Molecular pathogen detection in biosolids with a focus on quantitative PCR using propidium monoazide for viable cell enumeration

Sewage sludge is the solid, organic material remaining after wastewater is treated and discharged from a wastewater treatment plant. Sludge is treated to stabilize the organic matter and reduce the amount of human pathogens. Once government regulations are met, including material quality standards (e.g., E. coli levels and heavy metal content) sludge is termed "biosolids", which may be disposed of by land application according to regulations. Live-culture techniques have traditionally been used to enumerate select pathogens and/or indicator organisms to demonstrate compliance with regulatory requirements. However, these methods may result in underestimates of viable microorganisms due to several problems, including their inability to detect viable but non-culturable (VBNC) cells. Real-time quantitative polymerase chain reaction (qPCR) is currently under investigation as a fast, sensitive, and specific molecular tool for enumeration of pathogens in biosolids. Its main limitation is that it amplifies all target DNAs, including that from non-viable cells. This can be overcome by coupling qPCR with propidium monoazide (PMA), a microbial membrane-impermeant dye that binds to extracellular DNA and DNA in dead or membrane-compromised cells, inhibiting its amplification. PMA has successfully been used to monitor the presence of viable pathogens in several different matrices. In this review the use of PMA-qPCR is discussed as a suitable approach for viable microbial enumeration in biosolids. Recommendations for optimization of the method are made, with a focus on DNA extraction, dilution of sample turbidity, reagent concentration, and light exposure time.