By: Erik Wentzel

 Identifying Characteristics

The above left picture shows cells of thiobacilli growing in a "floc" of cellular material and sulfur in water.


Taxonomic Description

The picture to the left shows yellow and red zones below a hot spring in Yellowstone National Park, where large populations of thermal-tolerant thiobacilli have grown by oxidizing reduced sulfur and iron in the water.

The genus Thiobacillus can be broken into two groups. The first of these are those that grow only at neutral pH values. These are responsible for the oxidation of elemental sulfur in the reaction:

S- + 202 + 2H2O = 2H2SO4

This process yields 236 kcal of energy. The reduced sulfur compounds are complexed with a sulfhydryl group on a tripeptide glutathione. It is then oxidized to sulfite with the help of the enzyme sulfide oxidase. There is an apparent disparity in the lifestyles of these organisms. It lies in the fact that they produce sulfuric acid, but find it toxic. Some of those organisms growing at lower pH values can however, utilize Fe2+ as an electron donor.

The five most described members of the Thiobacillus species are: Thiobacillus thioparus, Thiobacillus denitrificans, Thiobacillus thiooxidans, Thiobacillus intermedius, and Thiobacillus ferrooxidans.

Thiobacillus thioparus exhibits the following reaction:

N-C-S- + 2O2 + 2H2O = SO42+ NH4+ + CO2 + 220 kcal

Thiobacillus denitrificans differs in that it can utilize NO3 instead of O2. This denitrification is shown in the following chemical equation:

2NO3- + S + H2O + CaCO3 = CaSO4 + N2

Thiobacillus thiooxidans has a much more acidic growth range. It grows beat between 2 to 5. It is also strictly aerobic, and is motile.

Thiobacillus intermedius is a facultative chemolithotroph with a pH range of 3 to 7. It growth is powered by S2O32-, which acts as an electron donor, and it is stimulated by the presence of organic matter.

Thiobacillus ferrooxidans is a unique organism when taken in the paradigm of Thiobacillus. It is strictly aerobic and it has a pH growth range of 1.5 to 5. It can oxidize Fe2+.


Isolation and Ecology

There are numerous growth media for the Thiobacillus genus. First, and foremost, are the broths. There are four major solutions of T2 media. They are separated into A, B, C, and D. Broth A is composed of Na2S2, 5H2O, KNO3, and NH4Cl. Solution B has KH2PO4, while C has NaCO3, and D has MgSO4, 7H2O, FeSO4, 7H2O, and trace metals. There is also a T3 agar and broth, and Thiobacillus thioparus grows well on thiocyanate.

The oxidation of sulfur that Thiobacillus executes can be very important commercially for the mining industry. The coal spoil piles that are left after strip mining operation are acted upon, and the byproduct is sulfuric acid. The left over is then termed acid mine drainage, and can be a problem in areas already devastated by the loss of wildlife, and habitat.

The corrosion damage of a gas pipeline by Thiobacillus ferrooxidans. The corroded hole in the pipe is obvious.

Another important commercial problem due to the action of Thiobacillus, Thiobacillus ferrooxidans in particular, is the corrosion of pipes and wells. Because the energy yield of the iron oxidation is so low, very large amounts of Fe3+ are produced. This forms highly insoluble ferric oxyhydroxides that can build up and clog pipes.

Other Sources of Information

Paul, E.A., Clark, F.E. 1996. Soil Microbiology and Biochemistry. Academic Press. San Diego, CA. 340 pp.

Chapelle, Francis H. 1993. Ground-Water Microbiology and Geochemistry. John Wiley & Sons, Inc. New York, NY. Pp 98-99.

Atlas, Ronald M., Parks, Lawrence C. 1993. Handbook of Microbiological Media. CRC Press. Boca Raton, FL. 1079 pp.

Holt, J.G., N.R. Krieg, P.H.A. Sneath, J.T. Staley, and S.T. Williams. 1994. Bergy’s Manual of Determinative Microbiology. Ninth Ed. Williams and Wilkins. Baltimore, MD. 787 pp.

Some Other Helpful Sites

American Society of Microbiology:

Institute of Microbial Ecology: