THE SALMONELLA-DUBLIN VIRULENCE PLASMID MEDIATES SYSTEMIC BUT NOT ENTERIC PHASES OF SALMONELLOSIS IN CATTLE

Plasmid-bearing and plasmid-free isolates and a plasmid-cured strain o f Salmonella dublin were compared for virulence in calves, The plasmid -bearing strains were highly virulent, causing severe enteric and syst emic disease with high mortality, In contrast, the plasmid-free strain s caused diarrhea but only low mortality, The infection kinetics of a wild-type and a derivative plasmid-cured strain were compared, Both st rains were isolated in high numbers from intestinal sites at 3 and 6 d ays after oral challenge and were isolated at comparable frequencies f rom systemic sites at 3 days, but not at 6 days, when the wild-type st rain was predominant, The strains were equally invasive in intestinal epithelia with and without Peyer's patch and elicited comparable secre tory and inflammatory responses and intestinal pathology in ligated il eal loops, The effect of the virulence plasmid on growth kinetics and on the outer membrane protein profile was assessed in an in vivo growt h chamber, The virulence plasmid did not influence either extracellula r growth or the expression of major outer membrane proteins, These obs ervations demonstrate that the virulence plasmid is not involved in ei ther the enteric phase of infection or the systemic dissemination of S . dublin but probably mediates the persistence of S. dublin at systemi c sites.