Glutamate Decarboxylase-Dependent Acid Resistance in Brucella spp.: Distribution and Contribution to Fitness under Extremely Acidic Conditions

Abstract

ABSTRACTBrucellais an expanding genus of major zoonotic pathogens, including at least 10 genetically very close species occupying a wide range of niches from soil to wildlife, livestock, and humans. Recently, we have shown that in the new speciesBrucella microti, the glutamate decarboxylase (Gad)-dependent system (GAD system) contributes to survival at a pH of 2.5 and also to infection in mice by the oral route. In order to study the functionality of the GAD system in the genusBrucella, 47 isolates, representative of all known species and strains of this genus, and 16 strains of the closest neighbor genus,Ochrobactrum, were studied using microbiological, biochemical, and genetic approaches. In agreement with the genome sequences, the GAD system of classical species was not functional, unlike that of most strains ofBrucella ceti,Brucella pinnipedialis, and newly described species (B. microti,Brucella inopinataBO1,B. inopinata-like BO2, andBrucellasp. isolated from bullfrogs). In the presence of glutamate, these species were more acid resistantin vitrothan classical terrestrial brucellae. Expression intransof thegadlocus from representativeBrucellaspecies in theEscherichia coliMG1655 mutant strain lacking the GAD system restored the acid-resistant phenotype. The highly conserved GAD system of the newly described or atypicalBrucellaspecies may play an important role in their adaptation to acidic external and host environments. Furthermore, the GAD phenotype was shown to be a useful diagnostic tool to distinguish these latterBrucellastrains fromOchrobactrumand from classical terrestrial pathogenicBrucellaspecies, which are GAD negative.