7th joint conference "Joint Microbiology & Infection Conference 2024" of the DGHM and VAAM

Abstract: Running cold and hot water in buildings is a widely established commodity. Except for legionellae, little attention has been given to the microbiology of domestic hot-water installations (DHWIs). In many countries, regulations consider hot water as heated drinking water that must comply (cold) drinking water (DW) standards. Only few reports exist on the microbiome in DHWIs. The lack of both, information and hygienic regulations, asks for a broad microbiological investigation of DHWI. Using flow cytometric (FCM) total cell count (TCC) determination, FCM-fingerprinting, and 16S rRNA-gene-based metagenomic analysis, the characteristics and composition of bacterial communities in hot water from associated boilers was studied in 14 inhouse DW installations in Switzerland and Austria. At all locations, 1.3 to 8-fold enhanced TCC were recorded in hot water compared to the cold DW. FCM-fingerprints of cold and corresponding hot water indicated different composition of cold- and hot-water microbiomes. Also, hot water in each boiler had its own individual FCM-fingerprint. 16S-rRNA-gene-based community analysis confirmed the marked differences in microbiome composition. Generally, bacterial diversity in cold DW was broad, that in hot water was restricted, with thermophilic strains from the families Hydrogenophilaceae, Nitrosomonadaceae and Thermaceae dominating. Batch growth assays, consisting of heated cold DW and inoculated with hot water, resulted in immediate cell growth, suggesting hot-water boilers as autonomously operating, thermophilic bioreactors. When cold DW was used as inoculum no significant growth was observed. The generation of assimilable organic carbon from dissolved organic carbon due to heating appeared to be the driver for growth of thermophilic microbial communities in hot-water systems. Despite consumers have been exposed to hot-water microbiota for a long time, with the exception of Legionella, no major pathogens have so far been associated with hot-water use. The role of harmless thermophiles and their interaction with potential human pathogens at elevated temperatures in DHWIs remains to be investigated (see Water Research 253 (2024): 121109).

Active thermophilic bacterial communities thrive in domestic hot-water boilers

Raum 5-6

Alexander Kirschner

Invited Speaker