Determination of sulfur and iron oxidation bacteria by the Most Probable Number (MPN) technique

The number and the activity of acidophilic bacteria present in bioleaching processes is one of the most important parameters to understand the behavio ur of a leaching process. The most frequently isolated species from these e cosystems are strains of Thiobacillus ferrooxidans, Leptospirillum ferrooxi dans and Thiobacillus thiooxidans. A widely recognized difficulty in quanti fying these autotrophic bacteria is problems with growth on solid medium. T his work presents a modified methodology of the Most Probable Number techni que (MPN) to enumerate the active bacterial populations with the capacity t o oxidize Fe(II) and reduced inorganic sulfur compounds. Ln both cases, spe cific reagents were used to read the positive and negative tubes. The prese nce of ferric ion in the cultures of ferrous iron oxidizing bacteria was de tected using 1% solution of potassium thiocyanate. For the sulfur oxidizing bacteria, the sulphuric acid generation was determined with 0.1% thymol bl ue solution. The methods were developed using pure cultures of Thiobacillus ferrooxidans growing on ferrous sulfate or in sodium tetrathionate solutio n; a culture of Leptospirillum ferrooxidans was also utilized to set up the technique for ferrous iron oxidizing bacteria. Later the MPN tecniques wer e utilized to estimate the active populations of ferrous and sulfur oxidizi ng bacteria present in samples of pregnant solutions of the copper mineral bioleaching operations in the north of Chile.