BvrR/BvrS is a well characterized two-component regulatory system that controls the expression of genes essential for Brucella abortus invasion to non-phagocytic cells [11, 12]. High level of identity is present between B. melitensis ChvI/ChvG (encoded by BMEI2036 and BMEI2035, respectively) and the B.

abortus BvrR/BvrS proteins [17]. In our study, no transcriptional change was observed in BMEI2036/I02035 ORFs between the most (late-log AG-14699 phase) and the least (stationary growth phase) invasive cultures. Likely, Brucella maintain a basal expression level of the regulatory locus, as a change in the phosphorylation of the protein required for activity rather than transcription. Twenty ORFs dedicated to signal transduction were identified in B. melitensis genome [19]. The importance of some of them in Brucella virulence had been characterized

lately, including blxR, vjbR, ftcR and bvrR/bvrS [12, 45, 50–52]. However, their contribution to internalization in non-phagocytic cells is less known. Recently, mutants with defective expression in two transcriptional regulators (vjbR BTK inhibitor and bvrR/S) had an altered pattern in initial host:pathogen interaction due to surface modifications [12, 45]. Future identification of the target genes of these regulators would clarify Brucella physiology, metabolism and virulence regulation. Several motility-related genes were more highly expressed at late-log phase compared to stationary phase, including kinesin-like protein, chemotaxis MotD protein

and genes related to flagellum apparatus synthesis and functions, e.g. flagellin itself (96.6-fold). Flagellin has been well-characterized as a contributor to bacterial virulence through chemotaxis and adhesion to and invasion of host cells [53]. The extent to which flagellar machinery participates in the process of invasion seems to depend at least partly on the species of bacteria and/or the host cell type. For instance, flagellar-associated motility in Salmonella is not required but accelerates invasion of Caco-2 colonic epithelial cells [54], whereas the invasion of Acanthamoeba astronyxis by Burkholderia pseudomallei absolutely Sitaxentan requires an intact flagellum apparatus [55]. In the case of B. melitensis, a previous study demonstrated that expression of flagella is growth curve-dependent and required for persistent disease in a mouse model but not for invasion in cellular models [20]. That study reported that a functional flagellum was assembled in early-log growth phase cultures but not at later time points. In our study, we did not analyze gene expression at early time points of the growth curve, but the results indicated that some flagellar genes were expressed more in late-log phase cultures as compared to stationary phase cultures. The differences in flagellum gene expression between the study of Fretin et al.