Bacillus thuringiensis (Bt) is a soil-dwelling bacterium used as a biopesticide, producing proteins toxic to various insect larvae. With diverse strains targeting specific pest groups, it has been utilized in biological control since its discovery over a century ago, and is considered safe for humans and beneficial insects. Effective application techniques and storage considerations are vital for maximizing its pest control efficacy in agricultural practices.

1. Bacillus thuringiensis
Prasanna R Kovath
Assistant Professor
Department of Biotechnology
St. Mary's College Thrissur

2. • Among prokaryotes, bacteria of the genus Bacillus (family
Bacillaceae) have been used in the microbial control of pests. In this
genus, the species Bacillus thuringiensis(Bt) is distinguished by its
biopesticide potential.
• Bt is a microbe naturally found in soil. It makes proteins that are toxic
to immature insects (larvae).
• There are many types of Bt. Each targets different insect groups.
Target insects include beetles, mosquitoes, black flies, caterpillars, and
moths.
• With Bt pesticides, routine testing is required to ensure that unwanted
toxins and microbes are not present.
• Bt has been registered for use in pesticides by the US Environmental
Protection Agency (EPA) since 1961.

3. History
• Bt was discovered by Ishiwatta in 1902 in reared Bombyx mori L.
(Lepidoptera,Bombycidae) in Japan.
• Later it was reisolated by Berliner, in 1911, from Ephestia kuehniella
Zeller (Lepidoptera, Pyralidae) larvae in Thuringia, which gave rise to
its current name.
• According to Van Frankenhuyzen (1993), the first biological control
trials of Bt were conducted against Ostrinia nubilalis Hübner
(Lepidoptera, Pyralidae) between 1920 and 1930 in Europe.
• Between 1930 and 1940, several trials were carried out with other
lepidopteran species in Europe and in the USA.
• Regarding the biological control of insects today, Bt is the mostly used
microorganism worldwide.

4. • Bt is a ubiquitous bacterium with a large enzyme complement, which
allows it to be found in a variety of sites, such as: soil, insects and
their habitats, stored products, plants, forest, and aquatic
environments.
• It can remain latent in the environment even in adverse conditions for
its development.
• Its entomopathogenic activity is highly dependent on the parasporal
inclusion body that forms during sporulation, which consists of Cry
proteins that are encoded by cry genes.

5. • Parasporal inclusion bodies (crystals) are composed of proteins of
varying quantity and quality according to the bacterial strain.
• Strains containing these crystals were measured as being toxic and
specific to larvae of Lepidoptera, Diptera, Coleoptera, Nematoida,
Protozoa,Trematoda, Acari, Hymenoptera, Hemiptera, Orthoptera,
Isoptera, and Mallophaga.
Transmission electron micrograph of a longitudinal section
of Bacillus thuringiensis towards the end of sporulation,
showing the spore (black ovoid structure) and the protein
crystal with insecticidal properties (bipyramidal inclusion).
Photo: from Institut Pasteur,
Station Centrale de Microscopie Électronique

6. Cellular Morphology and Physiology
• Bt is an aerobic Gram-positive and rod-shaped bacterium, with a
vegetative cell of 1.0–1.2 μm wide and 3.0–5.0 μm in length, usually
mobile by means of peritrichous flagella.
• The spore of this bacterium has an ellipsoidal shape but mostly are
cylindrical and is located in the central or paracentral region when
inside the mother cell.
• The species is non-strict aerobic with a temperature range of growth
between 10–5 ºC and 40–45 °C.
• The main characteristic that distinguishes this species from the
others of the same genus is the intracellular presence of a protein
crystal.

7. • Most strains are catalase-positive,oxidase negative, casein, gelatin and
starch are hydrolyzed. Voges-Proskauer- positive and citrate is utilized
as sole carbon source.
• Nitrate is reduced and tyrosine is decomposed. Phenylalanine is not
deamineted.
• Most strains utilize saccharose and other sugars

8. Bacillus thuringiensis:
(a, b) selective culture medium,
(c) gram staining,
(d) interferential
phase contrast micrograph, and
(e) transmission electron micrograph,
Bar = μm

9. • Bt has two distinct phases during cell development:
vegetative cellular division and spore formation
• The development of the spore and crystal involves seven distinct
stages:
• (a) phase I – formation of axial filaments
• (b) phase II – formation of forespore septum
• (c) phase III – first appearance of parasporal crystals and formation
of a forespore
• (d) phases IV to VI – formation of exospore, primordial cell wall, and
spore nucleoid transformation
• (e) phase VII – maturation of spores and cell lysis

