ABSTRACT
We
used multilocus sequence typing (MLST) to characterize phylogenetic
relationships for a collection of Bacillus cereus group
strains isolated from forest soil in the Paris area during a mild
winter. This collection contains multiple strains isolated from the
same soil sample and strains isolated from samples from different
sites. We characterized 115 strains of this collection and 19 other
strains based on the sequences of the clpC, dinB,
gdpD, panC, purF, and yhfL loci.
The number of alleles ranged from 36 to 53, and a total of 93 allelic
profiles or sequence types were distinguished. We identified three
major strain clusters—C, T, and W—based on the
comparison of individual gene sequences or concatenated sequences. Some
less representative clusters and subclusters were also distinguished.
Analysis of the MLST data using the concept of clonal complexes led to
the identification of two, five, and three such groups in clusters C,
T, and W, respectively. Some of the forest isolates were closely
related to independently isolated psychrotrophic strains. Systematic
testing of the strains of this collection showed that almost all the
strains that were able to grow at a low temperature (6°C)
belonged to cluster W. Most of these strains, including three
independently isolated strains, belong to two clonal complexes and are
therefore very closely related genetically. These clonal complexes
represent strains corresponding to the previously identified species
Bacillus weihenstephanensis. Most of the other strains of our
collection, including some from the W cluster, are not psychrotrophic.
B. weihenstephanensis (cluster W) strains appear to comprise
an effectively sexual population, whereas Bacillus
thuringiensis (cluster T) and B. cereus (cluster C) have
clonal population
structures.
Multiple-Locus Sequence Typing Analysis of Bacillus cereus and Bacillus thuringiensis Reveals Separate Clustering and a Distinct Population Structure of Psychrotrophic Strains