Hot Spring Bacterium Yields Heat-Stable Amylase With Industrial Promise

Deep beneath the religiously significant waters of the Tuva thermal spring in Gujarat, India, a microscopic inhabitant has captured the attention of biotechnologists. Researchers at Gujarat University have isolated a thermotolerant bacterium, Bacillus stercoris THS-K1, that produces a remarkably heat-stable alpha-amylase, an enzyme capable of cleaving the alpha-1,4 glycosidic bonds that hold starch molecules together. The study, published in Discover Biotechnology, describes not only the discovery of this previously unreported strain but also a rigorous statistical optimisation of its enzyme production and the first comprehensive molecular characterisation of the amylase gene from this organism. The findings could reshape how industry approaches starch processing, one of the most enzyme-intensive operations in the world.

Alpha-amylases are the workhorses of the industrial enzyme market, accounting for roughly 30 percent of global enzyme production. These endo-1,4-alpha-D-glucan glucohydrolases randomly hydrolyse the internal bonds of starch chains, depolymerising polysaccharides into reducing sugars in the alpha-configuration. Unlike glucoamylases, which peel glucose units from the non-reducing ends of starch molecules step by step, alpha-amylases act throughout the polymer, making them invaluable for rapid starch liquefaction. Both enzyme classes are calcium-dependent metalloenzymes, with calcium ions playing a critical structural role. Today, they underpin processes ranging from food manufacturing and detergent formulation to wastewater treatment and emerging clinical applications, and demand for them continues to climb.

The commercial enzymes currently used in starch saccharification, which operates at 70 to 110 degrees Celsius, are mesophilic GH-13 amylases that lack sufficient heat tolerance and specificity. This forces manufacturers to make costly pH adjustments and to supplement reactions with calcium. Thermal stability offers real economic rewards: substrates dissolve more readily at high temperature, medium viscosity drops, microbial contamination risk falls, and accompanying non-enzymatic reactions accelerate. For these reasons, the hunt is on for calcium-independent, thermostable, acid-stable amylases that can function near starch’s native pH of approximately 4.5, a combination that would make starch processing cheaper and more energy-efficient. The enzyme from THS-K1, with its broad pH tolerance and robust thermal endurance, is a serious candidate.

Optimising enzyme production from microbial sources is a bioprocess engineering challenge in itself. The researchers began with one-variable-at-a-time screening but quickly moved to statistical designs that capture interactions between factors. A Plackett-Burman design assessed ten medium parameters, including starch concentration, calcium chloride, beef extract, sodium chloride, peptone, pH, yeast extract, and phosphate salts, at two levels each. Regression analysis produced a first-order polynomial model with an R-squared value of 0.79 and a highly significant F-value of 14.39. Three variables emerged as dominant: soluble starch concentration, medium pH, and yeast extract concentration. Starch serves as both inducer and carbon source, while yeast extract supplies a complex blend of nitrogenous compounds and growth factors that inorganic salts cannot replicate.

With the key factors identified, the team applied a Central Composite Design under the Response Surface Methodology framework. Twenty experimental runs, each performed in triplicate, explored combinations of starch from 1.5 to 2.5 grams percent, pH from 6.5 to 7.5, and yeast extract from 0.1 to 1 grams percent. A second-order polynomial model fitted the data with adjusted and experimental R-squared values of 0.99 and 0.98 respectively, indicating an exceptionally strong agreement between predicted and observed responses. Three-dimensional surface plots confirmed meaningful interactions between the variables, with starch and pH jointly shaping the response landscape. Under the optimum conditions, a medium containing 2 percent soluble starch, 0.55 percent yeast extract, calcium chloride, phosphate salts, peptone, magnesium sulphate, and sodium chloride at pH 7 and 50 degrees Celsius for 72 hours, enzyme production rose from 1.1 units per millilitre to 4.94 plus or minus 0.05 units per millilitre, an increase of roughly 4.5-fold.

