Digital PCR to detect and quantify heteroresistance in drug resistant Mycobacterium tuberculosis

Drug resistance in Mycobacterium tuberculosis presents an enormous public health threat. It is typically defined as >1% of drug resistant colonies using the agar proportion method. Detecting small numbers of drug resistant Tb in a population, also known as heteroresistance, is challenging with current methodologies. Here we have utilized digital PCR to detect heteroresistance within M. tuberculosis populations with excellent accuracy versus the agar proportion method. We designed dual TaqMan-MGB probes to detect wild-type and mutant sequences of katG (315), rpoB (531), gyrA (94,95) and rrs (1401), genes that associate with resistance to isoniazid, rifampin, fluoroquinolone, and aminoglycoside respectively. We generated heteroresistant mixtures of susceptible and extensively drug resistant Tb, followed by DNA extraction and digital PCR. Digital PCR yielded a close approximation to agar proportion's percentages of resistant colonies, and yielded 100% concordance with agar proportion's susceptible/resistant results. Indeed, the digital PCR method was able to identify mutant sequence in mixtures containing as little as 1000∶1 susceptible:resistant Tb. By contrast, real-time PCR or PCR followed by Sanger sequencing were less sensitive and had little resolution to detect heteroresistance, requiring fully 1∶1 or 10∶1 susceptible:resistant ratios in order to detect resistance. Our assay can also work in sputum so long as sufficient quantities of Tb are present (>1000 cfu/ml). This work demonstrates the utility of digital PCR to detect and quantify heteroresistance in drug resistant Tb, which may be useful to inform treatment decisions faster than agar proportion.