Distribution of phenotypes among Bacillus thuringiensis strains

Authors: Martin, Phyllis; Gundersen-Rindal, Dawn; Blackburn, Michael - Mike

Submitted to: Systematic and Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2010
Publication Date: 6/1/2010
Citation: Martin, P.A., Gundersen, D.E., Blackburn, M.B. 2010. Distribution of phenotypes among Bacillus thuringiensis strains. Systematic and Applied Microbiology. 33:204-208.

Interpretive Summary: Insects cause billions of dollars in loss to crops in terms of both control and yield reduction. Bacteria such as Bt can be used to control insect pests, but it is often difficult to predict which ones will be effective. We developed a rapid way of classifying individual strains of Bt based on 6 chemical compounds the bacteria can utilize for growth. We found that the combination of chemicals used by a particular strain of the bacteria was often related to the types of insects that the strain could kill. The most common variety identified by our system, accounting for almost 25% of all strains, is much more likely to kill mosquitos, while strains that used the chemical urea were typically toxic to caterpillars of pests such as the gypsy moth or cabbage looper. Bt strains that were non-toxic to insects were often capable of using a specific combination chemical compounds for growth. These findings are of interest to scientists who track Bt survival in the environment as well as those who are interested in distinguishing among Bt isolates and predicting toxicity to insects.

Technical Abstract: An extensive collection of Bacillus thuringiensis isolates from around the world were phenotypically profiled using standard biochemical tests. Six phenotypic traits occurred in 20-86% of the isolates and were useful in distinguishing isolates: production of urease (U; 20.5% of isolates), hydrolysis of esculin (E; 32.3% of isolates), acid production from salicin (A; 37.4% of isolates), acid production from sucrose (S; 34.0% of isolates), production of phospholipase C (L; 79.7% of isolates), and hydrolysis of starch (T; 85.8% of isolates). With the exception of acid production from salicin and hydrolysis of esculin, which were associated, the traits assorted independently. Of the 64 possible combinations of these 6 phenotypic characteristics, 15 combinations accounted for ca. 80% of all isolates, with the most common phenotype being TL (23.6% of isolates). Surprisingly, while the biochemical traits generally assorted independently, certain phenotypic traits associated with the parasporal crystal were correlated with certain combinations of biochemical traits. Crystals that remained attached to spores (which tended to be non-toxic to insects) were highly correlated with the phenotypes that included both L and S. Of the 15 most abundant phenotypes characterizing B. thuringiensis strains, amorphous crystals were associated with TLE, TL, T and Ø (the absence of positive biochemical traits). Amorphous crystal types displayed a distinct bias toward toxicity to dipteran species of insects. Although all common phenotypes included B. thuringiensis isolates producing bipyramidal crystals toxic to lepidopteran insects, those with the highest abundance of crystals displayed phenotypes TLU, TLUA, TLUAE, and TLAE.