Connecting the cell cycle and the pathogenicity of Brucella abortus: when a specific bacterial cell cycle arrest in G1 phase takes part in the colonization of mammalian cells

The bacterial cell cycle is a developmental process starting with newborn bacteria that progress to the predivisional status in which they eventually generate two daughter cells. Along their development, the newborns grow and replicate their genome that is in turn faithfully transmitted to the two siblings. The bacterial cell cycle has been extensively investigated in the alphaproteobacterium Caulobacter crescentus, which produces a small non-replicating (G1 phase) swarmer cell for the colonization of new niches in aquatic environments at each cell division. The pathogen Brucella abortus is also part of the alphaproteobacteria group but displays a singular cell cycle compared to C. crescentus. Opposite to C. crescentus, B. abortus growth mode is unipolar resulting in the asymmetric distribution of the cell wall material in the predivisionnal cell. This asymmetry is further exacerbated by an asymmetric division that generates a large cell composed of “old” cell wall material and a small cell made of a new envelope material. In addition, B. abortus also contains a genome divided into several replicons each encoding its own segregating apparatus. Among those replicons is included a chromid (a combination of chromosomal and plasmidic features) carrying a RepAB-based segregation system for which the localization has not been not assessed so far. During this PhD thesis, we characterized the mechanisms of unipolar growth and chromosomes segregation of B. abortus. Concerning the unipolar growth study, we localized the only monofunctional transpeptidase penicillin-binding protein (FtsI) identified in B. abortus that creates links between the different peptidoglycan strands at every growth zone. The localization of FtsI revealed that this protein is present at the growing pole during the unipolar elongation and at the constriction site during the division. These data suggest the presence of a unique growth machinery performing both the elongation (at the new pole) and the division (at the constriction site) in two different periods of the cell cycle. We therefore propose that B. abortus and other rhizobiales unipolar growth mode relies on a simple process involving only one machinery used after the loss of the genes involved in the formation of the core of the elongation machinery such as mreB or pbp2. We investigated the replication status of B. abortus by generating fluorescent reporters of the segregation markers (ParB and RepB) and the replication origins (ori) and terminators (ter). We showed that both chromosomes were oriented along the cell length axis and that oriI was strictly polarly localized while oriII was not anchored to the poles, similarly to plasmids. Moreover, we showed that the replication and segregation of chromosome I are initiated before the duplication of chromosome II origin. As B. abortus is a facultative intracellular pathogen characterized by a striking proliferation arrest during the first phase of its infectious process, we envisioned a potential link between its cell cycle and its infectivity. Importantly, our chromosomal reporters indicated that the newly generated non-replicating bacteria (newborns) are predominant during the non-proliferative stage of the infection. As the bacterial cell cycle appears to be blocked during this infectious period, we propose that the B. abortus newborns constitute the major cell type able to colonize host cells. These data demonstrate thus that only a subset of the Brucella population is dedicated to the prospection of new environmental niches similarly to the swarmer G1 blocked cells in C. crescentus. Overall, this work demonstrates the need to investigate bacterial cell cycle to improve our knowledge of bacterial pathogenicity, since G1 block may represent a general strategy of resistance form in stressful conditions encountered by the intracellular pathogens.