Isolation, Identification And Biological Characteristics Of Salmonella Typhimurium In Pork

Salmonella is a kind of enteric pathogen and one of the major sources of food-borne salmonellosis both in human beings and animals. The World Health Organization has listed Salmonella as the severe and moderate risk of food borne pathogens. According to the statistics, Salmonella infection can cause the most serious bacterium food poisoning in number of cases and population all over the world. In China’s inland areas,70%～80% of the bacterial food poisoning are caused by Salmonella and 90% of which are due to the contaminated meat or animal products. Pork is the main animal food for the masses of residents in our country, and Salmonella contamination in the slaughter process is one of the key points of pork food safety problem. Therefore, the research of Salmonella pollution in the slaughter process and its pathogenic mechanism is of great significance.In this study, a S. typhimurium strain was isolated from a pig carcass in Yangzhou slaughterhouse, and its biological characteristics and the responses of host cells against Salmonella in the early stage of infection were analyzed. These will be helpful to control and prevent foodborne poisoning caused by Salmonella.1. Isolation and identification of a S. typhimurium strainPork food contaminated with Salmonella during the process of slaughter is one of the major causes of food poisoning. In this study, a S. typhimurium strain was isolated from the pig carcass in Yangzhou slaughterhouse and identified by biochemical and serologic assay. The drug sensitivity assay indicated that this isolate was sensitive to cephalosporins, but not to ampicillin. It was found that the mice could be killed after 5×105 cfu per mouse intraperitoneal injection. After intraperitoneally injected with the dose of 1 x 104 cfu per mouse, no dead mice were found, but the obvious pathological damages and inflammation reactions in liver and spleen were shown. The colonization ability of this isolate was strong. The bacteria count got the peak one week after infection and bacteria could still be separated from liver and spleen four weeks later. Besides, compared with the control, the organ coefficients and the malondialdehyde levels in liver increased significantly (P<0.01), while the superoxide dismutase levels in liver were reduced after a rise. In serum, the cereal third transaminase and the aspartate transaminase levels were increased (P<0.01), while the alkaline phosphatase, total protein and albumin levels were decreased (P<0.01). After infection, the isolate could induce the secretion of cytokines (interferon-γ, tumor necrosis factor and monocyte chemoattractant protein-1) in serum and the activation of T lymphocytes in spleen. Through the above analysis, we can get a preliminary understanding of the biological characteristics of this isolate.2. Responses of mouse neutrophils and B lymphocytes against Salmonella in the early stage of in vitro infection detected by flow cytometrySalmonella is a facultative intracellular bacteria. After infection, it can survive in the host cells and lead to long-term carrier state through regulating cell function, which brings a serious threat to human and animal health. The interactions between Salmonella and macrophages have been extensively characterized in various studies, while the other host cells were often overlooked. In this study, the neutrophilic granulocytes (NGs) and B lymphocytes were selected to analyze dynamically the responses against Salmonella in the early stage of in vitro infection. The results showed that the NGs (Gr-1+) and B cells (B220+) both were global and their growth were non-adherent. After infection, no obvious apoptotic NGs were found, but the population of dead cells increased. The intracellular reactive oxygen species (ROS)、mitochondrial membrane potential (MMP) and calcium ion concentration ([Ca2+]) of NGs were increased and the expression of co-stimulatory molecules on NGs surface were also upregulated slightly. However, the apoptosis and death rates of B lymphocytes had no obvious change after infection, and the intracellular ROS、MMP and CD86 expression level of B cells decreased when compared with the control, except [Ca2+]. These indicated that the interaction of salmonella with diffenent host cells was a complex process.Through the biolobical characterics analysis of the S. typhimurium isolate and the responses of NGs and B lymphocytes against Salmonella in the early stage of infection, it will be helpful for the study of pathogenic mechanism and the defense against Salmonella. In addition, we found that the responses of NGs and B cells against Salmonella in the early stage of infection were different. Further research are needed to interpret these.