In order to analyze the growth of Thiobacillus thiooxidans in the basal Starkey's medium supplemented with solid particles of sulfur, CS₂ method was developed to measure the cells adhering to the sulfur particles; the cells were totally released from the sulfur particles into the liquid phase by shaking of the culture with carbon disulfide, so that the total amount of cells existing in the culture could be measured quantitatively. The growth of Thiobacillus thiooxidans was remarkably enhanced by the addition of a trace amount of heavy metal ions to the basal Starkey's medium. It was found that, at the initial phase of growth, practically all the cells were attached on the surface of sulfur particles and, with progress of the culture, an increasing number of cells appeared in the liquid phase. At the stationary phase of growth, about equal amounts of cells were distributed on the sulfur particles and in the liquid phase.
Among the trace elements added, molybdate was found to be the very substance to accelerate the growth of Thiobacillus thiooxidans. In the presence of molybdate (below 0.3ppm as molybdenum) the specific growth rate and the final cell yield increased 2- and 4-fold, respectively, compared to those in its absence. This stimulative effect could not be substituted by tungstate, chromate, or vanadate.