Fate of Bacillus thuringiensis in soil

1
Fate of Bacillus thuringiensis in soil

• The topics to be discussed in this presentation
• Background
• Functions/Structure of Bt toxin
• Fate in Soil

2
Background

• Definition Gram () soil bacterium
• cry gene ? delta endotoxin ? protein crystal
• History
• Subspecies
• Bt subspecies  kurstaki - controls various types
  of lepidopterous insects. (Most commonly used)
• Bt subspecies  israelensis - effective against
  mosquitoes, blackflies' and some midges. 
• Bt subspecies tenebrionis - effective against
  certain beetle (chrysomelids) species and the
  boll weevil. 
• Bt subspecies Japonensis - effective against many
  species of scarabid beetles. 
• Bt subspecies aizawai -  used against wax moth
  larvae in honeycombs.

3
Pros and Cons of genetically engineering crops
with Bt

• Pros
• Stable during several years of storage
• Resistance of the crystals to inactivation by UV
  light
• Safety of preparation
• Bt is not known to cause injury to plants

• Cons
• Specificity of strains activity against some
  pests, but poor activity against others
• Takes longer to kill pests, perceived as less
  effective
• Fate in Soil/Safety
• More frequent applications

4
Structure/Function of Bt

• Structure
• Bt forms spore and crystal protein inclusion that
  consist of d-endotoxins
• 3 domains of the cry protein
• Mode of Action
• Kills by Collodial Osmotic Lysis

5
Cry Protein Structure

• Domain 1 responsible for inserting into the gut
  membrane and creating a pore where ions can pass
  freely
• Domain 2 responsible for binding to the
  receptors on the epithelial lining of the midgut
• Domain 3 responsible to protect the endotoxin
  from cleavage by gut proteases, or may be
  involved in ion channel formation, receptor
  binding, and insect specificity

6
Fate in Soil

• 2 pathways into soil
• Root exudates
• Biomass of plants amended with soil
• Growth
• Mobility
• Toxicity

• Length of stay in soil
• Free toxin vs. bound toxin
• Accumulation

7
Mobility

• Vertical Movement
• cry protein has a tendency to stick to clay
  and/or organic matter
• Bt is able to survive in soil in which it is not
  an indigenous bacterium for more than seven years
• Horizontal Gene Transfer
• Important because repeated exposure of an insect
  population to a pathogen exerts strong selective
  pressure for resistant mutants
• Results Found by EPA

8
Toxicity

• Free and bound toxins were insecticidal to the
  larvae of tobacco hornworm
• Saxena Stotzky 180 days
• LC50s higher for free toxin than bound toxin
• Free or bound have no effect on the growth of a
  spectrum of bacteria, fungi, and algae. This
  result is in agreement with many observations

9
Growth

• Can compete successfully for added nutrients with
  an active soil biota
• Can support growth of microorganisms
• free toxin can be readily utilized as a source of
  carbon and nitrogen
• Bound toxin cannot utilized as a source of
  carbon, slightly as a source of nitrogen, and
  does not support growth in the absence of
  exogenous sources of both available carbon and
  nitrogen
• confirmed by measurements of oxygen uptake

10
Conclusion

• Overall effect on microorganisms of Bt toxin in
  soil
• The results indicate that Bt bacteria do not
  affect culturable heterotrophic bacteria
  populations
• does Bt affect carbon cycling functional groups
  populations
• Future Implications