Seasonal Dynamics in the Number and Composition of Coliform Bacteria in Drinking Water Reservoirs

Worldwide, surface waters like lakes and reservoirs are one of the major sources for drinking water production, especially in regions with water scarcity. In the last decades, they have undergone significant changes due to climate change. This includes not only an increase of the water temperature but also microbiological changes. In recent years, increased numbers of coliform bacteria have been observed in these surface waters. In our monitoring study we analyzed two drinking water reservoirs (Klingenberg and Kleine Kinzig Reservoir) over a two-year period in 2018 and 2019. We detected high numbers of coliform bacteria up to 2.4 x 104 bacteria per 100 ml during summer months, representing an increase of four orders of magnitude compared to winter. Diversity decreased to one or two species that dominated the entire water body, namely Enterobacter asburiae and Lelliottia spp., depending on the reservoir. Interestingly, the same, very closely related strains have been found in several reservoirs from different regions. Fecal indicator bacteria Escherichia coli and enterococci could only be detected in low concentrations. Furthermore, fecal marker genes were not detected in the reservoir, indicating that high concentrations of coliform bacteria were not due to fecal contamination. Microbial community revealed Frankiales and Burkholderiales as dominant orders. Enterobacterales, however, only had a frequency of 0.04% within the microbial community, which is not significantly affected by the extreme change in coliform bacteria number. Redundancy analysis revealed water temperature, oxygen as well as nutrients and metals (phosphate, manganese) as factors affecting the dominant species. We conclude that this sudden increase of coliform bacteria is an autochthonic process that can be considered as a mass proliferation or "coliform bloom" within the reservoir. It is correlated to higher water temperatures in summer and is therefore expected to occur more frequently in the near future, challenging drinking water production. HighlightsO_LIColiform bacteria proliferate in drinking water reservoirs to values above 104 per 100 ml C_LIO_LIThe genera Lelliottia and Enterobacter can form these "coliform blooms" C_LIO_LIMass proliferation is an autochthonic process, not related to fecal contaminations C_LIO_LIIt is related to water temperature and appears mainly in summer C_LIO_LIIt is expected to occur more often in future due to climate change C_LI Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=75 SRC="FIGDIR/small/428560v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@1f4a65borg.highwire.dtl.DTLVardef@d5dde3org.highwire.dtl.DTLVardef@da92b9org.highwire.dtl.DTLVardef@1d6c5a9_HPS_FORMAT_FIGEXP M_FIG C_FIG