Summary
Previously uncultured nitrite‐oxidizing bacteria affiliated to the genus
Nitrospira
have for the first time been successfully enriched from activated sludge from a municipal wastewater treatment plant. During the enrichment procedure, the abundance of the
Nitrospira
‐like bacteria increased to approximately 86% of the total bacterial population. This high degree of purification was achieved by a novel enrichment protocol, which exploits physiological features of
Nitrospira
‐like bacteria and includes the selective repression of coexisting
Nitrobacter
cells and heterotrophic contaminants by application of ampicillin in a final concentration of 50 µg ml
−1
. The enrichment process was monitored by electron microscopy, fluorescence
in situ
hybridization (FISH) with rRNA‐targeted probes and fatty acid profiling. Phylogenetic analysis of 16S rRNA gene sequences revealed that the enriched bacteria represent a novel
Nitrospira
species closely related to uncultured
Nitrospira
‐like bacteria previously found in wastewater treatment plants and nitrifying bioreactors. The enriched strain is provisionally classified as ‘
Candidatus
Nitrospira defluvii’.