Growth variation among Bacillus thuringiensis strains can affect screening procedures for supernatant‐secreted toxins against insect pests

Abstract

Methods of screening Bt collections on the basis of feeding bioassays can be misleading with regards to identifying more promising isolates for biocontrol purposes if physiological differences are not considered. The consequences and implications of these findings for the development of experimental systems that depend on toxicity bioassays to identify alternative Bt strains and entomotoxins with practical applicability have been discussed.

Requirement of Simultaneous Assessment of Crystal- and Supernatant-Related Entomotoxic Activities of Bacillus thuringiensis Strains for Biocontrol-Product Development

“…Based on previous studies at laboratory scale [28,31,32], this strain has also shown some promise as a source of useful entomotoxic activity secreted in the SN. The possibility of using the SN of Btt 01 as an alternative source of useful entomotoxins was thus verified by applying the residual liquid fraction of a large-scale fermentation, from the same time used to produce spores, in feeding bioassays.…”

“…Differences in proteotoxin secretion among isolates during bacterial growth have shown to interfere with the final toxic effects of the respective SNs [28]. It was thus necessary to determine whether a similar temporal regulation also occurred with secretion of low-weight metabolites, such as the β-exotoxins.…”

“…In this context, the use of Bt -based biopesticides is highlighted, because toxins of certain strains have successfully controlled fall armyworm, by either direct application or plant transformation [23,24,25]. Furthermore, only recently the tropical germplasm of Bt has started to be investigated for their potential in biocontrol of fall armyworm races, in the context of corn production in Brazil [26,27,28]. …”

“…This suggest the possibility of its use in similar concentrations to those of commercial products, especially the HD1-based biopesticides (LC 50 of 6 × 10 6 endospores mL −1 ) used to control this pest [29,30]. In addition, entomotoxic activities of proteinaceous nature that are secreted in its culture supernatant (SN) at the log phase have also been evaluated [28,31,32]. Taking this information into account, and aiming at improving the economic potential of exploring this strain for the biocontrol market, we evaluated entomotoxic activities against fall armyworm from pellet (crystals) and from SN.…”

Anthropogenic action shapes the evolutionary ecology of Bacillus thuringiensis: response to Ruan et al

“…To us, however, this process is, in fact, currently ongoing everywhere largely because bacteria are among the fastest evolving organisms on Earth (with generation times in hours), and because Bt's genome changes rapidly through mutation and incorporation of new genes by conjugation, transformation, and transduction. Such a high genetic and physiologic diversity in Bt (the raw material of evolution) was evidenced by significant temporal variation in protein/ insect-toxicity secretion in culture and the presence of alternative toxins among isolates from a single subspecies [8,9]. It is noteworthy that the metabolic expense of producing insecticidal crystal proteins does not seem to significantly alter the fitness of Bt, as suggested by the similar levels of plant-colonization ability shown by strains lacking those ICPs [10].…”