Identification of antibacterial type 6 sectretion system effectors in pectobacterium carotovorum subsp. Brasiliense 1692

Abstract

Pectobacterium carotovorum subsp. brasiliense 1692 is an economically important pathogen as it causes soft rot and blackleg of numerous agricultural crops, especially potato, in South Africa and in other parts of the world. In a multicellular environment, microorganisms interact with each other and protein secretion systems mediate many of these interactions. Survival often requires competitive interactions against closely and distantly related species. Although initially identified as an antieukaryotic virulence factor, the T6SS has mainly been implicated in interbacterial competition. Studies have shown that the T6SS of Pectobacterium carotovorum ssp. brasiliense 1692 (Pcb1692) is significantly upregulated upon in planta infection; however, the precise role during infection remains unknown. In this study, the role of the T6SS in Pcb1692 was investigated. In planta potato leaf competition assays indicate that no observable competition was mediated by the T6SS in Pcb1692. As the majority of T6SSs are involved in bacterial competition, a bioinformatics approach was used to identify whether the lack of T6SS-mediated competition was due to an absence of antibacterial effectors within the genome. Upon discovery of a number of putative antibacterial effectors, these were cloned and ectopically expressed in E. coli. Results indicate that individual expression of effectors, except the AHH nuclease, does not contribute towards bacterial growth inhibition. Altogether, this study has provided insight into the antibacterial role of the T6SS of Pcb1692, and results indicate that although the T6SS is significantly upregulated during infection, bacterial competition is not its foremost role within potato leaves.