Bacillus thuringiensis

A Microbial Biorealm page on the genus Bacillus thuringiensis

Bacillus thuringiensis[1]

Classification

Higher order taxa

Eubacteria (kingdom); Bacteria (domain); Firmicutes (phylum); Bacilli (class); Bacillales (order); Bacillaceae (family); Bacillus (genus); Bacillus cereus group

Species

Bacillus thuringiensis

Description and significance

Describe the appearance, habitat, etc. of the organism, and why it is important enough to have its genome sequenced. Describe how and where it was isolated. Include a picture or two (with sources) if you can find them.

B. thuringiensis is a gram positive, soil-dwelling, spore-forming, rod-shaped bacteria. Cellularly, it is 1 µm in width and 5 µm in length. It grows at body temperature and produces diamond-shaped crystal from its crystal proteins (cry proteins) and uses it to fend off insects, predators, and other pathogens.

Bacillus thuringiensis crystal[2]

It's genome was recently sequenced (February 23rd, 2007). It is critical to have B. thuringiensis' genome sequenced because it has many benefits for humans in the form of agriculture. It is used primarly as an insect pathogen and farmers frequently use it as a pesticide for their crops to. Having sequenced the entire genome, scientists can witness and determine how mutations can create different variant strains that may eventually grow stronger against antibiotic pests. B. thuringiensis pesticide S-layer (where the Cry protein and toxins lie) has no known negative effects on humans, vertebrates, or plants.

http://pmep.cce.cornell.edu/profiles/extoxnet/24d-captan/bt-ext.html

Genome structure

Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence? Does it have any plasmids? Are they important to the organism's lifestyle?

B. thuringiensis has a circular plasmid chromosome. Due to its 2% GC-content, it has more A/G base pairs. Compared to its bacterial relatives, Bt's genome is consiered medium-sized. It genome is only 5,475 nucleotides long, has approximately 2.4 to 5.7 million base pairs [6], and consists of only seven genes: pK1S1_p1, pK1S1_p2, pK1S1_p3, pK1S1_p4, pK1S1_p5, pK1S1_p6, and pK1S1_p7. [7]

Bacillus thuringiensis' plasmid genome map[3]

Cell structure and metabolism

Describe any interesting features and/or cell structures; how it gains energy; what important molecules it produces.

gram positive -> thick cell wall -> peptidoglycan + periplasm(ic space), inner membrane, outer membrane, et al. techioc....

The cell's production of the crystal is very regulated. here is the proces...

Bacillus thuringiensis crystal life cycle[4]

Ecology

Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.

Several studies have been done on animals to see what the effects of B. thuringiensis are. When treated with 10,000 mg of Bt per kg body weight, autopsy results on birds showed no pathology was attributed. High concentrations of Bt in fish and aquatic organisms failed to show any adverse effects . Experiments performed on other animals showed similar negative results. Conclusion: There seems to be no adverse effects on eukaryotic-celled organisms when exposed to Bt.[8]

Effects on the environment are largely considered mild. It is made from the soil and thrives at body temperature. When exposed to higher temperatures such as sunlight (UV light) however, its half-life greatly decreases to around 3.8 hours. As soon as it is exposed to a warmer-than-normal medium, B. thuringiensis' spores start deteriorating and loses viability within four days. Normally, a short half-life is bad. But in Bt's case, it is actually good in that the short half-life minimizes antibiotic resistance. An additional advantage of its rapid degradation stems from it being readily biodegradable. Possible future reseach in breaking down plastic bags? [9]

Pathology

How Bacillus thuringiensis kills a caterpillar[5]

How does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.

Common insect targets are "including moths, mosquitoes, blackflies, beetles, hoppers, aphids, wasps and bees as well as nematodes"[10]. The toxicity lies in the crystals. Here is a rough map of how B. thuringiensis causes disease.

1. B. thuringiensis is digested and the toxins are mixed with the high pH to bind specific receptors in the gut to attack the host insect.

2. This process punches holes in the gut-lining and thus, the insect becomes weak.

3. As the gut is continuing to break down, spores begin to germinate from the toxic crystals and other bacterial pathogens start to infect the host.

4. B. thuringiensis spores are continuously weakening the host and the insect dies soon thereafter.

Bacillus thuringiensis is very species-specific and does not work with every single host. There are different strains of B. thuringiensis that effect different hosts. For example, "B.t. aizawai (B.t.a.) is used against wax moth larvae in honeycombs; B.t. israelensis (B.t.i.) is effective against mosquitoes", et al. The key is that the host must eat B. thuringiensis "in the immature, feeding stage of development referred to as larvae [because] it is ineffective against adult insects". [11]

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

It was once thought that B. thuringiensis' method of toxology was via toxin-mediated disruption, which basically consisted of Bt killing the host by starvation or blood poison. However, recent reserach by scientists at the University of Wisconsin-Madison reported having other players involved.[12]