Rapid detection of diarrheagenic E. coli by real-time PCR

Enterovirulent Escherichia coli are among the most important causes of acute diarrhea in developing as well as in developed countries. We have adapted classical PCR to detect these organisms in stool specimens to real-time PCR using the LightCycler® (LC) SYBR Green format followed by melting curve analysis. With only two different cycling protocols we could detect enteropathogenic E. coli (EPEC) and verocytotoxin-producing E. coli (VTEC) (duplex assay for both Verotoxin 1 (VT1) and Verotoxin 2 (VT2)) in one run and enteroaggregative E. coli (EAEC), enteroinvasive E. coli (EIEC) and enterotoxigenic E. coli (ETEC) (duplex assay detecting both heat-stable enterotoxin (ST) and heat-labile enterotoxin (LT)) in another run. Using serial dilutions of control strains, the LC proved to be clearly more sensitive than conventional PCR for five out of seven investigated targets: VTEC (VT1 and VT2), ETEC (ST and LT) and EIEC. For EPEC and EAEC, LC and conventional PCR had identical sensitivities. With stool samples, we found an optimal agreement between LC-PCR and the conventional PCR when samples were tested in a 1:10 dilution. Only one specimen was discrepant, being repetitively positive for VT by LightCycler® but not by conventional PCR. Given the significantly higher sensitivity of the LC-PCR for the VT target (up to a 10−4 dilution factor by melting curve analysis and up to a 10−6 dilution factor following gel electrophoresis), this is probably a false negative result by conventional PCR. We conclude that LightCycler® PCR is more rapid, easier than and at least as sensitive as our conventional PCR for the detection of enterovirulent E. coli in stool specimens after culture on MacConkey.