Population Structure of theBacillus cereusGroup as Determined by Sequence Analysis of Six Housekeeping Genes and theplcRGene

Abstract

The population structure of the Bacillus cereus group (52 strains of B. anthracis, B. cereus, and B. thuringiensis) was investigated by sequencing seven gene fragments (rpoB, gyrB, pycA, mdh, mbl, mutS, and plcR). Most of the strains were classifiable into two large subgroups in six housekeeping gene trees but not in the plcR tree. In addition, several consistent clusters were identified, which were unrelated to species distinction. Moreover, interrelationships among these clusters were incongruent in each gene tree. The incongruence length difference test and split decomposition analyses also showed incongruences between genes, suggesting horizontal gene transfer. The plcR gene was observed to have characteristics that differed from those of the other genes in terms of phylogenetic topology and pattern of sequence diversity. Thus, we suggest that the evolutionary history of the PlcR regulon differs from those of the other chromosomal genes and that recombination of the plcR gene may be frequent. The homogeneity of B. anthracis, which is depicted as an independent lineage in phylogenetic trees, is suggested to be of recent origin or to be due to the narrow taxonomic definition of species.The Bacillus cereus group, which is a subdivision of the genus Bacillus, includes the closely related species Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis (22). However, their pathogenic potentials and disease spectrums are quite different despite their genetic relatedness. B. cereus is an opportunistic pathogen and causes several types of infections in humans. It is frequently isolated as a contaminant of milk, cereals, and various other foods, and it produces an emetic toxin and one or several enterotoxins. On the other hand, B. thuringiensis is primarily an insect pathogen, and it produces intracellularly insecticidal crystal toxins of different specificities during sporulation or in the stationary phase, which is the only established difference between it and B. cereus (7). B. anthracis causes the potentially lethal disease anthrax, and it has been identified as a nonhemolytic, nonmotile, penicillin-sensitive, encapsulated bacterium. B. anthracis is important in that it is considered a potential biological weapon (26). The genes causing the lethal effect of anthrax are located on two large virulence plasmids, pXO1 and pXO2 (26).The genomes of these three species show high levels of similarity; for example, they share almost identical 16S ribosomal DNA sequences (2), although an association of a distinct type of 16S ribosomal DNA sequence with B. anthracis was recently reported (30). Although several phenotypes (such as capsule, lack of hemolysis, lack of motility, and susceptibility to gamma phage) and biochemical tests can differentiate B. anthracis from B. cereus and B. thuringiensis (38), species delimitation is unclear. In fact, they were suggested to be one species based on a multilocus enzyme electrophoresis (MLEE) result (14) and by the presence of an S-layer on the cell surface (24). Therefore, an ...

Specificity and Polymorphism of the PlcR-PapR Quorum-Sensing System in theBacillus cereusGroup

“…However, it does not appear possible to establish a strict correlation between the four pherotypes that we have defined in this study and the phylogenetic trees based on the analysis of housekeeping genes. This conclusion is in agreement with the absence of a phylogenetic relation between plcR and housekeeping genes, as indicated by a recent MLST study (13). The lack of congruence between pherotypes and phylogenetic classifications has previously been observed for the competence quorum-sensing system in B. subtilis (27) and for the Agr system in Staphylococcus species (3).…”

Multiple-Locus Sequence Typing Analysis of Bacillus cereus and Bacillus thuringiensis Reveals Separate Clustering and a Distinct Population Structure of Psychrotrophic Strains

“…After calculating the I A for the entire collection and detecting lineages in the clusters corresponding to our C and T clusters, Helgason et al (16) and Priest et al (26) concluded that the whole Bacillus cereus group has a clonal or weakly clonal population structure. Ko et al (19) discovered an unusual feature of the plcR gene, presumably related to its horizontal transfer. Our work is original in that we studied a large number of strains isolated from a very limited geographical location (a forest near Versailles).…”

Population Structure of Francisella tularensis

“…Results are indicated in Table S2 in the supplemental material. For four gene combinations (dnaA-mutS, dnaA-tul4, dnaA-fopA, and tpiA-fopA), significant conflict between data sets was indicated by P values below the recommended threshold of 0.05 (4,8,15). When strains F59, F60, and F62 were excluded from these analyses, all P values were above 0.05 (not shown).…”