The Glutaminase-Dependent System Confers Extreme Acid Resistance to New Species and Atypical Strains of .

Neutralophilic bacteria have developed specific mechanisms to cope with the acid stress encountered in environments such as soil, fermented foods, and host compartments. In , the glutamate decarboxylase (Gad)-dependent system is extremely efficient: it requires the concerted action of glutamate decarboxylase (GadA/GadB) and of the glutamate (Glu)/γ-aminobutyrate antiporter, GadC. Notably, this system is operative also in new strains/species of , among which , but not in the "classical" species, with the exception of marine mammals strains. Recently, the glutaminase-dependent system (named AR2_Q), relying on the deamination of glutamine (Gln) into Glu and on GadC activity, was described in . In genomes, a putative glutaminase ()-coding gene is located downstream of the genes. We found that in these genes are expressed as a polycistronic transcript. Moreover, using a panel of genus-representative strains, we show that the AR2_Q system protects from extreme acid stress (pH ≤2.5), in the sole presence of Gln, only the species/strains predicted to have functional and . Indeed, mutagenesis approaches confirmed the involvement of and of in AR2_Q and that the acid-sensitive phenotype of can be ascribed to a Ser248Leu substitution in GlsA, leading to loss of glutaminase activity. Furthermore, we found that the gene BMI_II339, of unknown function and downstream of the operon, positively affects Gad- and GlsA-dependent AR. Thus, we identified novel determinants that allow newly discovered and marine mammals strains to be better adapted to face hostile acidic environments. As for significance, this work may contribute to the understanding of the host preferences of species and opens the way to alternative diagnostic targets in epidemiological surveillance of brucellosis.