Rapid detection of flagellated and non-flagellated Salmonella by targeting the common flagellar hook gene flgE.

ABSTRACT

Salmonella spp. can cause animal and human salmonellosis. In this study, we established a simple method to detect all Salmonella species by amplifying a specific region within the flgE gene encoding the flagellar hook protein. Our preliminary sequence analysis among flagella-associated genes of Salmonella revealed that although Salmonella Gallinarum and Salmonella Pullorum are lacking flagella, they did have flagella-associated genes, including flgE. To investigate in detail, a comparative flgE sequence analysis was conducted using different bacterial strains including flagellated and non-flagellated Salmonella as well as non-Salmonella strains. Two unique regions (481-529 bp and 721-775 bp of the reference sequence) within the flgE open reading frame were found to be highly conserved and specific to all Salmonella species. Next, we designed a pair of PCR primers (flgE-UP and flgE-LO) targeting the above two regions, and performed a flgE-tailored PCR using as template DNA prepared from a total of 76 bacterial strains (31 flagellated Salmonella strains, 26 non-flagellated Salmonella strains, and 19 other non-Salmonella bacteria strains). Results showed that specific positive bands with expected size were obtained from all Salmonella (including flagellated and non-flagellated Salmonella) strains, while no specific product was generated from non-Salmonella bacterial strains. PCR products from the positive bands were confirmed by DNA sequencing. The minimum detection amount for genomic DNA and bacteria cells reached 18.3 pg/µL and 100 colony-forming unit (CFU) per PCR reaction, respectively. Using the flgE-PCR method to detect Salmonella in artificially contaminated milk samples, as low as 1 CFU/mL Salmonella was detectable after an 8-h pre-culture. Meanwhile, the flgE-tailored PCR method was applied to evaluate 247 clinical samples infected with Salmonella from different chicken breeding farms. The detection results indicated that flgE-PCR could be used to specifically detect Salmonella in concordance with the traditional bacterial culture-based detection method. It is worthwhile noticed that identification results using flgE-tailored PCR should be completed within less than 1 day, expanding the result of much faster than the standard method, which took more than 5 days. Overall, the flgE-tailored PCR method can specifically detect flagellated and non-flagellated Salmonella and can serve as a powerful tool for rapid, simple, and sensitive detection of Salmonella species. KEY POINTS • Targeting flgE gene for all Salmonella spp. found. • The established PCR assay is used to specifically detect all Salmonella spp. • The PCR method is applied to detect clinical Salmonella spp. samples within less than 1 day.