Role of the glutamate decarboxylase(GAD)-dependent system of Brucella spp. in the resistance to extreme acid stess

Brucella is the causative agent of brucellosis, the major bacterial zoonosis worldwide. This infection is transmitted to humans primarily by ingestion of contaminated/unpasteurized dairy products. In the last years, new species and atypical strains of Brucella (as B. microti, B. inopinata and isolates from bullfrog) have been isolated. These are more acid-resistant (AR) than the classical Brucella species (as B. abortus, B. melitensis, B. suis). In several food-borne pathogens such as Escherichia coli, the GAD-dependent system is the most efficient for survival under extreme acid stress. Our team has demonstrated that also in B. microti this system allows survival at pH 2.5 and contributes to murine infection by oral route [Occhialini A. et al, 2012]. The existence of a potentially functional GAD system in the new Brucella species, hypothesized by an in silico analysis of their genome sequences, was investigated by bacteriological, biochemical and genetic approaches. In contrast to the classical Brucella species, new and atypical strains were GAD-positive, able to export GABA and AR in the presence of glutamate or glutamine. Functional complementation of an E. coli gad mutant with the gad loci from Brucella strains demonstrated strong homologies between the two systems. We suggest that a functional GAD system may contribute to improve the adaptability of new species of Brucella in certain natural habitats and/or in the gastrointestinal tract of their hosts [Damiano MA et al. Appl. Environ. Microbiol. 2015].