10. • These insecticidal proteins are synthesized after stage II of sporulation
and accumulate in the mother cell as a crystal,
which can account for up to 25% of dry weight of the sporulated
cells
• These crystals can have different forms:
• Bipyramidal, Pyramidal,cuboidal, Flat rhomboid, Spherical, and
Rectangular.
• The most common form is that of a Bipyramidal crystal.
• The crystals may contain one or more delta-endotoxins, or Cry
proteins, which have molecular weights between 30 kDa and 140 kDa,
and which are
converted into toxic peptides after ingestion by target pests (insects).

11. • In addition to Cry proteins, Bt isolates can synthesize proteins called
Cyt, which have in vitro cytolytic activity and in vivo specificity to
dipterans
• Cyt toxins are able to affect insect midgut cells and may increase the
insecticidal activity of certain Cry toxins
• Proteins known as beta-exotoxins are also synthesized by some Bt
isolates. One of these, thuringiensin is a nonspecific and thermostable
protein toxic to vertebrates Because it is nonspecific, isolates that are
capable of producing, it cannot be used in the production of
biopesticides,
• Beta-exotoxin has been identified in different subspecies Bt, such as
Bt tenebrionis Bt kenyae and Bt tolworthi Bt thuringiensis,Bt kurstaki
and Bt darmstadiensis

12. • Vip proteins are divided into four families according to their amino
acid
identity.
Vip1 and Vip2 proteins act as a binary toxin and are toxic to some
coleopteran and hemipteran species
• In addition to the aforementioned proteins, Bt can produce
phospholipases, proteases, chitinases, and enterotoxins.
• Enterotoxin is similar to that produced by B.cereus, whose ingestion in
food results in intoxication.
• Bt produces some classes of bacteriocins, which are important for the
control of pathogenic microorganisms and food contamination

13. Biochemistry and Molecular Biology : Bacillus thuringenesis
• In the pioneering studies found that the crystal-producing bacteria
could be subdivided into six biochemical groups.
• In addition, the reality of this subdivision was supported by the
presence of an H antigen that was specific to each group which helps
in differentiating aerobic bacteria from spore formers and is widely
applicable across the many genera.
• The commercially available biochemical tools mostly used for the
identification of Bacillus spp. and correlates are the API 20E and
50CHB systems (BioMerieux), the VITEK systems (BioMerieux), and
Biolog.

14. • In molecular biology studies, one of the most important aspects of the
Bacillus genus is its diversity.
• There are species that have a key role in medicine, industry and the
economy. Some examples are B. cereus, B. thuringiensis, B.
anthracis,B. mycoides, and B. pseudomycoides.
• Many studies based on molecular markers find it difficult to ultimately
separate B. cereus and Bt.
• The most obvious difference between these two species is the presence
of plasmids which encode toxic proteins for insects in Bt
• The differentiation between these species using molecular markers
continues to be a hard task, regardless of the methods used

15. • In the specific case of Bt, since its description in 1915, the main
method applied to the identification of subspecies was based on the
flagellar H antigen reaction
• Among the tools mostly used, high-lights include that based on 16S
rRNA, RAPD, RFLP, REP-PCR,ERIC-PCR, and MLST.
• All of them make it possible to detect differences between the
analyzed strains, but a debate remains on which of these tools are the
most appropriate to be used as a standard method to molecularly
characterize B. thuringiensis strains and to correlate the molecular
patterns with toxicity to different insect species.

16. • The 16S rRNA genes are considered one of the main molecular
markers for studies in bacteria diversity.
• This marker is widely used for phylogenetic analysis and for studies in
metagenomics
• These genes have both conserved and variable regions which make its
application possible for studies in different taxonomic levels. In recent
decades, its application significantly extended our knowledge about
diversity in prokaryotes.
• Data can be generated by sequencing some regions of this gene or by
using the RFLP technique.

17. • The main restriction of the use of 16S rRNA is the high similarity
between the sequences of closely related species (Christensen et al.
1998), as happens in the Bacillus genus.
• To overcome these limitations, recent works have been using 16S
rDNA analysis together with other markers, increasing the capacity to
differentiate Bt strains and other species of Bacillus.

