Contrasting drivers of nontuberculous mycobacteria in building hot and cold water systems.

Nontuberculous mycobacteria (NTM) are opportunistic pathogens frequently detected in building water supply systems (BWSS). This study investigated NTM levels and their correlations with water quality parameters in a hotel building in East China. Sampling was conducted at 15 selected sites and a total of 45 cold-water and 23 hot-water samples were collected from May to December. Water quality parameters, including temperature, pH, turbidity, total chlorine, total dissolved solids (TDS), dissolved organic carbon (DOC), UV254, heterotrophic plate count (HPC), total cell count (TCC), intact cell count (ICC), assimilable organic carbon (AOC), and heavy metals (As, Pb, Al, Cu, Zn), were measured. NTM was quantified by 16S rRNA gene qPCR. NTM occurrence followed the order: building hot water systems (BHWS) > building cold water systems (BCWS) > municipal water supply systems (MWSS). In BCWS, NTM were positively correlated with temperature (p < 0.01) and UV254 (p < 0.05), and negatively correlated with total chlorine (p < 0.01), TDS (p < 0.01), and pH (p < 0.05). In BHWS, NTM were positively correlated with pH (p < 0.01) and negatively correlated with TDS (p < 0.05). All significant correlations remained after false discovery rate (FDR) correction (q < 0.05). These correlative patterns suggest that temperature/disinfectant management in BCWS and pH/metal regulation in BHWS may mitigate NTM risks, though these hypotheses require intervention validation. Causality remains unconfirmed. The qPCR assay targets total 16S rRNA genes and does not distinguish between viable and non-viable cells, nor between pathogenic and non-pathogenic species; thus, the reported gene copy numbers may overestimate the actual health hazard.