Immobilization in alginate as a technique for the preservation of Bacillus thuringiensis var. israelensis for long-term preservation

Abstract

Technique for immobilization using sodium alginate as the matrix to preserve Bacillus thuringiensis var. israelensis isolates for long time storage was developed. Two strains of B. thuringiensis var. israelensis viz., VCRC B-17 and WHO standard strain IPS-82 were immobilized in alginate matrix and preserved at 4 °C and when tested both were found to have maintained excellent viability and mosquito larvicidal activity for 10 years. Mosquito larvicidal activity of B-17 and IPS-82 alginate beads, in term of LC₅₀ values before storage was 72.07 ng/ml and 47.07 ng/ml, respectively and after storage at 4 °C for a period of 1 to 10 years the values ranged from 69.88 to 73.86 ng/ml with a mean of 72.38 ng/ml and 45.32 to 48.60 ng/ml with a mean of 47.49 ng/ml, respectively. Similarly spore count of the beads of the respective strains was 4.37 × 10⁸ and 3.33 × 10¹⁰ CFU/mg before storage. After storage at 4 °C for a period of 1 to 10 years the counts of the beads of the respective strains ranged from 4.23 × 10⁸ to 4.83 × 10⁸ CFU/mg (mean of 4.49 × 10⁸ CFU/mg) and 3.2 × 10¹⁰ to 3.87 × 10¹⁰ CFU/mg (mean of 3.54 × 10¹⁰ CFU/mg). The alginate matrix immobilization technique has many advantages over free cells are that they enhance the stability of both spores and toxin against several physicochemical conditions and confer reduced susceptibility to contamination.