A gram-negative, non-motile, non-marine, nitrite-oxidizing bacterium was isolated from an enrichment culture initiated with a sample from a partially corroded area of an iron pipe of a heating system in Moscow, Russia. The cells were 0.9-2.2 microns x 0.2-0.4 microns in size. They were helical- to vibroid-shaped and often formed spirals with up to three turns 0.8-1.0 micron in width. The organism possessed an enlarged periplasmic space and lacked intracytoplasmic membranes and carboxysomes. The cells tended to excrete extracellular polymers, forming aggregates. The bacterium grew optimally at 39 degrees C and pH 7.6-8.0 in a mineral medium with nitrite as sole energy source and carbon dioxide as sole carbon source. The optimal nitrite concentration was 0.35 mM. Nitrite was oxidized to nitrate stoichiometrically. The doubling time was 12 h in a mineral medium with 7.5 mM nitrite. The cell yield was low; only 0.9 mg protein/l was formed during oxidation of 7.5 mM nitrite. Under anoxic conditions, hydrogen was used as electron donor with nitrate as electron acceptor. Organic matter (yeast extract, meat extract, peptone) supported neither mixotrophic nor heterotrophic growth. At concentrations as low as 0.75 g organic matter/l or higher, growth of nitrite-oxidizing cells was inhibited. The cells contained cytochromes of the b- and c-type. The G+C content of DNA was 56.9 +/- 0.4 mol%. The chemolithoautotrophic nitrite-oxidizer differed from the terrestrial members of the genus Nitrobacter with regard to morphology and substrate range and equaled Nitrospira marina in both characteristics. The isolated bacterium is designated as a new species of the genus Nitrospira.(ABSTRACT TRUNCATED AT 250 WORDS)