Quantitative PCR and culture evaluation for enterotoxigenic Escherichia coli (ETEC) associated diarrhea in volunteers

Recent evidence suggests that the abundance of enteric pathogens in the stool correlates with the presence of clinical diarrhea. We quantified the fecal pathogen after feeding enterotoxigenic Escherichia coli (ETEC) strain H10407 to 30 adult volunteers. Stools were collected daily and examined using qualitative and quantitative (Q) culture. DNA was isolated, and quantitative (Q) PCR targeting the heat-labile toxin (LT) gene was performed. Nine volunteers developed diarrhea. Among 131 stool specimens with complete data, pathogen abundance by QPCR was strongly correlated with Qculture, ρ = 0.61, P < 0.0001. Receiver operating characteristic curve analysis comparing quantitative data against diarrhea status suggested cut-points, based on a maximum Youden Index, of 2.8 × 10⁴ LT gene copies and 1.8 × 10⁷ CFU. Based on these cut-points, QPCR had a sensitivity and specificity compared with diarrheal status of 0.75 and 0.87, respectively, and an OR of 20.0 (95% CI 5.7-70.2), whereas Qculture had a sensitivity and specificity of 0.73 and 0.91, respectively, and an OR of 28.6 (95% CI 7.7-106.6). Qculture had a sensitivity and specificity of 0.82 and 0.48, respectively and an OR of 4.4 (95% CI 1.2-16.0). The correlation between Qculture and QPCR was highest in diarrheal specimens, and both quantitative methods demonstrated stronger association with diarrhea than qualitative culture. Quantifying ETEC using quantitative culture or QPCR and the use of a clinically relevant threshold gives a more definite diagnosis of ETEC-associated diarrhea compared with qualitative culture.