Two-component signal transduction in Bacillus cereus and closely related bacteria

Bacillus cereus is a Gram-positive pathogen that is recognised as an important cause of food-borne disease worldwide. Within the genus Bacillus, B. cereus and its closest relatives form a homogeneous subdivision that has been termed the B. cereus group. This group includes B. anthracis, a pathogen that can cause anthrax in mammals, and B. thuringiensis, an insect pathogen that is used as an insecticide worldwide. Members of the B. cereus group can adapt to a wide range of environmental challenges. In bacteria, these challenges are generally monitored by two-component systems (TCS), which consist of a histidine kinase (HK) and a partner response regulator (RR). Upon sensing a specific environmental stimulus, the HK activates its cognate RR, which in turn controls the expression of genes that are involved in the appropriate response. This thesis describes the functional analysis of TCSs in the B. cereus group. By using in silico techniques, 50-58 HKs and 48-52 RRs were identified in eight different B. cereus group genomes. Biological functions, including the involvement in sporulation, biofilm formation and host-microbe interactions were predicted for these TCS proteins. A phylogenetic footprinting approach was developed and used to identify specific binding sites and target genes for over 50% of the B. cereus group DNA-binding RRs. These predictions allowed relating several RRs to a minimal regulon and thereby to a characteristic transcriptional response. To further support these predictions, the transcriptomes of two B. cereus TCS deletion mutants (ΔyvrHG and ΔyufLM) were analysed and compared with the transcriptome of wild-type B. cereus. This revealed that the minimal regulon predictions were correct for the two respective TCSs. Furthermore, the predicted biological roles for these TCSs, including roles in antibiotic resistance (YvrHG) and fumarate metabolism (YufLM), were supported by phenotypic tests. Besides the many “classical” HKs and RRs detected in the B. cereus group, several a-typical TCS proteins were found. These included five RRs without a DNA-binding output domain and two hybrid HKs (HK-RR fusions). Genome analyses revealed that one of the hybrid HK-encoding genes (BC1008) is located in a conserved gene cluster that also encodes the a-typical RR RsbY. In B. cereus, RsbY is known to activate the key stress-responsive sigma factor σB. As a partner HK for RsbY was still “missing”, the role of BC1008 in the σB-mediated stress response was tested. Indeed, a bc1008 deletion strain appeared incapable of inducing σB and its associated regulon upon stress conditions and appeared impaired in its heat adaptive response. In addition, truncation of the BC1008 fused RR receiver domain indicated that this domain plays a role in fine-tuning BC1008 activity. A comparative genome analysis further indicated that BC1008-type hybrid HKs control σB-like sigma factors in at least several other Gram-positive bacteria, including Geobacillus, Paenibacillus and actinobacteria. In summary, the research described in this thesis contributes to our understanding of B. cereus adaptive responses through TCSs. This knowledge may be applied for the development of novel intervention strategies for an improved control of B. cereus in food production environments.