Abstract
Bioinsecticides with lower concentrations of endospores/crystals and without loss of efficiency are economically advantageous for pest biocontrol. In addition to Cry proteins, other Bacillus thuringiensis (Bt) toxins in culture supernatants (SN) have biocontrol potential (e.g., Vip3A, Cry1I, Sip1), whereas others are unwanted (β-exotoxins), as they display widespread toxicity across taxa. A strain simultaneously providing distinct toxin activities in crystals and SN would be desirable for bioinsecticides development; however, strains secreting β-exotoxins should be discarded, independently of other useful entomotoxins. Entomotoxicity of crystals and SN from a Brazilian Bt tolworthi strain (Btt01) was tested against Spodoptera frugiperda to assess the potential for biocontrol-product development based on more than one type of toxin/activity. Tests showed that 107 endospores mL−1 caused >80% of larvae mortality, suggesting Btt01 may be used in similar concentrations as those of other Bt-based biopesticides. When it was applied to cornfields, a significant 60% reduction of larvae infestation was observed. However, bioassays with Btt01 SN revealed a thermostable toxic activity. Physicochemical characterization strongly suggests the presence of unwanted β-exotoxins, with isolate-specific temporal variation in its secretion. Knowledge of the temporal pattern of secretion/activity in culture for all forms of toxins produced by a single strain is required to both detect useful activities and avoid the potential lack of identification of undesirable toxins. These findings are discussed in the contexts of commercial Bt product development, advantages of multiple-activity strains, and care and handling recommended for large-scale fermentation systems.
Highlights
Bacillus thuringiensis (Berliner) (Bt) is an entomopathogenic bacterium found in many different environments, producing a variety of Cry proteins during sporulation that are selectively toxic to various insect pests [1,2,3]
We tested a standardized concentration of 107 endospores mL−1 for this strain, which is similar to other commercial Bt-based products and preserves satisfactory biocontrol effects
The work here reported is an example of a comprehensive sequence of steps required for a full characterization of a promising biocontrol B. thuringiensis (Bt) isolate, such that both its advantageous aspects and drawbacks are discussed in the context of a most appropriate management of the production system
Summary
Bacillus thuringiensis (Berliner) (Bt) is an entomopathogenic bacterium found in many different environments, producing a variety of Cry proteins during sporulation that are selectively toxic to various insect pests [1,2,3]. Once ingested by the target insect, these proteins cause osmotic imbalance and intestinal paralysis which lead to inanition, sepsis and death [4]. These characteristics have been successfully explored in the past 60 years in the field of biological control of insect pests. Not all of these alternative Bt toxins can be used for pest control, as it is the case with β-exotoxin (Thuringiensin) This is a thermostable compound of low molecular weight (701 Da) that is analogous to adenine or uracil nucleotides [10,11], showing its toxic action by interfering with processes of DNA-dependent RNA polymerization [12]
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