18. • The analysis of repetitive regions of the genome is one of the tools
successfully applied to analyze the diversity among the species of Bacillus.
• Its use has made possible the differentiation between Bt and B. cereus strains.
• The study of these regions is named fingerprint analysis and is based on
amplification by PCR of the regions repetitive enterobacterial palindromic
(REP) and enterobacterial repetitive intergenic consensus (ERIC) sequence.
• The genetic variability found in these sequences makes its successful
application possible to study the intra- and interspecific diversity of species
belonging to the genus Bacillus.
• Its efficiency to differentiate the species is greater than the one found in the
gene for 16S rRNA.

19. • The additional advantages of this technique are its easy and quick
implementation, its simplicity, and the fact of that the results are
reliable and reproducible.
• REP-PCR technique to differentiate strains from different samples,is
also used as atool for identification.

20. • Multilocus sequence analysis (MLSA) study the genetic diversity in
pathogenic microorganisms.
• It is based on the analysis of (housekeeping) genes that are expressed
in a constitutive way, in other words, genes which have a central role
in maintaining the cell’s metabolism.
• The alleles are identified through the sequencing of internal fragments
of these genes.
• The new variants are created by mutations, synonymous or not, in the
nucleotide sequence.
• This technique is considered an excellent tool to study the inter- and
intraspecific genetic variability and to study the strains evolution.

21. • Nowadays, despite of the great number of studies, the main way of
differentiating B. cereus from Bt is the presence of proteins (Bt toxins)
active against insect species.
• The studies also cannot associate molecular patterns of the
chromosomal DNA with the insecticidal activity.
• The best way to identify and characterize new strains of Bt is to
analyze the plasmid genes responsible for the synthesis of these toxins.

22. • Besides molecular patterns based on using nucleic acids, some chemical
markers are considered important for the analysis and description of the
inter- and intraspecific variability in bacteria.
• Among the chemotaxonomic tools applied to the identification
of bacteria, highlights include :
 fatty acid methyl ester (FAME)
 matrix assisted laser desorption/ionization (MALDI) time-of-flight mass
spectrometry.

23. • The FAME technique is based on the analysis of short-chain fatty
acids, which have between 9 and 20 carbons.
• The production of the fatty acids is considered a specific property of
each microorganism and can be driven by environmental factors.
• These compounds are important for the adaptation process of
microorganisms.
• All these properties have made the fatty acids analysis an important
tool for the taxonomy of bacteria.
• Fatty acids analysis is also a good marker for studies of the adaptation
in Bacillus species, as their composition varies according to the
environment.

24. • The MALDI technique is considered one of the most important tools
for the study of microorganisms.
• It is based on the use of a mass spectrophotometer to analyze the
spectral patterns of ions present in macromolecules.
• In this technique, all proteins of the bacteria cell provide a specific
pattern (fingerprint) which can be statistically analyzed.
• The results are obtained more quickly, and the process has lower cost,
is less laborious, and has the same efficiency as the traditional
techniques that use 16S rRNA (RFLP, AFLP, and sequencing).

26. Mode of Action of Bt Toxins
• The first impact of Cry toxins on the insect is cessation of feeding due
to paralysis of the gut and mouthparts
• In addition to gut paralysis, midgut cells swell leading to an ion
imbalance and death
• The molecular events leading to Cry toxin-mediated insect death are
controversial , but the accepted initial steps are as follows:
• The Bt Cry and Cyt proteins require solubilization in the insect midgut
to produce protoxins that are typically about 130 kDa, 70 kDa or 27
kDa for Cyt.
• These in turn are proteolytically cleaved at the C-terminus and/or at
the N-terminus by midgut proteases, generating the activated core
toxin.

27. • The toxin then crosses the peritrophic matrix and binds to receptors in
the apical membrane of the midgut cells, with receptor binding being
an important determinant of toxin specificity.
• Toxin insertion into the epithelial membrane forms ion channels or
pores, leading to lysis of the cells, damage to the midgut epithelial
tissue, and death of the larva

29. TIPS FOR APPLYING BT
• Make sure you have the right strain for the pest you want to control
• A pH greater than 8 is what activates the toxin in the insect’s gut
• You will have better control if the larvae are small. There will be less
damage to the plants because the little insects won’t eat as much. If
you treat larger larvae, they will eat more of the plants and cause
greater damage. Also, they may morph into the reproductive phase and
become insensitive to the toxin.
• The spray will be more effective if you add a spreader or sticker to the
tank mix. Use the spray within 12 hours of mixing. Make sure that you
are spraying both the top and bottom surfaces of the leaves.

