AgfC and AgfE facilitate extracellular thin aggregative fimbriae synthesis in Salmonella Enteritidis

摘要

Salmonella thin aggregative fimbriae (Tafi; curli) are important in pathogenesis and biofilm formation; however, less is known of their structure and morphogenesis. In the Salmonella agfBAC Tafi operon, the transcription and role of agfC have been elusive. In this study, agfBAC transcripts were detected using a sensitive reverse transcriptase technique. Native AgfC was not detected using polyclonal antibodies generated against purified hexahistidine-tagged AgfC; however, in trans expression revealed that AgfC was localized to the periplasm as a mature form. An isogenic ΔagfC mutant displayed an abundance of 20?nm fibres, in addition to native Tafi (5–7?nm), and had an increase in cell surface hydrophobicity. Purified 20?nm fibres were depolymerized under exceptionally stringent conditions to release what proved to be AgfA subunits. This revealed that the 20?nm fibres represented a different form of Tafi. The role of AgfC in Tafi assembly was investigated further using an antibody-capture assay of isogenic Δagf mutants. A soluble antibody-accessible form of AgfA was captured in wild-type (wt), ΔagfB and ΔagfF strains, in support of the extracellular nucleation–precipitation pathway of Tafi assembly, but not in ΔagfC or ΔagfE mutants. This indicates that AgfC and AgfE are important for AgfA extracellular assembly, facilitating the synthesis of Tafi.