Effect of Arsenic on Fluoride Tolerance in Microbacterium paraoxydans Strain IR-1

Abstract

Fluoride (F) and arsenic (As) are two major contaminants of water and soil systems around the globe, causing potential toxicity to humans, plants, animals, and microbes. These contaminated soil systems can be restored by microorganisms that can tolerate toxic stress and provide rapid mineralization of soil, organic matter, and contaminants, using various tolerance mechanisms. Thus, the present study was undertaken with the arsenic hyper-tolerant bacterium Microbacterium paraoxydans strain IR-1 to determine its tolerance and toxicity to increasing doses of fluoride, either individually or in combination with arsenic, in terms of growth inhibition using a toxicity unit model. The minimum inhibitory concentration (MIC)and half maximal inhibitory concentration (IC₅₀) values for fluoride increased, from 9 g/L to 11 g/L and from 5.91 ± 0.1 g/L to 6.32 ± 0.028 g/L, respectively, in the combination (F + As) group. The statistical comparison of observed and expected additive toxicities, with respect to toxicity unit (TU difference), using Student's t-test, was found to be highly significant (p < 0.001). This suggests the antagonistic effect of arsenic on fluoride toxicity to the strain IR-1. The unique stress tolerance of IR-1 ensures its survival as well as preponderance in fluoride and arsenic co-contaminated sites, thus paving the way for its possible application in the natural or artificial remediation of toxicant-exposed degraded soil systems.

Keywords: Microbacterium sp.; antagonistic effect; arsenic; fluoride; minimum inhibitory concentration; toxicity unit.

Figure 1

Comparative analysis of fluoride and arsenic toxicity on the growth of M. paraoxydans strain IR-1 (in 48 h). The figure depicts the maximum growth in the control group (without any toxicant—100% growth), and a gradual decrease in the growth of the strain when exposed to arsenic (Group I) and fluoride (Group II) individually. But there was a highly significant (p < 0.001) increase in the growth of the strain in the combination group (Group III—As + F,) as per the statistical comparison using Student’s t-test with Group I and Group II. All the experiments were performed in triplicate and error bars denote standard error.

Figure 2

Regression lines representing the growth inhibition of M. paraoxydans IR-1 in presence of arsenic (Group I, ■), fluoride (Group II, ▲), and the combination (Group III, ●). The inhibitory concentration of the combination group was higher than the individual exposure to arsenic and fluoride. IC₅₀ values for all the three groups were calculated from the respective regression line.