Closely Related Brucella Species Widely Differ in their Vegetative and Intracellular Growth

Growth rate is a key prokaryotic trait that allows for estimating fitness and understanding cell metabolism. While it has been well studied in model organisms, there is limited data on slow-growing bacteria. In particular, there is a lack of quantitative studies on Brucella species. This genus includes important microorganisms that are causative agents 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, B. melitensis, B. suis, B. ovis, and B. canis). Strikingly, each species displayed distinct growth parameters in culture. Doubling time (DT) ranged from 2.7 hs^-1 in B. suis to 18 h^-1 for B. ovis. In the context of intracellular infection of J774A.1 phagocytic cells, DT was longer, but it widely varied across species, closely correlating with the growth observed in vitro. Overall, and despite high genome similarity, we also found species-specific growth parameters in the intracellular cell cycle.