30. • While it can survive for years in the ground if adsorbed to soil particles,
Bt is rapidly inactivated by the UV radiation in sunlight. Many people
spray their plants in the evening, so the toxin can work overnight before
being inactivated by the sun the next day.
• The bacteria are sensitive to temperature and must be stored at 50-60 F.
Do not expose the bacteria to hot or cold temperatures, which can kill
them.
• Additives, such as sticking or wetting agents, are often useful in a Bt
application to improve performance, allowing it to cover foliage more
thoroughly and to resist washing off.

31. • Unlike most insecticides, which target a broad spectrum of species,
including both pests and beneficial insects, Bt is toxic to a narrow
range of insects.
• Research suggests that Bt does not harm the natural enemies of
insects, nor does it impair honeybees and other pollinators critical to
agroecological systems.
• Bt integrates well with other natural controls and is used for integrated
pest management by many organic farmers.

32. • The use of insect-resistant Bt plants can potentially reduce use of
chemical insecticide sprays, which are extremely toxic and expensive.
• Bt toxin applied as an insecticide or consumed with GMO food crops
is considered nontoxic to humans and other mammals because they
lack the digestive enzymes needed to activate the Bt protein crystals.
• However, any introduction of new genetic material is potentially a
source for allergens, and, for this reason, certain strains of Bt are not
approved for human consumption.

33. Bt Type: Controls:
Bacillus thuringiensis kurstaki (Btk) Most caterpillars
Bacillus thuringiensis israelensis (Bti) Mosquitoes, flies, fungus gnats
Bacillus thuringiensis San Diego Specific beetles
Bacillus thuringiensis tenebrionis Specific beetles
Bacillus thuringiensis aizawai (Bta) Some caterpillars
• The effectiveness of Bt may be reduced after two or three
years of storage. Dry formulations last longer than liquid
formulations.
• Bt products should be stored out of sunlight and in cool, dry
conditions.

34. • If you use Bt on your farm, the EPA will require you to take some
steps to prevent resistance. One way is to alternate its use with
synthetic insecticides.
• Another is to rotate your crops. Since different kinds of insects feed on
different crops, you would be using a different type of Bt.
• The large variety of Bacillus thuringiensis strains available enables
gardeners, farmers, and mosquito control experts to control an array of
insect pests.
• Since the strains are highly specific to the insects targeted, residual
effects against other organisms are not a concern.

35. • What happens to Bacillus thuringiensis (Bt) when it enters the
body?
• When eaten, Bt is confined to the gut. It does not reproduce, and the
toxin is broken down like other proteins in the diet. Bt leaves the body
within 2 to 3 days.
• If breathed in, Bt can move to the lungs, blood, lymph, and
kidneys. Bt is then attacked by the immune system. Levels
of Bt decrease quickly one day after exposure.

36. What happens to Bacillus thuringiensis (Bt) in the
environment?
• Toxins created by Bt are rapidly broken down by sunlight and in acidic
soil. Other microbes in soil can also break it down. Bt does not readily
leach in soil.
• It typically remains in the top several inches of soil. Bt remains
dormant in most natural soil conditions. However, there has been some
reproduction in nutrient rich soils. On the soil surface,
dormant Bt cells last only a few days.
• However, below the soil surface, they can last for months or years.
The half-life in unfavorable soil is about 4 months. Bt toxins break
down much faster. In one study, 12% remained after 15 days.
• In water, Bt does not readily reproduce. A study found Bt toxins in the
air were broken down rapidly by sunlight. Forty-one percent (41%) of
the toxin remained after 24 hours. On plant surfaces, sunlight breaks
down Bt; the half-life of Bt toxins is 1-4 days.

37. Reference
• https://www.mdpi.com/2072-6651/6/10/3005/htm
• https://gardenerspath.com/how-to/organic/bacillus-thuringiensis/
• http://npic.orst.edu/factsheets/btgen.html#products
• https://www.researchgate.net/publication/41714537
• Microbial Pest Control Agent BACILLUS THURINGIENSIS (Published under the joint sponsorship
of the United Nations Environment Programme, the International Labour Organisation, and the World Health
Organization, and produced within the framework of the Inter-Organization Programme for the Sound
Management of Chemicals)
• Bacillus thuringiensis as a Specific, Safe, and Effective Tool for Insect Pest Control (ROH, JONG YUL, JAE
YOUNG CHOI, MING SHUN LI, BYUNG RAE JIN1, AND YEON HO JE*)
• https://www.researchgate.net/publication/318152732