Co-production of parasporal crystal toxins and antimicrobial substances by <i>Bacillus thuringiensis</i> BAR 3

Abstract

Co-production of antimicrobial substances and insecticidal compounds by Bacillus thuringiensis BAR 3 was investigated. The cell free supernatant (CFS) of B. thuringiensis showed inhibitory activities against both Gram positive (B. thuringiensis IFO13866 and Staphylococcus aureus ATCC 25923) and Gram negative (Proteus mirabilis ATCC 29906) bacteria. Proteus mirabilis ATCC 29906 was the most sensitive to the CFS and the inhibition zone diameter was 10.10 mm. Maximum production of both the endotoxin and antimicrobial substances were achieved when the basal medium (3 g/L beef extract, 5g/L peptone and 8g/L NaCl) was supplemented with 10 g/L glucose, 3 g/L yeast extract, 2 g/L KH2PO4, 1 g/L MgSO₄, at pH of 7. The antimicrobial activity of the CFS was stable over a wide range of pH (2-8) and temperature (30-100°C) but the activity was completely lost after autoclaving for 15 min at 121°C. The antimicrobial activity was unaffected by amylase and catalase but was nearly completely denatured by protease and proteinase K. Under the optimum culture condition, the total Bacillius thuringiensis cell count was 1.30 × 10¹² (cfu/ml) with 97% sporulation (spore count was 1.26×10¹² cfu/ml) and the LC₅₀ was 0.096 mg/L against third instar larvar of Culex pipens.

Keywords: Bio-insecticide, anti-microbial substances, Bacillus thuringiensis, endotoxin, sporulation.