Bacillus thuringiensis

The most extensively analyze of this group comprises the gram-positive bacteria, B. thuringiensis (1:489). A soil microorganism, B. thuringiensis occurs naturally throughout the world (4:1993). The bacteria is a facultative anaerobe: i.e., under anaerobic conditions, it can use nitrate as an electron acceptor. During its post-exponential developing phase, B. thuringiensis produces an intracellular inclusion body. This proteinaceous crystal is usually bipyramidal in shape. Upon lysis of the fuss cell, both the spore and the inclusion separate and are released into the environment (i.e., the soil, ponds, streams) (6:364). inside these areas, the larvae of mosquitos and blackflies feed on small particulate matter. Ingestion of B. thuringiensis' inclusion is followed by midgut solubilization and the concomitant release of toxic proteins. These "protoxins" are thusly activated by immature proteases. The activated toxins disrupt the larval midgut epithelium's structural integrity. Inevitably, this sequence of events leads to plant louse death (3:615).

The incapacitation of its host insect provides B. thuringiensis with several survival benefits. In addition to ingesting the bacteria's toxic crystallized inclusion, insect larvae typically ingest the spore as well. The biochemical events which cancel out the insect provide the bacteria with a unique ecological niche. B. thuringiensis is essentially a soil microorganism. Not only is the insect

1. Aronson, A. I. The two faces of Bacillus thuringiensis: insecticidal proteins and post-exponential survival. molecular Microbiology. 7:489-496; 1993.

7. Prince, R. C. At least one Bacillus thuringiensis toxin forms ion-selective pores in membranes. Trends in Biochemical Sciences. 15:2-3; 1990.

In addition to B. thuringiensis spore and (-endotoxin, many an(prenominal) another(prenominal) of the pesticide formulations also contain various additional active ingredients. much(prenominal) additives may, for example, include both fungicides and chemical insecticides. These products, therefore, are generally employ to target an array of insect species.

Additional developments involving biopesticides might adjoin their inactivation rates.

The toxicity of (-endotoxin crystals largely depends on the rate at which they are ingested. In many cases, the feeding zone for targeted insects is each just below the water's surface or at somewhat greater depth. Future formulations might be designed to lodge suspended within these zones (2:4).

The genes for B. thuringiensis proteins often occur on plasmids. These bacteria generally hire an array of plasmid-encoded protoxin genes. The number and size of these plasmids order of battle considerable variation. For the most part though, subspecies that contain many plasmids tend to have a broader size range. These subspecies may, for example, have plasmids in the following size groups: 4-6 megadaltons (MDa), 30 MDa, and 100 MDa. In contrast, certain other B. thuringiensis subspecies have only a few of the larger plasmids (2:11).

5. Porter, A. G.; Davidson, E. W.; Liu, J. W. Mosquitocidal toxins of bacilli and their genetic manipulation for effective biological control of mosquitoes. Microbiological Reviews. 57:838-861; 1993.

Various experimental evidence has show that Bacillus thuringiensis toxins bind to the apical brush border membrane of midgut columnar cells. This binding, however, depends on the specific polypeptide involved. For example
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