Arsenite and Arsenate Detection in Bacteria

The bacteria were pre-cultured overnight at 35 °C in a chemically defined medium (CDM)^{67 (link)}. After that, bacterial suspension was spread uniformly on CDM agar-containing culture plates (the experiment was repeated thrice, each time with ten replicates for each isolate). The CDM used comprised of K₂HPO₄ 10.0 g L⁻¹, Na₂SO₄ 1.0 g L⁻¹, NH₄Cl 1.0 g L⁻¹, MgSO₄ 2.0 g L⁻¹, CaCl₂·2H₂O 6.5 g L⁻¹, Fe₂SO₄·7H₂O 0.4 g L⁻¹, and NaHCO₃ 0.8 g L⁻¹. The pH of CDM used for this experiment was adjusted around 7.2 ± 0.2 at 25 °C. After 18–20 h of visible growth at 35 °C, 0.1 M AgNO₃ solution was poured on the agar plates to submerge the colonies^{68 (link),69} . The reaction between AgNO₃ and arsenite or arsenate leads to the formation of a coloured precipitate. The appearance of a brown precipitate indicates the existence of silver arsenate (Ag₃AsO₄) in the agar plate. On the contrary, a yellowish-white deposit suggests the presence of silver arsenite (Ag₃AsO₃)^{68 (link)}.