ARSENIC TOXICITY IS NOT DUE TO A DIRECT EFFECT ON THE OXIDATION OF REDUCED INORGANIC SULFUR-COMPOUNDS BY THIOBACILLUS-CALDUS

Thiobacillus caldus is a moderately thermophilic acidophile which has been implicated in the biooxidation of arsenic containing mineral sulf ides. The toxic effects of arsenic on this bacterium are presented her e. Addition of arsenite to a growing culture of T. caldus caused a tra nsient increase in the optical density of the culture while causing a simultaneous decrease in cell viability. The increase in optical densi ty was shown to be due to the formation of extracellular sulfur. The o xidation rates of tetrathionate and thiosulfate were decreased by incr easing concentrations of arsenite, while in a culture induced to arsen ic resistance the rates were not as adversely effected. Sulfur oxidati on was also inhibited to the same extent as tetrathionate oxidation, w ith the oxidation of solid sulfur being slightly more effected than th e oxidation of sulfur dissolved in acetone. Thus, bactericidal arsenit e causes a transient formation of extracellular sulfur in the culture supernatant of T. caldus yet the toxicity of arsenite is not due to di rect inhibitory effects on reduced inorganic sulfur compound oxidation by these bacteria.