratory disease in broiler chickens, and experimental aerosol exposure of chickens to this strain induced disease similar to that
seen in the field. In order to establish whether the virulent phenotype
of this strain was associated with carriage of particular plasmids,
four plasmid-cured derivatives, each lacking two or more of the
plasmids carried by the wild-type strain, were assessed for virulence.
Virulence was found to be associated with one large plasmid, pVM01.
Plasmid pVM01 was marked by introduction of the transposon
TnphoA, carrying kanamycin resistance, and was then cloned
by transformation of E. coli strain DH5
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Colonization of the Respiratory Tract by a Virulent
Strain of Avian Escherichia coli Requires Carriage of a
Conjugative Plasmid
. The cloned
plasmid was then reintroduced by conjugation into an avirulent
plasmid-cured derivative of strain E3 which lacked pVM01. The conjugant
was shown to be as virulent as the wild-type strain E3, establishing that this plasmid is required for virulence following aerosol exposure.
This virulence plasmid conferred expression of a hydroxamate siderophore, but not colicins, on both strain E3 and strain DH5
. Carriage of this plasmid was required for strain E3 to colonize the
respiratory tracts of chickens but was not necessary for colonization of the gastrointestinal tract. However, the virulence plasmid did not
confer virulence, or the capacity to colonize the respiratory tract, on
strain DH5
. Thus, these studies have established that infection of
chickens with E. coli strain E3 by the respiratory route is
dependent on carriage of a conjugative virulence plasmid, which confers
the capacity to colonize specifically the respiratory tract and which
also carries genes for expression of a hydroxymate siderophore. These
findings will facilitate identification of the specific genes required
for virulence in these pathogens.
*
Corresponding author. Mailing address: Veterinary
Preclinical Centre, The University of Melbourne, Parkville, Victoria
3052, Australia. Phone: 61 3 93447342. Fax: 61 3 9344 7374. E-mail: g.browning{at}vet.unimelb.edu.au.
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