The intracellular pathogenic bacteria belonging to the genus Brucella must cope with acidic stress as they penetrate the host via the gastrointestinal route, and again during the initial stages of intracellular infection. Following their uptake by the host cell, Brucella replicate inside a membrane-bound compartment—the Brucella-containing vacuole—whose acidification is essential for the survival of the pathogen. Therefore, identifying the genes that contribute to the survival of Brucella in acidic environments will greatly assist our understanding of its molecular pathogenic mechanisms. In a comparative transcriptomic analysis of the attenuated vaccine Brucella melitensis strain Rev.1 against the virulent strain 16M in cultures grown under either neutral or acidic conditions, we found 403 genes that respond differently to acidic conditions in the two strains (FDR < 0.05, fold change ≥ 2), involved in crucial cellular processes, including metabolic, biosynthetic, and transport processes. Further analysis of the RNA-seq data of 16M by comparing it to published transcriptomic data of this strain from both an in cellulo and an in vivo model, revealed 588 genes that were exclusively differentially expressed in 16M grown under acidic versus neutral pH conditions, including 286 up regulated genes and 302 down regulated genes that were not differentially expressed in either the in cellulo or the in vivo model. Among them we detected 13 key genes that are known to be associated with a bacterial response to acidic stress and were highly up regulated under acidic conditions in our study. These genes provide new molecular insights into the mechanisms underlying the acid-resistance of Brucella within its host and may facilitate the design of new and improved brucellosis vaccines.

Keywords: Brucella melitensis, Acid stress, RNA-Seq, Transcriptomic analyses, Virulence