Question

Bt toxin protein crystals present in the bacterium Bacillus thuringiensis, do not kill the bacteria themselves because
(a) Bacteria are resistance to the toxin
(b) Toxins occurs as inactive proteins in bacteria
(c) Bacteria enclose toxins in a special sac
(d) None of these

Answer 1

Hint: They are a Gram-positive, soil-dwelling bacterium, commonly used as a biological pesticide. They also arise naturally in the gut of caterpillars of diverse types of moths and butterflies, also on leaf surfaces, aquatic environments, animal feces, insect-rich environments, and flour mills and grain storage facilities.

Complete step by step answer:
Soil bacterium Bt (Bacillus thuringiensis) makes proteins, which destroy definite insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles), and dipterans (flies, mosquitoes). Bacillus thuringiensis forms some intracellular protein crystals. These crystals contain a toxic insecticidal protein. The Bt toxicant protein is present in the form of an inactive toxin and is changed into an active toxin due to the alkaline pH of the alimentary tract that solubilities the crystals. The activated toxin sticks to the top surface of the midgut epithelial cells and produces spores which give rise to cell swelling and lysis and finally cause the death of the insect.

Additional information: In most strains of B. thuringiensis, the cry genes are present on a plasmid. B. thuringiensis is a crucial store of Cry toxins for the manufacturing of biological insecticides and insect-resistant genetically modified crops. When insects ingest toxin crystals, their alkaline digestive tracts alter the insoluble crystals, making them soluble and therefore responsive to being cut with proteases present within the insect gut, which liberates the toxin from the crystal. The Cry toxin is then placed into the insect gut cell wall, paralyzing the alimentary canal and making a pore. The insect ends up eating and starves to death. Live Bt bacteria can also colonize the insect, which may grant an end. The midgut bacteria of susceptible larvae could also be required for B. thuringiensis insecticidal activity.
So, the right answer is 'Toxins occur as inactive proteins in bacteria'.

Note: B. thuringiensis was firstly observed in silkworms by Japanese sericulture engineer Ishiwatari Shigetane in 1902. The use of Bt toxins as plant-incorporated protectants gives rise to the necessity for an extensive evaluation of their safety to be used in foods and potential unintended results on the environment.