The role of a 54-kb plasmid in the virulence of Enteritidis strains of phage type 4 was examined in mice, young chickens and adult laying-hens. Although the plasmid was essential for full expression of virulence in mice, differences in expression of virulence by this 54-kb plasmid were noted among strains; thus, introduction of the plasmid to a naturally occurring strain that lacked it did not make that strain virulent. In newly hatched chickens, virulence of an Enteritidis strain of phage type 4 by oral or parenteral routes was unrelated to possession of this plasmid which, similarly, played no role in infection in egg-laying hens. When a strain of Enteritidis phage type 4 and a plasmid-cured strain derived from it were given orally to chickens, both strains were equally invasive and their patterns of localisation in spleen, liver and ovaries were similar and they were isolated at similar frequencies from cultured, laid eggs.


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