A pathological study on the virulence of Salmonella choleraesuis associated with 50-kilobase plasmid in mice.

The virulence of Salmonella choleraesuis associated with 50-kilobase plasmid was assessed by the pathology produced in mice. The strains used in this study were plasmid-contained RF-1, plasmid-cured 31N-1 and plasmid-reintroduced 31N-1-T. Mice were intraperitoneally inoculated with sublethal doses of these bacteria and chronologically necropsied. Pathologically, the plasmid-positive (P+) cases were distinctly different from the plasmid-negative (P-) ones, especially in the peritoneal fluids and peritoneum. The P+ cases were characterized by extensive, granulomatous peritoneal lesions composed of abundant macrophages frequently with neutrophils, sero-fibrinous exudates and vasculitis and the bacterial antigens were detected in such sites. By contrast, the peritonea in the P- cases were mildly involved and had scattered granulomas. The bacterial antigens were hardly visible in all tissues examined. It is suggested that spread preparations of the mesentery may be a useful tool for observing the distribution of peritoneal lesions.