Microbes In Human Welfare

Bacillus thuringiensis, often abbreviated as Bt, is a soil bacterium widely used as a biological insecticide because during sporulation it produces protein crystals containing a toxic insecticidal protein (Bt toxin). When susceptible insect larvae ingest these crystals, the alkaline pH of their gut solubilises the protein into its active toxic form, which binds to the gut lining and kills the larvae, while the toxin remains harmless to humans and other animals. Pseudomonas fluorescens is incorrect because, although it acts as a biocontrol agent against certain plant pathogens, it does not produce the insecticidal crystal proteins described here. Rhizobium leguminosarum is wrong because it is a nitrogen-fixing symbiont of legume roots functioning as a biofertilizer, not an insecticide. Clostridium botulinum is incorrect since it produces a dangerous neurotoxin causing botulism food poisoning and is not employed as a crop biopesticide. As stated in NCERT Class 12, Chapter 10 (Microbes in Human Welfare), Bt formulations are sprayed to control butterfly caterpillars. As a consistency check, only Bacillus thuringiensis matches both crystal-protein production and larva-specific insecticidal action, confirming it as the correct biocontrol microbe.