Genetic Differentiation between Sympatric Populations of Bacillus cereus and Bacillus thuringiensis

Author:

Vilas-Boas Gislayne12,Sanchis Vincent13,Lereclus Didier13,Lemos Manoel Victor F.2,Bourguet Denis1

Affiliation:

1. Unité de Recherches de Lutte Biologique, Institut National de la Recherche Agronomique, La Minière, 78 285 Guyancourt

2. Departamento de Biologia Aplicada à Agropecuária, FCAV/UNESP, Jaboticabal/SP 14884-900, Brazil

3. Unité de Biochimie Microbienne, CNRS (URA 2172), Institut Pasteur, 75724 Paris Cedex, France

Abstract

ABSTRACT Little is known about genetic exchanges in natural populations of bacteria of the spore-forming Bacillus cereus group, because no population genetics studies have been performed with local sympatric populations. We isolated strains of Bacillus thuringiensis and B. cereus from small samples of soil collected at the same time from two separate geographical sites, one within the forest and the other at the edge of the forest. A total of 100 B. cereus and 98 B. thuringiensis strains were isolated and characterized by electrophoresis to determine allelic composition at nine enzymatic loci. We observed genetic differentiation between populations of B. cereus and B. thuringiensis . Populations of a given Bacillus species— B. thuringiensis or B. cereus —were genetically more similar to each other than to populations of the other Bacillus species. Hemolytic activity provided further evidence of this genetic divergence, which remained evident even if putative clones were removed from the data set. Our results suggest that the rate of gene flow was higher between strains of the same species, but that exchanges between B. cereus and B. thuringiensis were nonetheless possible. Linkage disequilibrium analysis revealed sufficient recombination for B. cereus populations to be considered panmictic units. In B. thuringiensis, the balance between clonal proliferation and recombination seemed to depend on location. Overall, our data indicate that it is not important for risk assessment purposes to determine whether B. cereus and B. thuringiensis belong to a single or two species. Assessment of the biosafety of pest control based on B. thuringiensis requires evaluation of the extent of genetic exchange between strains in realistic natural conditions.

Publisher

American Society for Microbiology

Subject

Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology

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