Genetic Differentiation between Sympatric Populations of Bacillus cereus and Bacillus thuringiensis

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.

Population genetics of microbial pathogens estimated from multilocus sequence typing (MLST) data

“…This could be due to variable selective pressures on the genome and/or temporal/geographical structuring generated by random genetic drift, which would not be surprising considering the wide distribution and temporal dispersion of the isolates analyzed. These data support previous conclusions for low rates of recombination for B. cereus (Vilas-Boas et al, 2002), S. pyogenes Feil et al, 2001) and S. agalactiae (Jones et al, 2003). Finally, B. pseudomallei (and closely related species), M. catarrhalis, Staphylococcus epidermidis, Vibrio vulnificus, Campylobacter jejuni, Enterococcus faecium, E. coli, and S. aureus showed consistently low mean levels of ρ( ≤15).…”

Identification of Bacillus anthracis by rpoB Sequence Analysis and Multiplex PCR

“…2). Previous studies that used MLEE separated strains of the B. cereus group into two subgroups (9,11,34). However, because of the incongruent strains studied, it is unclear whether the two main clusters observed in the present study correspond to the subgroups described by these earlier studies.…”

Adaptation and incipient sympatric speciation of Bacillus simplex under microclimatic contrast at “Evolution Canyons” I and II, Israel

“…Parasexual recombination (lateral gene transfer) in bacteria, the level of which could be probably environmentally selected (56,57), could exert different but not mutually exclusive effects. First, it could initiate speciation by entering adaptive genetic information into the recipients' genome, allowing for an adaptive boost of the recombinant (53,58).…”