Assessment of differences between DNA content of cell-cultured and freely suspended oocysts of Cryptosporidium parvum and their suitability as DNA standards in qPCR

BACKGROUND: Although more modern methods are available, quantitative PCR (qPCR) is reproducible, sensitive and specific with instruments and expertise readily available in many laboratories. As such, the use of qPCR in Cryptosporidium research is well established and still widely used by researchers globally. This method depends upon the generation of standards at different concentrations to generate standard curves subsequently used for the quantification of DNA. METHODS: We assessed four types of DNA template used to generate standard curves in drug screening studies involving Cryptosporidium spp.: (i) serially diluted Cryptosporidium parvum oocysts (10⁶–1); (ii) diluted template DNA from pure oocysts (×10–×10⁶ dilution of 10⁶ oocyst DNA template); (iii) oocysts incubated in human ileocecal adenocarcinoma (HCT-8) cells (10⁵–1 and 5 × 10⁴–50); and (iv) diluted DNA template (5 × 10⁴) from cell culture incubated parasites (×10–×1000). RESULTS: Serial dilutions of both cell culture and pure oocyst suspension DNA template yielded better linearity than cell culture derived standards, with dilutions of 10⁶ oocysts exhibiting similar quantification cycle (Cq) values to those obtained from DNA template dilutions of 10⁶ oocysts. In contrast, cell culture incubated oocysts demonstrated significantly higher DNA content than equivalent freely suspended oocysts and diluted DNA template from both cell culture derived and freely suspended oocysts across numerous concentrations. CONCLUSIONS: For many studies involving Cryptosporidium, only relative DNA content is required and as such, the superior linearity afforded by freely suspended oocysts and diluted DNA template (from either cell culture derived standards or freely suspended oocysts) will allow for more accurate relative quantification in each assay. Parasite division in the cell culture standards likely explains the higher DNA content found. These standards, therefore, have the potential to more accurately reflect DNA content in cell culture assays, and despite more modern methods available for absolute quantification, i.e. droplet digital PCR (ddPCR), the ubiquity of qPCR for the foreseeable future encourages further investigation into the reduced linearity observed in these standards such as varying oocyst seeding density, non-linear growth rates and assay efficiency.