High-resolution melting analysis: A novel approach for clade differentiation in Pythium insidiosum and pythiosis

Pythium insidiosum causes life-threatening human pythiosis. Based on phylogenetic analysis using internal transcribed spacer (ITS) region, mitochondrial cytochrome C oxidase II (COX2) gene, intergenic spacer (IGS) region and exo-1,3-β-glucanase gene (exo1), P. insidiosum is classified into clade ATH, BTH, and CTH related to geographic distribution. At present, polymerase chain reaction in any of these specific regions with DNA sequencing is the only technique to provide clade diagnosis. In this study, P. insidiosum-specific primers targeting COX2 gene were designed and used in real-time quantitative polymerase chain reaction (qPCR) with subsequent high-resolution melting (HRM) to provide rapid identification as well as clade classification for P. insidiosum. Based on the qPCR-HRM method, 15 P. insidiosum isolates could be differentiated from 28 related organisms with 100% specificity and 1 pg limit of detection. This technique was, in addition, directly tested on clinical samples from proved human pythiosis cases: nine corneal scrapes and six arterial clots. The qPCR-HRM results of all nine corneal samples were a 100% match with the results from the conventional PCR at clade level. However, the qPCR-HRM results of arterial clot samples were only matched with the nucleotide sequencing results from the conventional PCR at species level. In conclusion, the qPCR-HRM is a simple one closed tube, inexpensive and user-friendly method to identify P. insidiosum into clade level.