Purification followed a classical but carefully executed pipeline. The extracellular enzyme was first precipitated with ammonium sulphate, with the active fraction recovered between 50 and 70 percent saturation, then dialysed overnight through a 10 kilodalton cutoff membrane. Size exclusion chromatography on a Bio-Gel P-100 column separated proteins by molecular mass, and a subsequent ion exchange step on DEAE-cellulose, eluted with a sodium chloride gradient, polished the preparation. Together, these steps achieved a 12.8-fold purification with a 25.14 percent yield. SDS-polyacrylamide gel electrophoresis revealed a single band at approximately 25 kilodaltons, a small size for an alpha-amylase, and zymographic analysis, in which a clear band appeared against a dark blue iodine-stained starch background, confirmed that this band was the active enzyme.

Characterisation of the purified enzyme revealed a profile well suited to demanding industrial conditions. Maximum catalytic activity, reaching 3.8 units per millilitre, occurred at 50 plus or minus 1 degree Celsius, although the enzyme remained active across a wide range from 40 to 90 degrees Celsius. The optimum pH was 7, with activity sustained from pH 5 through 10, an unusually broad working window. Thermal stability testing showed half-lives of 13 hours at 50 degrees, 6 hours at 60 degrees, 2 hours at 70 degrees, and an impressive 1 hour and 53 minutes even at 80 degrees. In pH stability trials, the enzyme endured half-lives of 15 to 16 hours between pH 6 and 8 and still retained activity after 8 hours at pH 9. Kinetic analysis using Lineweaver-Burk plots yielded a Michaelis constant of 0.033 milligrams per millilitre, indicating strong substrate affinity, a maximum velocity of 4.18 micromoles per millilitre per minute, a turnover number of 8.71 per second, and a catalytic efficiency of 4.28 times 10 to the fourth per molar per second.

Metal ion profiling added another dimension to the enzyme’s industrial credentials. Divalent cations including copper, iron, calcium, and magnesium significantly enhanced activity, consistent with the metalloenzyme nature of amylases, while monovalent potassium and sodium ions exerted a slight inhibition. To understand the molecular basis of these properties, the team extracted genomic DNA, amplified the amylase gene with Bacillus-specific primers AmyF1 and AmyR1, and recovered a fragment of approximately 1.7 kilobases. Sanger sequencing and BLASTx analysis confirmed that the sequence encodes an alpha-amylase of the conserved cd11315 domain, and translation predicted a polypeptide of roughly 368 amino acids with high sequence identity to the alpha-amylase of Bacillus subtilis. A phylogenetic tree built with the UPGMA method and 1000 bootstrap replicates placed the enzyme firmly within the Bacillus amylase family.

Homology modelling using the SWISS-MODEL server, based on the crystal structure of the Bacillus sp. KR-8104 amylase (PDB entry 3DC0), produced a three-dimensional model showing 61.66 percent sequence identity with a QMEAN quality score of 0.73, indicating reliable local geometry. Ramachandran plotting confirmed that the majority of residues occupy favoured regions, validating the model for structure-function analysis. Comparison with the template revealed multiple amino acid substitutions that the authors suggest may underpin the enzyme’s enhanced thermal stability. Together, the statistical optimisation, biochemical characterisation, kinetic analysis, and in silico modelling represent the first integrated molecular portrait of an amylase from Bacillus stercoris isolated from an Indian hot spring. With its ability to withstand elevated temperatures, function across a broad pH spectrum, and digest starch efficiently, the THS-K1 enzyme and its Amy gene offer a promising foundation for recombinant production and protein engineering aimed at cheaper, greener starch saccharification.

Subject of Research: Bioprocess optimisation and molecular characterisation of a thermostable alpha-amylase from thermotolerant Bacillus stercoris THS-K1

Article Title: Bioprocess optimisation and molecular characterisation of alpha-amylase from thermotolerant Bacillus stercoris THS-K1

Article References: Padia, H. H., Jain, K. S., Goswami, V. C., & Tipre, D. R. (2025). Bioprocess optimisation and molecular characterisation of alpha-amylase from thermotolerant Bacillus stercoris THS-K1. Discover Biotechnology, 2(1), Article 35. https://doi.org/10.1007/s44340-025-00044-9

Image Credits: AI Generated

DOI: 10.1007/s44340-025-00044-9

Keywords: alpha-amylase, Bacillus stercoris, thermostable enzyme, hot spring microbiology, response surface methodology, Plackett-Burman design, central composite design, enzyme purification, starch saccharification, Amy gene, homology modelling, bioprocess optimisation