Systematic evaluation of potential pathogenicity of Salmonella Indiana

Salmonella Indiana has emerged in recent years as an important zoonotic pathogen, but its pathogenicity has not been fully elucidated. In this study, using in vivo and in vitro animal and cellular experimental model systems, we evaluated the pathogenicity of Salmonella Indiana (S. Indiana) compared with three other serotypes of Salmonella, S. Enteritidis, S. Typhimurium and S. Thompson. The animal experiments included observations of clinical symptoms, pathological changes and determination of median lethal dose in mice. The adhesion and invasiveness and intracellular proliferative capacity of Salmonella in vitro were measured with the murine macrophage-like cell line RAW264.7 cells and the human colon adenocarcinoma cell line Caco-2 cells. The results of animal experiments showed that S. Indiana, S. Enteritidis, S. Typhimurium and S. Thompson caused histopathological changes in most organs to varying degrees, primarily in the liver and intestine of mice. The gross lesions included white necrotic foci on the liver surface with different levels. The histopathological changes of monocyte/macrophage infiltration and coagulative necrosis were observed in the liver. Intestinal villi became short and were sloughed off, and lymphocyte infiltration was found in the submucosa. Compared with the other serotypes, the pathological changes caused by S. Indiana were slighter and had a relatively high median lethal dose in mice. The results of adhesion and invasion tests showed that the intracellular growth trend of most Salmonella strains was positively correlated with the number of pathogens adhering to and invading cells. Compared with the strains of the other three serotypes, most S. Indiana strains exhibited significantly lower adhesion and invasiveness to RAW264.7 and Caco-2 cells within 30 min. Most S. Indiana strains displayed twice to four times lower intracellular proliferation within 24 h in RAW264.7 cells. In conclusion, S. Indiana was pathogenic, but its pathogenicity was lower than that of S. Typhimurium and S. Enteritidis, and was similar to that of S. Thompson.