Chromosomally encoded arsenical resistance of the moderately thermophilic acidophile Acidithiobacillus caldus

Arsenical resistance is important to bioleaching microorganisms because the se organisms release arsenic from minerals such as arsenopyrite during biol eaching. The acidophile Acidithiobacillus caldus KU was found to be resista nt to the arsenical ions arsenate, arsenite, and antimony via an inducible, chromosomally encoded resistance mechanism. Because no apparent alteration of the toxic ions was observed, Acidithiobacillus (At.) caldus was tested to determine if it was resistant as a result of decreased accumulation of t oxic ions. Reduced accumulation of arsenate and arsenite by induced At. cal dus cells supported this hypothesis. It was also found that, with the addit ion of an energy source, induced At. caldus could transport arsenate and ar senite out of the cell against a concentration gradient. The lack of efflux in the absence of an added energy source and in the presence of inhibitors suggested that efflux was energy dependent. Induced At. caldus also expres sed arsenate reductase activity, indicating that At. caldus has an arsenica l resistance mechanism that is analogous to previously described systems fr om other Bacteria. Southern hybridization analysis showed that At. caldus a nd other gram-negative acidophiles carry an Escherichia coli arsB homologue on the chromosome.