Caractérisation des opérons ARN ribosomiques et des prophages comme facteurs potentiels de la plasticité génomique chez Bacillus cereus

Le groupe Bacillus cereus comprend six bactéries phénotypiquement distinctes : B. anthracis, agent responsable de la maladie du charbon ; B. cereus, pathogène opportuniste capable de contaminer les produits alimentaires ; B. thuringiensis, bio-pesticide utilisé pour lutter contre les insectes ; B. weihenstephanensis, qui possède la capacité de croître à basse température et B. mycoides et B. pseudomycoides, qui forment des colonies de forme mycoïdale. Les membres du groupe B. cereus influent de manière notable sur les activités humaines. Il est important de caractériser les différences génétiques entre ces bactéries, ainsi que les mécanismes de plasticité génomique qui contribuent à la variabilité des souches du groupe B. cereus.Au cours de cette étude, la phase finale du séquençage complet du génome de la souche type B. cereus ATCC 14579 a été achevée. Nous avons étudié deux facteurs potentiels de la plasticité génomique : les opérons ARN ribosomiques et les phages tempérés. Nous avons caractérisé la variabilité du nombre d'opérons ribosomiques. Nos travaux ont mis en évidence l'existence de 2 types d'opérons ribosomiques au sein du groupe B. cereus. Nous avons identifié et caractérisé deux types de phages tempérés inductibles chez la souche B. cereus ATCC 14579 et chez des souches proches. Un mécanisme d'interaction entre ces deus types de phages a été proposé. Nos travaux soulignent également le rôle des phages dans la plasticité du génome de B. cereus. Enfin, l'approche MLST nous a permis de caractériser un large échantillon de souches du groupe B. cereus, et de mettre en évidence l'existence de 2 complexes clonaux regroupant la plupart des souches psychrotrophes.The Bacillus cereus group includes six phenotypically distinct bacteria : B. anthracis, the causative agent of anthrax ; B. cereus, an opportunistic food-borne pathogen ; B. thuringiensisi, a biopesticide used to struggle against insects ; B. weihenstephanensis, which is able to grow at low temperature (7ʿC) ; B. mycoides and B. pseudomycoides which form rhizoidal colonies. Members of the B. cereus group have a great impact on human activities. It is important to characterise the genetic differences between these bacteria as well as the effective mechanisms of genome plasticity which contribute to the variability of the strains.The finishing of the complete genome sequencing of the type strain B. cereus ATCC 14579 was achieved. We characterised two potentially important factors in genome plasticity in the B. cereus group : ribosomal RNA operons and temperate phages. We identified the number and the structural variability of ribosomal RNA operons of several strains of the B. cereus group, revealing the existence of two distinct types of ribosomal RNA operons. We identified and characterised two types of inducible prophages in the strain B. cereus ATCC 14579 and in some related strains. A general mode of interrelation between these temperate phages was proposed. Our work highlighted the role of temperate phages in the genome plasticity of the B. cereus group bacteria. We characterised a representative collection of strains from diverse origins MLST approach, revealing that most of the psychrotrophic strains of the B. cereus group are genetically closely related.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

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

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