Colonization of the respiratory tract by a virulent strain of avian Escherichia coli requires carriage of a conjugative plasmid

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 esta blish whether the virulent phenotype of this strain was associated with car riage 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 alpha. The cloned plasmid was then reintroduced by conjugat ion into an avirulent plasmid-cured derivative of strain E3 which lacked pV M01. 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 side rophore, but not colicins, on both strain E3 and strain DH5 alpha. Carriage of this plasmid was required for strain E3 to colonize the respiratory tra cts of chickens but was not necessary for colonization of the gastrointesti nal tract. However, the virulence plasmid did not confer virulence, or the capacity to colonize the respiratory tract, on strain DH5 alpha. Thus, thes e studies have established that infection of chickens with E. coli strain E 3 by the respiratory route is dependent on carriage of a conjugative virule nce plasmid, which confers the capacity to colonize specifically the respir atory tract and which also carries genes for expression of a hydroxymate si derophore. These findings will facilitate identification of the specific ge nes required for virulence in these pathogens.