Enumeration and characterization of arsenate-resistant bacteria in arsenic free soils

The numbers of arsenate-resistant bacteria present in four different soils were determined. Counts were typically in the order of 106 arsenate-resistant colony forming units g−1 of soil, accounting from 7 to 50% of the total viable count. Arsenate-resistant bacteria obtained from woodland soil and leaf litter were further characterized. 16S rRNA sequence analysis revealed that these bacterial isolates represented six bacterial groups including the Proteobacteria, Bacteroidetes, and Firmicutes. Almost all of these isolates were bacteria that have not previously been shown to tolerate arsenic. Some of these bacteria were capable of growth when exposed to high levels of arsenate (over 250 mM), although arsenite tolerance was much lower. These results support the suggestion that arsenate-resistant bacteria are common in natural environments and may play an important role in arsenic cycling in soils.