Quorum Sensing and Self-Quorum Quenching in the Intracellular Pathogen<i>Brucellamelitensis</i>

<div><p><i>Brucella</i> quorum sensing has been described as an important regulatory system controlling crucial virulence determinants such as the VirB type IV secretion system and the flagellar genes. However, the basis of quorum sensing, namely the production of autoinducers in <i>Brucella</i> has been questioned. Here, we report data obtained from the use of a genetic tool allowing the <i>in situ</i> detection of long-chain <i>N</i>-acyl-homoserine lactones (AHL) activity at single bacterium level in <i>Brucella melitensis</i>. These data are consistent with an intrinsic production of AHL by <i>B. melitensis</i> in low concentration both during <i>in vitro</i> growth and macrophage infection. Moreover, we identified a protein, named AibP, which is homologous to the AHL-acylases of various bacterial species. <i>In vitro</i> and during infection, expression of <i>aibP</i> coincided with a decrease in endogenous AHL activity within <i>B. melitensis</i>, suggesting that AibP could efficiently impair AHL accumulation. Furthermore, we showed that deletion of <i>aibP</i> in <i>B. melitensis</i> resulted in enhanced <i>virB</i> genes expression and VirB8 production as well as in a reduced flagellar genes expression and production of FlgE (hook protein) and FliC (flagellin) <i>in vitro</i>. Altogether, these results suggest that AHL-dependent quorum sensing and AHL-quorum quenching coexist in <i>Brucella</i>, at least to regulate its virulence.</p></div

Primers used in this work.

<p>Primers used in this work.</p

<i>B. melitensis</i> both produces and degrades long-chain AHLs during macrophage infection.

<p>(<b>A and B</b>) <i>B. melitensis</i> wt and ▵<i>aibP</i> QS reporter strains were used to infect monolayers of RAW264.7 murine macrophages. Prior to (0h) and during infection, bacteria or cells were fixed, bacteria were labelled with a monoclonal A76-12G12 anti-LPS antibody and DNA was labelled with DAPI. (A) Immunofluorescence micrographs are representative from at least two independent experiments. Bacteria LPS appears in red, DNA in blue. Scale bar, 10 µm. (B) The percentage of GFP(ASV)-positive bacteria at different times post-infection was determined as described in the Material and Methods section. Error bars represent the standard deviation from two independent experiments. Data have been analyzed by ANOVA I after testing the homogeneity of variance (Bartlett). * and ** denote significant differences (<i>P</i> < 0.05 and <i>P</i> < 0.01) in relation to wt bacteria prior to infection (0h) while # denotes a significant difference (<i>P</i> < 0.05) in relation to ▵<i>aibP</i> bacteria prior to infection. (<b>C</b>) Intracellular replication of <i>B. melitensis</i> wt and ▵<i>aibP</i> strains in RAW264.7 murine macrophages. At indicated times, cells were lysed and intracellular colony forming units (CFUs) were determined. Error bars represent the standard deviation of triplicates in one representative experiment out of three. (<b>D</b>) RAW264.7 macrophages were infected with <i>B. melitensis</i> wt in the presence of C12-HSL or ACN (negative control) and treated as described (C). ** and *** denote significant (<i>P</i> < 0.01 and <i>P</i> < 0.001 respectively) differences in relation to infection by wt bacteria in the presence of CAN (Bartlett and ANOVA I analysis).</p

Self-quorum quenching regulates <i>virB</i> genes expression.

<p>(<b>A</b>) <i>B. melitensis</i> wt and ▵<i>aibP</i> strains both carrying the pBBR <i>PvirB</i>-<i>gfp</i>(ASV) plasmid were grown in 2YT and GFP(ASV) fluorescence intensity was measured at indicated phases of growth by flow cytometry (5×10⁴ events acquired). Results are representative of two independent experiments. (<b>B</b>) The relative abundance of <i>virB1</i>, <i>virB2</i>, and <i>virB8</i> mRNAs was determined by qRT-PCR on RNA isolated from bacteria harvested at the early exponential phase of growth in 2YT supplemented or not with exogenous C12-HSL (5 µM). Deletion of <i>aibP</i> results in significant upregulation of <i>virB</i> genes (P<0.001 in Student’s t test), whereas exogenous C12-HSL significantly downregulates their expression (P<0.001 in Student’s t test). ▵<i>aibP PaibP</i> on the right panel is the complemented strain. Results are representative of two independent experiments. Error bars represent standard deviation from biological triplicates. (<b>C</b>) (Left panel) Western Blot analysis of VirB8 production performed on whole protein lysates of bacteria harvested at the indicated phases of growth in 2YT. ▵<i>aibP PaibP</i> is the complemented strain. (Right panel) Bacteria were harvested in the log phase of growth in 2YT in the absence (–) or in the presence of C12-HSL. The VirB8 protein was detected at its expected size (26,5 kDa). Detection of PrlR or Omp89 proteins was used to normalize total protein content.</p

Validation of the specificity and sensitivity of the QS reporter system in <i>Brucella melitensis</i> 16M.

