Genetic Analysis of the CDI Pathway from Burkholderia pseudomallei 1026b

Contact-dependent growth inhibition (CDI) is a mode of inter-bacterial competition mediated by the CdiB/CdiA family of two-partner secretion systems. CdiA binds to receptors on susceptible target bacteria, then delivers a toxin domain derived from its C-terminus. Studies with Escherichia coli suggest the existence of multiple CDI growth-inhibition pathways, whereby different systems exploit distinct target-cell proteins to deliver and activate toxins. Here, we explore the CDI pathway in Burkholderia using the CDI(II) (Bp1026b) system encoded on chromosome II of Burkholderia pseudomallei 1026b as a model. We took a genetic approach and selected Burkholderia thailandensis E264 mutants that are resistant to growth inhibition by CDI(II) (Bp1026b). We identified mutations in three genes, BTH_I0359, BTH_II0599, and BTH_I0986, each of which confers resistance to CDI(II) (Bp1026b). BTH_I0359 encodes a small peptide of unknown function, whereas BTH_II0599 encodes a predicted inner membrane transport protein of the major facilitator superfamily. The inner membrane localization of BTH_II0599 suggests that it may facilitate translocation of CdiA-CT(II) (Bp1026b) toxin from the periplasm into the cytoplasm of target cells. BTH_I0986 encodes a putative transglycosylase involved in lipopolysaccharide (LPS) synthesis. ∆BTH_I0986 mutants have altered LPS structure and do not interact with CDI(+) inhibitor cells to the same extent as BTH_I0986(+) cells, suggesting that LPS could function as a receptor for CdiA(II) (Bp1026b). Although ∆BTH_I0359, ∆BTH_II0599, and ∆BTH_I0986 mutations confer resistance to CDI(II) (Bp1026b), they provide no protection against the CDI(E264) system deployed by B. thailandensis E264. Together, these findings demonstrate that CDI growth-inhibition pathways are distinct and can differ significantly even between closely related species.