mallei [16,17,49]. No cellular phenotype was evident following infection with ΔbopC or ΔbopE deletion mutants, and the ΔbopACE triple effector mutant was indistinguishable from the ΔbopA single deletion strain. As with bopE and bopC, no roles were observed for the BsaN-regulated effector candidate loci BPSS1513-1514 in cell-based virulence assays. BPSS1513 encodes
a hypothetical protein and BPSS1514 is annotated as folE, a predicted GTP cyclohydrolase. Based on their genomic organization, the transcription of these loci is likely driven from the promoter upstream of BPSS1512 tssM. The see more secretion of HA-tagged BPSS1513 was not detected in in vitro secretion assays, although it is possible that the epitope tag could have interfered with secretion of BPSS1513, or that the assay was not performed at conditions buy SHP099 permissive for secretion. It is
intriguing why these three genes are placed under BsaN/BicA regulation by the bacterium. One possibility could be that they are important under specific stress conditions or during chronic infection. Conclusions Elucidating the scope of the BsaN regulon significantly enhances our understanding of B. pseudomallei pathogenic mechanisms. BsaN orchestrates the temporal and spatial expression of virulence determinants during progression through the intracellular lifecycle, EPZ5676 mw promoting endosome escape and possibly evasion of autophagy through activation of T3SS3 effector loci, facilitating cell-cell spread by activation of T6SS1 and the bim intracellular motility loci, and suppressing cellular immunity via the action of the TssM ubiquitin hydrolase. BsaN also suppresses other loci that are potentially counterproductive following intracellular localization, such as the fla1 flagellar motility and chemotaxis locus, which could lead to activation of cellular immunity pathways through PAMP recognition. It is likely that the BsaN regulon and other virulence determinants that promote pathogenesis in higher mammals have been shaped primarily as a result of interactions with free-living
protozoa, similar to what is believed to be the case for L. pneumophila . Indeed, many of the same BsaN-regulated systems, namely T3SS and T6SS, are thought to act as “anti-predation determinants” that facilitate endosome escape and promote survival within bacteriovorus amoebae by manipulating eukaryotic pathways that are enough conserved from protists to humans . The dual regulatory roles of BsaN – that of an activator and a suppressor – indicate that it is a key node in a regulatory program that successfully enables an environmental saprophyte to transition from the soil to surviving intracellularly. Methods Bacterial strains and culture conditions Bacterial strains are listed in Table 3. Plasmids are listed in Table 4 and Additional file 1: Table S2. The B. pseudomallei wild-type strains used in this study are clinical isolates KHW. Plasmids were introduced into E. coli DH5α and S17-1  strains by electro- or chemical-transformation.