Bacillus thuringiensis (Bt) is shown here as smooth bacterial cells dividing at the top of a 35-hour colony.

Bacillus thuringiensis (Bt) is shown here as smooth bacterial cells dividing at the top of a 35-hour colony. As the bacteria grow older, they start producing spores to survive the depletion of their nutrients. The sporulating bacteria also produce protein crystals at a later stage of their development. Such proteins are toxic to many important plant pests, such as the caterpillars of butterflies and moths (e.g. gypsy moth, tent caterpillar, tobacco hornworm), mosquito larvae, blackfly larvae, etc. but they do not affect other wildlife and are safe to humans. For this reason, Bt is used as an insecticidal microorganism for biological control of harmful insects in agriculture, forestry, and horticulture. Enhanced SEM. Image width: 8 micrometres, bar: 1 micrometre, magnification: 12, 500x if the image is printed 10cm wide.