A study on the role of exopolysaccharide in conferring acid tolerance in Bacillus sp.
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Date
2018-01
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AAU, Jorhat
Abstract
Soil bacteria have evolved various mechanisms to adapt to stress environmental conditions such as temperature, salinity, drought and low pH condition of soil. Among the several environmental stress conditions, soil acidity an important factor influencing physicochemical and biological properties of soil along with microbial diversity and crop production is an emerging issue of immense concern due to its wide spread distribution across the globe. Although low soil pH restricts the number and diversity of bacteria, it is known that some soil bacteria are able to thrive in such conditions having evolved various mechanisms including production of biofilm to circumvent acid stress. Bacterial exopolysaccharide (EPS) are high-molecular-weight complex polymers composed of sugar moieties that form the main component of the biofilm which aid the bacteria to colonize substrata. In the present study, a total of 28 isolates were identified and characterized as acid tolerant EPS producing bacteria among which, B. amyloliquefaciens p16 produce the highest EPS (219.96 μg/ml). A culture medium containing sucrose (3.5%) as carbon source with pH 5.0 and incubated for 24 hrs was optimal for maximum production of EPS. The HPLC analysis of monomeric units of EPS produced by B. amyloliquefaciens p16 revealed the abundance of galactose at pH 7.0 which however, changed to arabinose when shifted to acidic condition (pH to 5.0 and 4.5). The isolate B. amyloliquefaciens p16, significantly improved soil physical properties in terms of greater soil aggregation (80.59 mm diameter aggregates) and water holding capacity (53.90%) when inoculated into soil over the control (31 mm diameter aggregates and 18.21%, respectively). The differential expression of epsA and epsB, the first two genes of the eps operon showed a 7 and 9 fold increased expression in pH 5.0 compared to pH 7.0 respectively. Disruption of the epsB gene in B. amyloliquefaciens p16 using integration vector pMUTIN4 generated mutants that produced significantly lesser EPS (33.23 μg/ml) when compared to the WT (223.87 μg/ml). The generated mutant of B. amyloliquefaciens p16 lacking the wrinkled morphology had an extended lag phase of 24 hrs and was barely able to survive in acidic medium (pH 4.5) unlike that of the WT type. Soil inoculated with generated mutants formed smaller soil aggregates (42.41±1.70 mm) and had decreased water holding capacity (27.67±1.94%) compared to the WT (80.59± 0.22 mm and 53.90± 1.66%, respectively). This study indicates that EPS secreted by acid tolerant bacteria (B. amyloliquefaciens p16) imparts acid tolerance and also aids in improving the soil physical structure through increased soil aggregation and water holding capacity.