Isolation of a sulfur-oxidizing Streptomyces sp. from deteriorating bridge structures and its role in concrete deterioration

• Microorganisms located on the surface of concrete at a Texas bridge site were cultured and found to lower pH to 3.4.
• A strain of Streptomyces sp. was isolated from the mixed culture and shown to oxidize thiosulfate to sulfuric acid.
• Concrete exposed to mixed culture and pure Streptomyces sp. culture showed chemical and physical signs of deterioration.

Concrete samples obtained from deteriorated bridge structures in Texas were cultured in growth medium containing thiosulfate and investigated for microbial production of acid and subsequent biodeterioration of concrete. Mixed cultures obtained from a corroded concrete oxidized thiosulfate to sulfuric acid causing the pH to drop from 6.3 to 3.4. Interestingly, the decrease in pH was minimal when the organisms were cultured in the presence of concrete and thiosulfate in spite of an 84% oxidation of thiosulfate to sulfuric acid, presumably due to the buffering capacity of fresh concrete. Biodeterioration of concrete was also evident from the 2-fold higher amount of calcium leached from concrete compared to un-inoculated controls. Further, a pure culture was isolated and identified as a Streptomyces sp. The isolate oxidized more than 96% of the added thiosulfate to sulfate (sulfuric acid) and decreased the culture pH from 6.4 to 3.5. Under acidic conditions, an appreciable amount of calcium leached from concrete (nearly 4-fold higher than the control) suggesting deterioration of concrete. Scanning electron micrographs of the concrete exposed to both mixed cultures as well as Streptomyces sp. revealed distinct cracks, presumably due to the formation of expandable products, gypsum and ettringite.