<p>(<b>a</b>) Immunofluorescence of the <i>B. melitensis</i> control strain incubated 4 hours with C12-HSL (1 µM) and labelled with monoclonal A76-12G12 anti-LPS antibody (red). No GFP(ASV) signal is detected. (<b>from b to e</b>) Observation of GFP(ASV) production by the <i>B. melitensis</i> reporter strain after a 4h incubation with various concentrations of synthetic C12-HSL; (<b>b</b>) 1nM, (<b>c</b>) 10nM, (<b>d</b>) 100nM, (<b>e</b>) 1 µM; scale bar 5 µm. (<b>f</b>) Measurement of GFP(ASV) fluorescence intensity by flow cytometry (5×10⁴ events acquired) in the <i>B. melitensis</i> QS reporter strain fixed after a 4h-incubation with 0.1nM or 1nM of C12-HSL or 3-oxo-C12HSL. The <i>B. melitensis</i> control strain was used as a negative control (black dotted line). The results are representative of at least two independent experiments.</p

Transient production of long-chain AHLs by <i>B. melitensis</i> in liquid culture.

<p>The <i>B. melitensis</i> QS reporter strain was grown in 2YT and cell density was determined when fluorescence intensity of GFP(ASV) was assessed by flow cytometry (5×10⁴ events acquired). (<b>A</b>) Growth curve of the <i>B. melitensis</i> QS reporter strain. Numbers represent the 4 distinct phases of growth. OD₆₀₀, optical density at 600nm. (<b>B</b>) Histograms of GFP(ASV) fluorescence intensity representative of the growth phases represented in A. The <i>B. melitensis</i> control strain was used as a negative control. In (<b>B2</b>), the peak of GFP(ASV) fluorescence intensity due to endogenous AHLs was compared with results obtained after a 4h-incubation of the <i>B. melitensis</i> QS reporter strain with synthetic C12-HSL or 3-oxo-C12-HSL (bacteria from the early log phase were used). The insets show differential interference contrast (DIC) and FITC fluorescence microscopy of (from top to bottom) the negative control strain, the QS reporter strain in the absence of exogenous AHL and incubated with C12-HSL 0.1nM, with C12-HSL 1nM, and with 3-oxo-C12-HSL 0.1 nM respectively. Results are representative of three independent experiments.</p

Global Analysis of Quorum Sensing Targets in the Intracellular Pathogen <i>Brucella melitensis</i> 16 M

Many pathogenic bacteria use a regulatory process termed quorum sensing (QS) to produce and detect small diffusible molecules to synchronize gene expression within a population. In Gram-negative bacteria, the detection of, and response to, these molecules depends on transcriptional regulators belonging to the LuxR family. Such a system has been discovered in the intracellular pathogen <i>Brucella melitensis</i>, a Gram-negative bacterium responsible for brucellosis, a worldwide zoonosis that remains a serious public health concern in countries were the disease is endemic. Genes encoding two LuxR-type regulators, VjbR and BabR, have been identified in the genome of <i>B. melitensis</i> 16 M. A Δ<i>vjbR</i> mutant is highly attenuated in all experimental models of infection tested, suggesting a crucial role for QS in the virulence of <i>Brucella</i>. At present, no function has been attributed to BabR. The experiments described in this report indicate that 5% of the genes in the <i>B. melitensis</i> 16 M genome are regulated by VjbR and/or BabR, suggesting that QS is a global regulatory system in this bacterium. The overlap between BabR and VjbR targets suggest a cross-talk between these two regulators. Our results also demonstrate that VjbR and BabR regulate many genes and/or proteins involved in stress response, metabolism, and virulence, including those potentially involved in the adaptation of <i>Brucella</i> to the oxidative, pH, and nutritional stresses encountered within the host. These findings highlight the involvement of QS as a major regulatory system in <i>Brucella</i> and lead us to suggest that this regulatory system could participate in the spatial and sequential adaptation of <i>Brucella</i> strains to the host environment

Global Analysis of Quorum Sensing Targets in the Intracellular Pathogen <i>Brucella melitensis</i> 16 M

