Entomopathogenic Bacteria

Biological insecticides based on entomopathogenic bacteria represent 95% of the microbial insecticides market and are based mainly on one bacterium species: Bacillus thuringiensis (Bt). There are several Bt-based products for the control of mosquitoes that transmit human diseases, and also for the control of the most important lepidopteran and coleopteran crop and forest pests. Insecticide products are also commercially available based on two other entomopathogenic bacteria: Serratia entomophila and Bacillus sphaericus for the control of grass grub and mosquitoes, respectively.

Most of the entomopathogenic bacteria invade their host after ingestion by the larvae causing the disruption of the midgut tissue, followed by septicemia. In most cases these bacterial pathogens rely on insecticidal toxins during the pathogenic process but also produce an array of virulence factors that contribute to the killing of the insect.2 4

8.2.1 Bacillus thuringiensis

Bacillus thuringiensis (Bt) is a member of the Bacillus cereus group of bacteria that also include B. cereus and B. anthracis among others.2 Bt is distinguished from the other members of the B. cereus group for its entomopathogenic properties due to the production of insecticidal 8-endotoxin proteins during the sporulation phase of growth-forming crystal inclusions (see Figure 8.1). These 8-endotoxins, named Cry and Cyt, are highly specific to their target insect, innocuous to humans, other vertebrates and plants, and completely biodegradable. Therefore, they represent a viable alternative for the control of

Figure 8.1 Scanning electron microscopy observation of the spore and crystal mixtures produced by a B. thuringiensis subsp kurstaki strain that produces Cry1A crystal proteins. Arrows point to crystal inclusions (cry).

insect pests in agriculture and disease vectors of importance in human public health.2,5

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