Shigella is the causative agent of Shigellosis in humans, and comprises four species, S. boydii, S. dysenteriae, S. flexneri, and S. sonnei. Shigella species possess a virulence plasmid of approx 220 kb encoding virulent determinants which enables it to invade the colonic mucosa. Enteroinvasive E. coli (EIEC) is closely related to Shigella spp. and contains the same virulence plasmid. Traditionally Shigella has be identified by culture and biochemical tests
At Dorevitch Pathology we are using a new molecular diagnostic tool the Faecal Multiplex Realtime PCR using Tib Molibiol LightMix® kits on the Roche LightCycler platform, for the detection of 5 different bacterial species from faecal specimens. In this study we compared the sensitivity of traditional culture detection of Shigella spp. with the multiplex PCR which detects the ipaH gene, part of the virulence plasmid. We included 61 specimens in our analysis and found that the PCR was more sensitive in the identification of Shigella spp.
The low sensitivity of traditional culture methods compared to PCR can be due to changes in the pH of the original specimen, the use of antibiotics and difficulties detecting low numbers.
There are some limitations of the Multiplex PCR method. Positive results may be caused by residual DNA detected from non-viable organisms. This method is unable to differentiate between Shigella spp. and EIEC as they both possess the same target gene ipaH. Additional testing by culture can assist in distinguishing between these two organisms.
Through the introduction of this technology it has enabled improved testing within the traditional culture method by the inclusion of new media to isolated Shigella spp. more effectively.