Bacterial cultures of Acidithiobacillus thiooxidans are isolated, maintained and identified and used in the treatment of materials containing sulfur-compounds, such as contaminated and/or spent catalysts with elemental sulfur (S). Bacterial cultures of Acidithiobacillus thiooxidans exhibit sulfur-oxidizing activity particularly useful in the transformation of elemental sulfur (S) to sulfates (SO4), a compound soluble in water (H2O) and usable in industry. The bacterial cultures of Acidithiobacillus thiooxidans are mainly used as a biological or biotechnological procedure for the treatment of contaminated and/or spent catalysts with elemental sulfur (S) hazardous contaminated wastes that are mainly, but not exclusively, from the Claus process that operates at environmental conditions; does not impact the environment or ecosystem; and recovers 91-100% of the elemental sulfur (S) in sulfate form (SO4).