Bacillus thuringiensis Toxins: An Overview of Their Biocidal Activity

Abstract

Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

Specificity determinants for Cry insecticidal proteins: Insights from their mode of action

“…However, when examined, there is a lack of direct correlation between structure and activity against specific targets, i.e. the same Cry toxin can be active against taxonomically diverse insects and Cry toxins with diverse binding determinants (domains II and/or III) may be active against the same insect (Palma et al, 2014).…”

Genetically engineered (modified) crops (Bacillus thuringiensis crops) and the world controversy on their safety

“…Interestingly, Palma et al (2014) reported that 700 Bt cry genes (Cry proteins) have been identified in the past decades. While many Cry proteins have pesticidal properties against insect pests in agriculture, others have no known invertebrate targets and have been termed "parasporins".…”

Structural classification of insecticidal proteins – Towards an in silico characterisation of novel toxins

“…Significant progress has been made in recent years in the elucidation of new toxin structures and the potential to derive reliable structures from modelling approaches is discussed elsewhere in this issue (Berry and Board, 2016). Similarly, while there are a lot of data available on the target specificity of individual toxins (Palma et al, 2014a;van Frankenhuyzen, 2009), many of these are contradictory, thus compounding efforts to derive meaningful associations. The area where least information is known concerns the interaction of the toxin with the target cell (Vachon et al, 2012).…”

“…Within the 3-domain toxin family, we find toxins with activity against insects in several orders, principally amongst the Lepidoptera with fewer active against the orders Diptera and Coleoptera, and with small numbers active against Hymenoptera and Hemiptera as well as toxins affecting nematodes and gastropods (reviewed recently (Palma et al, 2014a)). …”