Many pathogenic bacteria use a regulatory process termed quorum sensing (QS) to produce and detect small diffusible molecules to synchronize gene expression within a population. In Gram-negative bacteria, the detection of, and response to, these molecules depends on transcriptional regulators belonging to the LuxR family. Such a system has been discovered in the intracellular pathogen <i>Brucella melitensis</i>, a Gram-negative bacterium responsible for brucellosis, a worldwide zoonosis that remains a serious public health concern in countries were the disease is endemic. Genes encoding two LuxR-type regulators, VjbR and BabR, have been identified in the genome of <i>B. melitensis</i> 16 M. A Δ<i>vjbR</i> mutant is highly attenuated in all experimental models of infection tested, suggesting a crucial role for QS in the virulence of <i>Brucella</i>. At present, no function has been attributed to BabR. The experiments described in this report indicate that 5% of the genes in the <i>B. melitensis</i> 16 M genome are regulated by VjbR and/or BabR, suggesting that QS is a global regulatory system in this bacterium. The overlap between BabR and VjbR targets suggest a cross-talk between these two regulators. Our results also demonstrate that VjbR and BabR regulate many genes and/or proteins involved in stress response, metabolism, and virulence, including those potentially involved in the adaptation of <i>Brucella</i> to the oxidative, pH, and nutritional stresses encountered within the host. These findings highlight the involvement of QS as a major regulatory system in <i>Brucella</i> and lead us to suggest that this regulatory system could participate in the spatial and sequential adaptation of <i>Brucella</i> strains to the host environment

Global Analysis of Quorum Sensing Targets in the Intracellular Pathogen <i>Brucella melitensis</i> 16 M

Many pathogenic bacteria use a regulatory process termed quorum sensing (QS) to produce and detect small diffusible molecules to synchronize gene expression within a population. In Gram-negative bacteria, the detection of, and response to, these molecules depends on transcriptional regulators belonging to the LuxR family. Such a system has been discovered in the intracellular pathogen <i>Brucella melitensis</i>, a Gram-negative bacterium responsible for brucellosis, a worldwide zoonosis that remains a serious public health concern in countries were the disease is endemic. Genes encoding two LuxR-type regulators, VjbR and BabR, have been identified in the genome of <i>B. melitensis</i> 16 M. A Δ<i>vjbR</i> mutant is highly attenuated in all experimental models of infection tested, suggesting a crucial role for QS in the virulence of <i>Brucella</i>. At present, no function has been attributed to BabR. The experiments described in this report indicate that 5% of the genes in the <i>B. melitensis</i> 16 M genome are regulated by VjbR and/or BabR, suggesting that QS is a global regulatory system in this bacterium. The overlap between BabR and VjbR targets suggest a cross-talk between these two regulators. Our results also demonstrate that VjbR and BabR regulate many genes and/or proteins involved in stress response, metabolism, and virulence, including those potentially involved in the adaptation of <i>Brucella</i> to the oxidative, pH, and nutritional stresses encountered within the host. These findings highlight the involvement of QS as a major regulatory system in <i>Brucella</i> and lead us to suggest that this regulatory system could participate in the spatial and sequential adaptation of <i>Brucella</i> strains to the host environment

Localization of PBPs and PBP2b during the vegetative cell cycle of <i>L</i>. <i>lactis</i>.

<p>(A) Labelling of <i>L</i>. <i>lactis</i> PBPs by Bocillin-FL staining. Membranes of wild type (WT), <i>pbp1a</i>, <i>pbp1b</i>, <i>pbp2a</i>, <i>pbp2</i> and <i>dacA</i> mutant cells were purified, incubated with Bocillin-FL (+ Bocillin-FL), and separated on SDS polyacrylamide gel. Bocillin-FL-labeled PBP bands were revealed by fluorescence scanning. Dotted lines indicate auto-fluorescent bands detected in wild-type extracts prior to Bocillin-FL staining. Colored arrowheads mark the absence of PBP1b (1b, black), PBP2a (2a, light blue), PBP2b (2b, red), PBP1a (1a, yellow) and DacA (purple) in the mutant profiles, except for PBP2x whose deleted mutant is not viable. (B) Localization of PBPs with respect to FtsZ during the cell cycle. Cells expressing FtsZ-Ve (NZ3900 [pGIBLD031]) were stained with Bocillin™650/665 and visualized by phase contrast (PC) and epifluorescence (FtsZ-Ve and Bocillin) microscopy. Merge shows the superimposition of both fluorescent patterns. Scale bar, 2μm. <i>L</i>. <i>lactis</i> cell cycle was reconstituted from individual cells taking the beginning of cell constriction (as visualized by phase contrast) as the demarcation between elongation-only and combined elongation + division. PBP staining by Bocillin™650/665 is depicted in red on the cell cycle diagram shown below the pictures. Scale bar, 2μm. (C) Cells expressing the Venus-PBP2b fusion (Ve-PBP2b) (NZ3900 [pGIBLD041]) were visualized by phase contrast (PC) and epifluorescence microscopy. Scale bar, 2μm. A complete cell cycle was reconstituted from representative cells as reported in panel B. Ve-PBP2b fluorescence pattern is depicted in yellow on the cell cycle diagram shown below the pictures. The two bottom panels show fluorescence intensity maps (in arbitrary units, A.U.) from low (blue) to high (red) intensity). Cells (<i>n</i> = 20) were chosen before and after cell constriction based on phase contrast imaging and their normalized Ve-PBP2b fluorescent profiles were superimposed.</p