Characterization of PdhS functions, an essential and polarly localized histidine kinase, in Brucella abortus

(eng) Several bacteria divide asymmetrically, one of the key processes leading to cellular differentiation. A huge number of complex shapes are observed among bacteria, where functionalities can be attributed to specialized cell types as observed in the prokaryotic model Caulobacter crescentus life cycle where a stalked and proliferative competent cell generates a motile flagellated cell able to find favorable replication niches. We are working on Brucella abortus, a facultative intracellular class III pathogen that is responsible for a worldwide zoonosis called Brucellosis, humans being accidental hosts. B. abortus divides asymmetrically, and following an old pole marker called PdhS, it has been suggested that a differentiation event was taking place during B. abortus cell cycle growing in bacteriological medium. PdhS, an essential histidine kinase, is involved in B. abortus cell cycle regulation and displays atypical features such as its large sensory domain. In order to better characterize PdhS, we used a domain mapping approach and identified minimal domains for protein-protein interaction (with PdhS itself or with a partner called FumC), for polar localization of for altered morphologies generation in B. abortus. We hypothesized that the slight morphological asymmetry observed in B. abortus could reflect a functional asymmetry generated after cytokinesis completion. We thus attempted to label the two different sibling cells using chemical compound and molecular markers. We showed that a mother cell, proliferative and non-invasive generates a virulent daughter cell that is blocked in its cell cycle. This shows that a differentiation event is taking place during a cellular invasion by B. abortus. Interestingly, the generation of the invasive daughter cell is observed at each division, using polar marker in growing bacteria. The generation of specialized cell types just after cytokinesis and despite the inheritance of an identical genetic material shows that clonal population of bacteria are more heterogeneous than previously thought. Responsive and stochastic events can trigger phenotypic heterogeneity and we propose that programmed differentiation along the B. abortus cell cycle is another way to generate phenotypic diversity.