###### Abstract
While water safety is strictly monitored in DWDSs, this is not the case to the same extent in DWIs. Deterioration of microbiological water quality has been frequently reported, especially in the \"last meters prior to consumption\" [15]. Testing for opportunistic pathogens in a research context regularly reveals surprises. In a large-scale German study from 2017, 12.8% of drinking water samples (\(n=22\),786) from public buildings were found positive for _Legionella_ spp. above the allowed limit of 100 cfu/100 mL [16]. The same study reported that 2.9% of samples (\(n=3468\)) were positive for _Pseudomonas_ spp. (\(\geq\)1 cfu/100 mL). Several factors contribute to the deteriorating microbiological water quality in premise plumbing systems, including the use of other pipe materials than in the DWDS, such as elastomeric materials like EPDM and other plastic materials that promote bacterial growth [17]. Additionally, water stagnates in plumbing systems most of the time, on average 23 h per day [18]. Stagnation can cause substantial increases in cell numbers [19; 20]. And eventually, cold water in a DWI is typically not as cold as in a DWDS, leading to accelerated regrowth [20]. Although excessive warming of cold water can often be mitigated by correcting poor practices in plumbing systems, the problem is, in part, also associated with warmer raw water in the summer in surface water treatment plants and further warming during transport in the DWDS. The latter problems can be assumed to increase in the future with climate change. Finally, the heating of water inside buildings for hot water production affects the microbiology and probably also the nutrient pool. Bacterial communities in hot water have been found to differ strongly from cold water communities [6; 21]. Water from hot water boilers was reported to have higher total cell numbers than inflowing cold water [22]. The authors discussed the existence of specialized thermophilic bacteria utilizing AOC generated from the thermal disintegration of TOC. Overall, existing scientific knowledge points to constant changes in the microbiome, with microbes adapting to new conditions on their way from raw water to the tap. Changes in the composition of the microbial community and also in its function can be sensitively measured using modern sequencing technologies (including metagenomics). Whether these changes affect biological stability depends, however, on its definition. Newer definitions imply biological stability to be reached when conditions prevent the replication of microorganisms of hygienic relevance or of microorganisms that lead to aesthetic (i.e., color, odor or taste) or technical (i.e., corrosion) problems (for a review, see [23]. Overall, a diverse set of biological stability parameters is available with important differences between groundwater and surface water [24]. In this study, we relate biological stability to the regrowth potential of the autochthonous heterotrophic bacteria contained in the corresponding water before and after regrowth in a seven-day batch growth test. This parameter is fundamentally linked to nutrient availability. The net increase in cell numbers in these seven days, as determined by flow cytometry, was expressed as the regrowth factor, whereas the cell numbers obtained after regrowth represent the maximal growth potential. The presented data were compiled from different individual projects performed over the last few years and comprise changes throughout the treatment of different surface waters (river, reservoir and canal water) together with changes during distribution. Finally, we looked into drinking water at the entry points of different buildings and the effect of water heating. Whereas most studies focus on individual aspects of drinking water treatment, the distribution or changes in DWIs, we try to give a holistic overview of changes in cell concentrations, regrowth potentials and the deduced relative ratios of dissolved and biologically fixed nutrients from source to tap.