IMPORTANCE OF THE SEROVAR-SPECIFIC PLASMID FOR VIRULENCE OF SALMONELLA STRAINS IN CALVES

To evaluate the influence of serovar-specific plasmids on salmonella v irulence in calves, experiments were performed involving infection, by the oral route, with mixtures of strains containing equal counts of a plasmid-carrying and a plasmid-free strain of the same serovar. The c oncentration ratio between the plasmid-carrying and the plasmid-free s train which had developed in the organs of the infected animals was us ed for a comparative evaluation of virulence and pathogenetic behaviou r of the strains. While in the S. typhimurium strains studied, the pre sence of the plasmid was accompanied by a significantly increased colo nization and multiplication of the agent in the host's body, examinati on of S. enteritidis and S. dublin revealed that the plasmid-free stra ins exhibited identical or even significantly higher bacterial counts than the plasmid-carrying strains in organs. The fact that plasmid-fre e salmonella strains with a high virulence for calves have been found demonstrates that the presence of a serovar-specific plasmid is not an indispensable requirement for the development of salmonellosis in cal ves.