Sef fimbria operon construction, expression, and function for direct rapid detection of Salmonella Enteritidis

Salmonella Enteritidis (SE) causes both horizontal and vertical transmission of diseases in poultry industry and is also one of the main causes of human food poisoning. Sequence analysis of the sef operon of poultry-derived Salmonella serotypes showed the presence of an entire sef operon in SE, whereas only sef pseudogenes were found in Salmonella Gallinarum and Salmonella Pullorum. Subsequently, the sef operon of SE was cloned into the pBR322 plasmid and expressed in a modified Escherichia coli strain SE5000. sef operon expression was demonstrated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western blot, agglutination assay, and transmission electron microscopy. The results showed that SE5000+Sef, but not SE5000+pBR322, could specifically react with SE-positive chicken serum in an agglutination assay, which could be clearly visualized by the naked eye within less than 2 min. In contrast, SE5000+Sef could not be recognized in Salmonella Gallinarum- and Salmonella Pullorum-positive chicken sera. Next, taking advantage of the exclusive presence of an entire sef operon in SE, we set up an agglutination-based detection system to monitor the dynamics of Sef-targeted antibody from SE-infected chicks for 47 days. Using the proposed detection method, SE was readily detectable starting from 2 weeks post-infection. Finally, we compared the proposed SE5000+Sef-based detection system with commercially available agglutination antigen using the classical bacterial isolation and identification procedure as reference. The results showed that the SE5000+Sef system was more consistent with the results of bacterial isolation and identification with almost 100% accuracy. We established a simple, sensitive, and cheap agglutination method for rapid and specific detection of SE-infected chickens, which can facilitate epidemiological investigation and eradication of SE infections. KEY POINTS: • Only the Salmonella Enteritidis serotype expressed Sef fimbriae in chicken infected with SE. • A rapid, large-scale method of detection by the naked eye of detection of SE-infected chicken is presented.