Bacterial diversity from the source to the tap: a comparative study based on 16S rRNA gene-DGGE and culture-dependent methods

This study aimed to assess the influence of water treatment and distribution on the bacterial communities with particular emphasis on tap water. Samples from the water treatment plant, the bulk supply distribution system and household taps, supplied by the same drinking water treatment plant, were analyzed using culture-dependent and culture-independent methods. Water treatment imposed alterations in the composition of the bacterial community, although this effect was more evident in the cultivable bacteria rather than among the total community assessed by 16S rRNA gene-denaturing gradient gel electrophoresis (DGGE) profiling. Water disinfection, mainly chlorination, promoted a reduction on bacterial diversity and cultivability, with a shift in the pattern of cultivable bacteria from predominantly Gram-negative to predominately Gram-positive and acid-fast. Downstream of the chlorination stages, tap water, in comparison with raw water, presented higher diversity indices and cultivability percentages. From the source to the tap, members of the Alpha-, Beta- and Gammaproteobacteria were the predominant lineages identified using 16S rRNA gene-DGGE analysis. Although with a lower coverage, the DGGE-based lineage identifications were in agreement with those found using 454-pyrosequencing analysis. Despite the effectiveness of water treatment to eliminate or inactivate most of the bacteria, Proteobacteria such as Acinetobacter, Bosea and Sphingomonadaceae may successfully colonize tap water.