A strictly anaerobic thiosulfate-reducing bacterium was isolated from a corroding offshore oil well in Congo and was designated strain SEBR 4207. Pure culture of the strain induced a very active pitting corrosion of mild steel, with penetration rates of up to 4 mm per year. This constitutes the first experimental evidence of the involvement of thiosulfate reduction in microbial corrosion of steel. Strain SEBR 4207 cells were vibrios (3 to 5 by 1 µm), stained gram negative, and possessed lateral flagella. Spores were not detected. Optimum growth occurred in the presence of 3% NaCl at pH 7.0 and 42°C. Strain SEBR 4207 utilized peptides and amino acids, but not sugars or fatty acids. It fermented serine, histidine, and Casamino Acids, whereas arginine, glutamate, leucine, isoleucine, alanine, valine, methionine, and asparagine were only used in the presence of thiosulfate. Peptides were fermented to acetate, isobutyrate, isovalerate, 2-methylbutyrate, H2, and CO2. The addition of either thiosulfate or sulfur but not sulfate increased peptide utilization, growth rate, and biomass; during growth, H2S was produced and a concomitant decrease in H2 was observed. The addition of either thiosulfate or sulfur also reversed H2 inhibition. 16S rRNA sequence analysis indicates that strain SEBR 4207 is distantly related to members of the genus (83% similarity). Because the phenotypic and phylogenetic characteristics cannot be assigned to any described genus, strain SEBR 4207 is designated as a new species of a new genus, gen. nov., sp. nov. Strain SEBR 4207 has been deposited in the Deutsche Sammlung von Mikroorganismen und zellkulturen GmbH (= DSM 11002).


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