Quantitation of Mycoplasma genitalium using droplet digital PCR

Mycoplasma genitalium is a sexually transmitted infection with increasing concerns around antimicrobial resistance. Droplet digital PCR (ddPCR) is a rapid quantification method with high precision that may be useful for absolute quantitation of bacteria in samples. This study aimed to develop a ddPCR assay for the quantification of M. genitalium. ddPCR targeting the gene mgpB was established and analysed using the QX100 ddPCR system. The assay was evaluated against quantitated DNA standards, and then in comparison to an established quantitative PCR performed on the Lightcycler 480 II. DNA template of increasing complexity was used, including synthetic double stranded DNA, DNA extracts from laboratory-cultured M. genitalium strains (n = 17) and DNA from M. genitalium-positive clinical samples (n = 21). There was a strong correlation between ddPCR concentration estimates and measured DNA standards (r2 = 0.997), and between ddPCR and qPCR quantitation for different templates (r2 ranging from 0.953 to 0.997). ddPCR reliably detected template in a range from <10 copies per reaction to >104 copies per reaction and demonstrated linearity over dilution series. Concentration estimates by ddPCR were reproducibly less than those determine by qPCR. ddPCR demonstrated precise and reproducible quantitation of M. genitalium with a variety of templates.