Comparison of two variants of Salmonella enterica serovar Heidelberg from the 2015-2017 outbreak in dairy beef calves

Salmonella is one of the top five foodborne pathogens and one of the leading causes of foodborne illness in humans in the United States and worldwide. In the United States, a 2015-2017 multistate Salmonella Heidelberg outbreak was linked to human contact with dairy beef calves. Typically, foodborne isolates of Salmonella cause disease in humans but the carrier food animal does not display any signs of disease. The 2015-2017 Heidelberg outbreak was unique because the isolates associated with the outbreak were recovered from ill humans as well as sick dairy calves. Two main variants of Salmonella Heidelberg were isolated from the intestines of calves during the outbreak and because greater morbidity and mortality in calves was associated with one variant compared to the other, the two isolates were deeply characterized to explore factors likely contributing to their contrasting disease severity. The Salmonella Heidelberg isolate with greater morbidity and mortality in dairy beef calves also had elevated expression of virulence genes and higher invasion of human and bovine epithelial cells, potentially enhancing this isolate's pathogenicity, and contributing to the ecological success in dairy calves.

沙门氏菌（Salmonella）是全球及美国范围内五大食源性致病菌之一，也是引发人类食源性疾病的主要诱因之一。2015至2017年，美国多州暴发海德堡沙门氏菌（Salmonella Heidelberg）疫情，该疫情与人类接触肉牛犊存在关联。通常而言，食源性分离得到的沙门氏菌可使人类致病，但作为致病菌携带者的食用动物并无任何疾病症状。本次2015-2017年的海德堡沙门氏菌疫情较为特殊：此次疫情相关的分离株同时从患病人类与患病肉牛犊体内被分离得到。疫情期间，研究人员从犊牛肠道内分离得到两株主要的海德堡沙门氏菌变异株；相较于另一株，其中一株可导致犊牛出现更高的发病率与死亡率，因此研究人员对两株分离株开展了深度表征，以探究可能造成二者疾病严重程度存在显著差异的相关因素。在肉牛犊中引发更高发病率与死亡率的海德堡沙门氏菌分离株，其毒力基因表达水平更高，对人类及牛上皮细胞的侵袭能力更强，这或可增强该分离株的致病性，并助力其在肉牛犊群体中实现生态定植与传播优势。