intI1 primer selection for class 1 integron integrase gene and transcript quantification - validation and application for monitoring intl1 gene abundance within septic tanks in Thailand
Antimicrobial resistance (AMR) poses serious global public health threat and wastewater treatment (WWT), including septic tanks, are a significant source of AMR genes to the environment. Environmental monitoring of broad-range AMR genes remains a challenge. The class-1 integron-integrase (intI1) gene has been proposed as a proxy for overall AMR abundance, but there is no consensus on the qPCR primer set to use. A systematic review of the literature found 65 primer sets. The coverage and specificity of each, including newly designed MGB-TaqMan primer-probe, was evaluated against a database of clinical and environmental intl1, intl1-like and non-intl1 sequences. Three primer sets were selected, laboratory validated for DNA and mRNA quantification and used to quantity intl1 gene abundance from household and healthcare conventional septic tanks (CST) and novel household Solar Septic Tanks (SST) in Thailand. Specificity of the intl1 septic tank amplicons showed that no primer set could distinguish between intl1 and intl-1 like sequences. Each primer set showed the same trends across septic tanks, with highest gene abundance in influent>sludge>effluent. There was no statistical difference between the same sample quantified by the three primer sets. However, when comparing gene abundances from the same primer set across septic tanks, statistical differences between different sample types were observed for one primer set but not the others. This may lead to different interpretation of risk associated with each reactor in spreading intl1 to the environment. Comparing reactor types, intI1 abundance in the effluent was lowest in the SST-household<CST-household<CST-healthcare. Depending on primer set used, 31 to 42% of intI1 was removed from effluent of the CST-household tank with accessible influent. Our study provided insight into the importance of intl1 primer choice. We propose the use of the validated set (F3-R3) for optimal intI1 quantification and towards the goal of achieving standardisation across environmental studies.