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

¹ Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia V8W 3P6, Canada
² Department of Microbiology and Infectious Diseases, University of Calgary, Calgary, Alberta T2N 4N1, Canada

Correspondence
W. W. Kay
wkay{at}uvic.ca

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 agfC 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 agf mutants. A soluble antibody-accessible form of AgfA was captured in wild-type (wt), agfB and agfF strains, in support of the extracellular nucleation–precipitation pathway of Tafi assembly, but not in agfC or agfE mutants. This indicates that AgfC and AgfE are important for AgfA extracellular assembly, facilitating the synthesis of Tafi.