Characterization of siderophore producing arsenic-resistant Staphylococcus sp. strain TA6 isolated from contaminated groundwater of Jorhat, Assam and its possible role in arsenic geocycle

Abstract

Background: Microorganisms specifically bacteria play a crucial role in arsenic mobilization and its distribution in aquatic systems. Although bacteria are well known for their active participation in the different biogeochemical cycles, the role of these bacteria in regulating the concentration of arsenic in Brahmaputra valley has not been investigated in detail.

Results: In this paper, we report the isolation of an arsenic resistant bacterium TA6 which can efficiently reduce arsenate. The isolate identified as Staphylococcus sp. TA6 based on the molecular and chemotaxonomic identification (FAME) showed resistance to the high concentration of both arsenate and arsenite (As(III) = 30 mM; As(V) = 250 mM), along with cross-tolerance to other heavy metals viz., Hg²⁺, Cd²⁺, Co²⁺, Ni²⁺_, Cr²⁺. The bacterium also had a high siderophore activity (78.7 ± 0.004 μmol) that positively correlated with its ability to resist arsenic. The isolate, Staphylococcus sp. TA6 displayed high bio-transformation ability and reduced 2 mM As(V) initially added into As(III) in a period of 72 h with 88.2% efficiency. The characterization of arsenate reductase enzyme with NADPH coupled assay showed the highest activity at pH 5.5 and temperature of 50 °C.

Conclusions: This study demonstrates the role of an isolate, Staphylococcus sp. TA6, in the biotransformation of arsenate to arsenite. The presence of ars operon along with the high activity of the arsenate reductase and siderophore production in this isolate may have played an important role in mobilizing arsenate to arsenite and thus increasing the toxicity of arsenic in the aquatic systems of the Brahmaputra valley.

Keywords: Arsenate reductase; Arsenic; Assam; Jorhat; Siderophore; Staphylococcus sp..

Fig. 1

The evolutionary history was inferred using the Neighbor-Joining method. The optimal tree with the sum of branch length = 0.90052 (~ 0.1) is shown. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (1000 replicates) are shown next to the branches. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Jukes-Cantor method and are in the units of the number of base substitutions per site. All positions containing gaps and missing data were eliminated

Fig. 2

Effect of arsenic on bacterial growth rate (a) Arsenite (As(III)) (b) Arsenate (As(V))

Fig. 3

(a) Bioconversion of arsenate [As (V)] to arsenite [As (III)]. (b) The rate of Biotransformation over an incubation period of 72 h

Fig. 4

Kinetic Profile of enzyme activity (Lineweaver Burk Plot)

Fig. 5

Arsenate Reductase enzyme activity (a) At different pH (pH 5.5 was found to be optimal for maximum activity) (b) At different Temperature (50 °C was found as the optimal temperature for maximum activity)

Fig. 6

Effect of siderophore on growth percentage and arsenate reduction in the modified LB medium containing (a) 5 mM arsenate (b) 10 mM arsenate

Fig. 7

Graphical representation of tentative schematic mechanism of arsenic mobilization by Staphylococcus sp. TA6. The process can be catagorised in four sequential steps (A) production of siderophore to scavenge iron from arsenopyrite ores (B) Entry of arsenate through pit/pst phosphate transport channel into the cell (C) conversion of arsenate to arsenite and (D) efflux of arsenite to the surrounding environment.