Brucella spp. are facultative anaerobic bacteria under denitrifying conditions

Bacteria of the genus Brucella are responsible for human and animal brucellosis worldwide and comprise the well-characterized, so-called classical species belonging to the core clade, with a host range limited to mammals, and the novel atypical strains mainly considered as non-zoonotic to date, but able to colonize non-traditional hosts such as amphibians. Despite marked phenotypical differences between the two groups, Brucella species display a high degree of genome identity, including genes encoding reductases involved in the denitrification pathway, a respiratory process able to reduce nitrate to nitrogen under anoxic conditions. Brucella spp. have always been considered as strictly aerobic pathogens. Our study, however, describes for the first time that atypical Brucella spp. grow well under anoxia, but in a strictly nitrate-dependent manner, whereas classical species show a heterogeneous behavior. The rate of anaerobic growth of the atypical species Brucella microti was dependent on nitrate concentration, highlighting a more efficient denitrification pathway with rapid consumption of intermediate nitrite, whereas the classical species Brucella suis exhibited lower, nitrate concentration-independent growth coupled with nitrite accumulation. Transcriptional studies performed under anoxic conditions revealed a higher expression of genes participating in the last three denitrification steps in B. microti, as compared to B. suis, which correlates with the rapid and efficient consumption of nitrite in this atypical species and its rapid growth.