Identification of Shigella species in stool specimens by DNA amplification of different loci of the Shigella virulence plasmid.

The sensitivity and specificity of the polymerase chain reaction (PCR) for detection of DNA sequences specific to Shigella spp. and enteroinvasive Escherichia coli (EIEC) in stools was evaluated. Stool specimens were obtained from patients with acute gastroenteritis before and after antibiotic treatment. Fecal material was pre-incubated in phosphate-buffered saline, gram-negative broth or brain heart infusion (BHI) broth, and DNA was extracted and amplified. Primers complementary to the ial or the virF loci of the 140 MDa plasmid of Shigella were evaluated. The highest sensitivity for detection of Shigella DNA in stools (higher than that of culture) was reached by pre-incubation of the fecal material in BHI broth and use of virF primers for amplification. The specificity of this PCR protocol was documented by the negative results obtained with non-Shigella enteric organisms. These findings point out the important diagnostic and epidemiologic potential of the virF-specific PCR protocol in the investigation of Shigella infections.