Members of the bacterial genus Sphingomonas (recently split into four genera), belonging to α-4-subclass of Proteobacteria, were isolated and characterised from water distribution network biofilms. Water temperature in the studied network, serving 200,000 people, is less than 5°C for about five months every winter. Sphingomonads, characterised using fluorescent oligonucleotide probes and fatty acid composition analysis (FAME), were a major group of bacteria among the distribution network biofilm isolates. Intact biofilms, grown on steel slides in a biofilm reactor fed with tap water, were detected in situ using fluorescence labelled oligonucleotide probes (FISH). Hybridisation with probes targeted on α-proteobacteria and sphingomonads was detected, but FISH on intact biofilms suffered from non-specific hybridisation and intensive autofluorescence, possibly due to extracellular material around the bacterial cells attached to biofilm. These preliminary results indicate that bacteria present in the distribution network biofilms in this study phylogenetically differ from those detected in more temperate regions.

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