Role of the glutamine-dependent acid resistance system of Brucella spp. in response to extreme acid stress

Background: The glutamate-dependent decarboxylase (GAD) system is the most potent acid resistance system (AR2) described in Escherichia coli. It consists of a glutamate decarboxylase enzyme and a glutamate/GABA antiporter. Our group showed that the GAD system is also operative in newly described and atypical Brucella species, among which Brucella microti. Recently, an AR2-Q system, based on deamination of glutamine to glutamate by the glutaminase YbaS, was also described in E. coli. In the genome of Brucella, a glsA gene homologous to ybaS is located just upstream and in same orientation as the gadB and gadC genes encoding the GAD system. Objectives: Using a panel of genus-representative strains, our aim was to demonstrate the existence of a functional AR2-Q system in Brucella spp and its contribution to survival at an extreme acid stress. Methods and Results: In contrast to the classical Brucella species studied, only the new and atypical strains were found AR in the presence of glutamine and glutamate. By RT-PCR experiments, the genes of the gadBC-glsA locus were found expressed as an operon. Functional complementation of a glsA mutant of B. microti with the ybaS gene of E. coli demonstrated strong homologies between the 2 enzymes. Acid resistance phenotypes of mutant strains of B. microti confirmed the role of the AR2-Q system in extreme glutamine-dependent AR at pH 2.5. Conclusion: The GAD and AR2-Q systems are functional in new and atypical species of Brucella. This study provides insight into the adaptation of these strains to extremely acidic environments.

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