BTk 101

The B. thuringiensis “species family” of bacteria (which includes B. cereus and B. anthracis) are distinguished by their ability to produce several different substances that are either immediately toxic to some life forms or toxic when placed in the proper environment. The table below gives the nomenclature for the three major types:


Classsification of Bt Toxins
Name Target Organism Production When toxic
beta-exotoxin toxic to many life forms exuded by active bacteria immediately
delta-endotoxin or protoxin generally toxic to certain insects crystalline protein formed on sporulation requires alkaline environment to convert to toxin
B. cereus type enterotoxin toxic to mammals – causes gastroenteritis exuded by active bacteria immediately

Bt strains are often identified by the toxins they produce. In particular, the delta-endotoxin, also called a crystalline protein protoxin, or cry, for short, comes in different varieties, usually called cryIA, cryIB, cryII, etc. cry proteins are the desirable insecticidal agent that is produced by Bt strains. The following diagram shows how Bt’s are generally identified:

The delta-endotoxin is produced when the Bt bacteria goes into a dormant state, or sporulates. It exists as a separate substance outside the spore. Many Bt-based pesticides include a large amount of the endotoxin as an additional ingredient to live bacteria and spores. This substance is inactive until placed in an alkaline (high pH) environment, specifically in the gut of many insects. It then reacts to form an active toxin that may then poison the insect depending on the activity of the particular cry protein against that insect.

Many Bt bacteria also produce one or two other toxins. Both are secreted by the bacteria in its non-spore (growing) life stage. The beta-exotoxin is quite toxic to many life forms. In fact, any Bt that produces beta-exotoxin cannot be used as an insecticidal agent in the US. Another toxin is B. cereus enterotoxin, so called because B. cereus produces much more of this toxin than most Bt strains. However, almost all Bt’s produce some amount of this toxin, which causes gastroenteritis (food poisoning) in humans.

Bt’s in general, in addition to producing the three toxins above, also exhibit cytotoxic, or cell-destroying, activity due to some other substances secreted by the active Bt cell. This same cytotoxicity is shared to a great extent with B. cereus.

Even though a particular insect may not be very susceptible to the protoxin, the spores may germinate in the insect gut after ingestion, and the toxicity of growing Bt cells may be enough to harm or kill the insect. This is a particular concern for organisms that live in the soil, where Bt spores may survive for years after spraying.


Btk is the kurstaki subspecies of the Bt bacteria. Btk is one of the subspecies found most often on leaves in Japan and somewhat in the US. The Btk HD-1 strain that is used in Foray 48B is not a naturally prevalent strain but was isolated after some amount of work that may have involved selective breeding for maximum protoxin production. HD-1 is probably not a genetically modified organism; however, there are a number of extant patents on GM Bt organisms and it is not out of the question that Bt sprays do or could soon contain GM bacteria. For example, see this patent assigned to Valent BioSciences, the manufacturer of the Foray pesticides.

HD-1 does not produce beta-exotoxin, and it produces cry1A protoxins. This means it is may not be too toxic, for example, to the larval stage of Diptera (flies) but it will certainly kill Lepidopterae larvae (butterflies and moths). Unfortunately, it also produces a not insignificant amount of enterotoxin, at a level of about 5% compared to a typical B. cereus bacterial culture. In addition, it, of course, exhibits the cytotoxic (or cytolytic) activity of many Bt and B. cereus bacteria.


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