Selenite enrichment broth to improve the sensitivity in molecular diagnostics of Salmonella

Abstract

Selenite enrichment broth (SEB) is used to optimize the recovery of Salmonella enterica subspecies enterica from stool samples. Compared to a direct culture approach, it enhances culture yield by reducing growth of faecal coliforms and faecal streptococci. Over the course of seven years from 2000 to 2017, 47,235 faecal samples were tested with a Salmonella PCR. We investigated the added value of using SEB in combination with faeces for DNA extraction, in order to improve the sensitivity of molecular diagnostics for detection of Salmonella. A Salmonella enterica subspecies enterica strain was tested for growth characteristics, with and without incubation in SEB, to determine the impact of Selenite enrichment in the Salmonella PCR. Retrospectively, a total of 102 Salmonella enterica subspecies enterica PCR positive faecal samples were re-analysed. DNA extraction was performed with the EasyMag® and MagNaPure96® system using three different input volumes of faeces and SEB. Prospectively, 114 Salmonella PCR positive faecal samples were retested within 2 days using five different input volumes for DNA extraction. Retrospectively, PCR that used SEB as part of input in the DNA extraction, 7/102 (7%) Salmonella PCR positive samples were additionally detected compared to no use of SEB. Of these, Salmonella enterica subspecies enterica serovariation Thompson, Enteritidis, 9,12:l.v and Senftenberg have been outbreak related in the past. Prospectively results were combined in collaboration with another microbiology laboratory, 15/114 (13.2%) additional specimens were detected with the Salmonella PCR, including processing Selenite enrichment broth. In conclusion, of the total 47,235 feacal samples, with SEB the prevalence of a positive PCR for Salmonella is 2.2%. Of these 2.2% positive Salmonella PCRs, 0.4% was not detected in culture. By using SEB an improved detection of Salmonella diagnostics could be realized and a substantial part of 13,2% additional Salmonella cases could be detected.

Keywords: Foodborne disease; Gastroenteritis; Multiplexed real-time PCR; Outbreaks; Salmonella enterica; Selenite enrichment.