Optimization of Quantitative PCR Methods for Enteropathogen Detection

Jie Liu , Jean Gratz, Caroline Amour, Rosemary Nshama, Thomas Walongo, Athanasia Maro, Esto Mduma, James Platts-Mills, Nadia Boisen, James Nataro, Doris M. Haverstick, Furqan Kabir, Paphavee Lertsethtakarn, Sasikorn Silapong, Pimmada Jeamwattanalert, Ladaporn Bodhidatta, Carl Mason, Sharmin Begum, Rashidul Haque, Ira Praharaj, Gagandeep Kang, Eric R. Houpt
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Detection and quantification of enteropathogens in stool specimens is useful for diagnosing the cause of diarrhea but is technically challenging. Here we evaluate several important determinants of quantification: specimen collection, nucleic acid extraction, and extraction and amplification efficiency. First, we evaluate the molecular detection and quantification of pathogens in rectal swabs versus stool, using paired flocked rectal swabs and whole stool collected from 129 children hospitalized with diarrhea in Tanzania. Swabs generally yielded a higher quantification cycle (Cq) (average 29.7, standard deviation 3.5 vs. 25.3 ± 2.9 from stool,P<0.001) but were still able to detect 80% of pathogens with a Cq < 30 in stool. Second, a simplified total nucleic acid (TNA) extraction procedure was compared to separate DNA and RNA extractions and showed 92% (318/344) sensitivity and 98% (951/968) specificity, with no difference in Cq value for the positive results (ΔCq(DNA+RNA-TNA) = -0.01 ± 1.17, P = 0.972, N = 318). Third, we devised a quantification scheme that adjusts pathogen quantity to the specimen’s extraction and amplification efficiency, and show that this better estimates the quantity of spiked specimens than the raw target Cq. In sum, these methods for enteropathogen quantification, stool sample collection, and nucleic acid extraction will be useful for laboratories studying enteric disease.