Population dynamics of soil microbes and diversity of Bacillus thuringiensis in agricultural and botanic garden soils of India

Abstract

Different microbial guilds of three rice fields (aerobic (dry or wet) laterite and red sandy, and anaerobic (flooded), clay-loam), a botanical garden (wet, aerobic, humus) and a sorghum (dry, aerobic, sandy) soil were investigated. Population (x106 cfu/g dr. soil) of the aerobic (3.8 - 26.2) and anaerobic (0.91 - 13.6) heterotrophic, aerobic (1.1 - 3.7) and anaerobic (0.8 - 1.3) spore forming, Gram (-)ve (0.3 - 0.7), phosphate solubilizing (0.01 - 0.06), asymbiotic N2-fixing (0.26 - 0.88), sulfur oxidizing (0.13 - 0.75), sulfate reducing (0.01 - 0.1), nitrifying (0.1 - 0.23) and denitrifying (0.04 - 0.25) bacteria; actinomycetes (0.01x104 cfu/g dr. soil), fungi (0.01 - 0.05x105 cfu/g dr. soils) and Bacillus thuringiensis (Bt) (0.13- 0.25x106 cfu/g dr. soil) population were variable in different soils. Nevertheless, proportions of Bt in relation to spore forming bacteria were comparable (index (5-9) x10-4) and they produced either monotypic (bipyramidal or spherical) or heterotypic (polymorphic-bipyramidal or bipyramidalrhomboidal) crystals although the bipyramidal crystal producing Bt were predominant in the soils. The bipyramidal crystal producing Bt were diverse in different ecologies, resistant to penicillin group of antibiotics and tolerated 5 - 6% NaCl. Phenotypic characters allowed to group the Bt isolates of botanic garden as B. thuringiensis subsp. coreanensis and as B. thuringiensis subsp. thompsoni/coreanensis, sorghum fields as B. thuringiensis subsp. finitimus and but those of rice fields as B. thuringiensis subsp. thuringiensis/ shandongiensis, finitimus and thompsoni/coreanensis. The isolates produced 25.78, 25.78, 86.26, 24.73, 68.0, 26.8 and 26.8 kDa proteins and equivalent to Cyt, Cry5 and Cry2 toxins effective against the insects of Diptera and Lepidoptera/Coleoptera.