Isolation of iron-oxidizing bacteria from corroded concretes of sewage treatment plants

Thirty-six strains of iron-oxidizing bacteria were isolated from corroded c oncrete samples obtained at eight sewage treatment plants in Japan. All of the strains isolated grew autotrophically in ferrous sulfate (3.0%), elemen tal sulfur (1.0%) and FeS (1.0%) media (pH 1.5). Washed intact cells of the 36 isolates had activities to oxidize both ferrous iron and elemental sulf ur. Strain SNA-5, a representative of the isolated strains, was a gram-nega tive, rod-shaped bacterium (0.5-0.6 x 0.9-1.5 mu m). The mean G+C content o f its DNA was 55.9 mol%. The pH and temperature optima for growth were 1.5 and 30 degrees C, and the bacterium had activity to assimilate (CO2)-C-14 i nto the cells when ferrous iron or elemental sulfur was used as a sole sour ce of energy. These results suggest that SNA-5 is Thiobacillus ferrooxidans strain. The pHs and numbers of iron-oxidizing bacteria in corroded concret e samples obtained by baring to depths of 0-1, 1-3, and 3-5 cm below the co ncrete surface were respectively 1.4, 1.7, and 2.0, and 1.2 x 10(8), 5 x 10 (7), and 5 x 10(6) cells/g concrete. The degree of corrosion in the sample obtained nearest to the surface was more severe than in the deeper samples. The findings indicated that the levels of acidification and corrosion of t he concrete structure corresponded with the number of iron-oxidizing bacter ia in a concrete sample. Sulfuric acid produced by the chemolithoautotrophi c sulfur-oxidizing bacterium Thiobacillus thiooxidansis known to induce con crete corrosion. Since not only T. thiooxidans but also T. ferrooxidans can oxidize reduced sulfur compounds and produce sulfuric acid, the results st rongly suggest that T. ferrooxidans as well as T. thiooxidans is involved i n concrete corrosion.