Abstract
Burkholderia cenocepacia causes chronic and life-threatening respiratory infections in immunocompromized people. The B. cenocepacia N-acyl-homoserine lactone (AHL)-dependent quorum sensing system relies on the production of AHLs by the synthases CepI and CciI while CepR, CciR and CepR2 control expression of many genes important for pathogenesis. Downstream from, and co-transcribed with cepI, lies BCAM1871 encoding a hypothetical protein that was uncharacterized prior to this study. Orthologs of B. cenocepacia BCAM1871 are uniquely found in Burkholderia spp and are conserved in their genomic locations in pathogenic Burkholderia. We observed significant effects on AHL activity upon mutation or overexpression of BCAM1871, although these effects were more subtle than those observed for CepI indicating BCAM1871 acts as an enhancer of AHL activity. Transcription of cepI, cepR and cciIR was significantly reduced in the BCAM1871 mutant. Swimming and swarming motilities as well as transcription of fliC, encoding flagellin, were significantly reduced in the BCAM1871 mutant. Protease activity and transcription of zmpA and zmpB, encoding extracellular zinc metalloproteases, were undetectable in the BCAM1871 mutant indicating a more significant effect of mutating BCAM1871 than cepI. Exogenous addition of OHL restored cepI, cepR and fliC transcription but had no effect on motility, protease activity or zmpA or zmpB transcription suggesting AHL-independent effects. The BCAM1871 mutant exhibited significantly reduced virulence in rat chronic respiratory and nematode infection models. Gene expression and phenotypic assays as well as vertebrate and invertebrate infection models showed that BCAM1871 significantly contributes to pathogenesis in B. cenocepacia.
Highlights
Diverse pathogenic bacteria often utilize cell-cell communication systems to communicate with neighboring cells
We found aidA transcription was significantly decreased in the BCAM1871 mutant compared to wild type and its expression was significantly influenced by 300 rM exogenous octanoyl-Lhomoserine lactone (OHL) in the medium (Fig. 8B)
Our data suggests BCAM1871 is an integral component of the B. cenocepacia QS network and its inactivation disrupts optimal timing and magnitude of cell-density dependent gene expression and virulence factor production, resulting in significantly reduced pathogenesis in rat and nematode infection models (Fig. 9)
Summary
Diverse pathogenic bacteria often utilize cell-cell communication (quorum sensing, QS) systems to communicate with neighboring cells. These QS systems enable organisms to produce and perceive chemical signals resulting in altered gene expression and phenotypes contributing to pathogenesis. In Gram-negative bacteria, the most widely-studied QS system involves production of N-acyl-homoserine lactones (AHLs) by synthases of the LuxI protein family During bacterial growth these AHLs are released from bacterial cells into the surrounding medium. There are LuxR homologs that are not genetically linked to an AHL synthase gene and these are called orphan or solo LuxR homologs [3]
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