Inside Out: the physiology of Brucella Vegetative and Intracellular Growth.

Growth rate is a key prokaryotic trait that allows estimating fitness and understanding cell metabolism. While in some clades it has been well studied in model organisms, there is little data on slow-growing bacteria. In particular, there is a lack of quantitative studies on the species of the Brucella genus. This is an important microorganism since it is the causative agent of brucellosis, one of the most widespread bacterial zoonoses affecting several species of animals, including humans. Brucella species exhibit approximately 97% genomic similarity. Despite this, Brucella species show different host preferences, zoonotic risks, and pathogenicity. After more than one hundred years of research, numerous aspects of Brucella biology, such as in vivo and in vitro growth remain poorly characterized. In this work, we analyzed vegetative and intracellular growth of the classical Brucella species (B. abortus 2308, B. melitensis 16M. B. suis 1330, B. ovis, and B. canis). Strikingly, each species displayed particular growth parameters in culture. Doubling time (DT) spanned from 2.7 hs-1 in B. suis to 18h-1 for B. ovis. In the context of intracellular infection of J774A.1 phagocytic cells, DT was higher, but it widely varied across species, closely correlating to the growth observed in vitro. Overall, despite high similarity of the genomes, we found species-specific growth parameters in the intracellular cell cycle.