Variation in the toxicity of arsenic compounds to microorganisms and the suppression of the inhibitory effects by phosphate.

The toxicity of potassium arsenate, as measured by retardation or inhibition of growth on solid nutrient media, showed wide variation among different fungi but was consistently reduced by the addition of large amounts of potassium phosphate, with both arsenic-sensitive and arsenic-tolerant fungi. Poria monticola was completely inhibited by 0.0025 M arsenate but was progressively less inhibited as the phosphate content of the medium increased and grew slowly at 0.04 M arsenate when 0.16 M KH(2)PO(4) was added. Cladosporium herbarum showed 36% reduction in growth at 0.08 M arsenate in a low-phosphate medium, but when 0.01 M KH(2)PO(4) was added, arsenate concentrations up to 0.64 M (at which the medium contains 4.8% As) caused no reduction in growth rate. Addition of phosphate also reduced the toxicity of potassium arsenite but not that of dimethyl sodium arsonate (sodium cacodylate). The counteracting effect of phosphate on arsenate toxicity was found to occur with every one of a wide variety of microorganisms tested. The author interprets the results as supporting the thesis that the fungitoxicity of arsenate is due to its competitive interference with phosphorus in oxidative phosphorylation and not to a reaction with the -SH groups of essential proteins. The latter mechanism is, however, probably operative with dimethyl sodium arsonate. The practical implications of the counter-inhibition phenomenon in laboratory investigations and standard tests of arsenical fungicides, in biochemical research, and in the commercial use of arsenical biocides are set out.