The Retrospective on Atypical Brucella Species Leads to Novel Definitions

The genus Brucella currently comprises twelve species of facultative intracellular bacteria with variable zoonotic potential. Six of them have been considered as classical, causing brucellosis in terrestrial mammalian hosts, with two species originated from marine mammals. In the past fifteen years, field research as well as improved pathogen detection and typing have allowed the identification of four new species, namely Brucella microti, Brucella inopinata, Brucella papionis, Brucella vulpis, and of numerous strains, isolated from a wide range of hosts, including for the first time cold-blooded animals. While their genome sequences are still highly similar to those of classical strains, some of them are characterized by atypical phenotypes such as higher growth rate, increased resistance to acid stress, motility, and lethality in the murine infection model. In our review, we provide an overview of state-of-the-art knowledge about these novel Brucella sp., with emphasis on their phylogenetic positions in the genus, their metabolic characteristics, acid stress resistance mechanisms, and their behavior in well-established in cellulo and in vivo infection models. Comparison of phylogenetic classification and phenotypical properties between classical and novel Brucella species and strains finally lead us to propose a more adapted terminology, distinguishing between core and non-core, and typical versus atypical brucellae, respectively.

Automated summary

The genus Brucella consists of twelve species of intracellular bacteria with varying zoonotic potential. While six of them are considered classical and known to cause brucellosis in terrestrial mammals, the remaining two species originate from marine mammals. Recent research has led to the discovery of four new species, as well as numerous strains isolated from a wide range of hosts, including cold-blooded animals. These newly identified species exhibit atypical phenotypes, such as higher growth rate, increased resistance to acid stress, motility, and lethality in murine infection models. This review provides an overview of the latest knowledge on these novel Brucella species, highlighting their phylogenetic positions, metabolic characteristics, acid stress resistance mechanisms, and behavior in well-established infection models. The comparison between classical and novel Brucella species and strains leads to a proposed more adapted terminology, distinguishing between core and non-core, and typical versus atypical brucellae.