The self-associating autotransporters (SAATs) are a group of glycosylated proteins from widespread E. coli pathogens that are transported to the cell surface by the type V secretion pathway. These virulence factors have central roles in bacterial aggregation and biofilm formation, which are important attributes for colonisation and persistence, which is extremely relevant to the areas of bacterial pathogenesis, nonsocomial infections and food sanitation. We recently elucidated the mechanism by which the SAAT Antigen43 from uropathogenic E. coli (UPEC) promotes bacterial aggregation/biofilm formation, by means of self-association between neighbouring cells1.
We sought to determine if all SAATs shared a common mechanism for facilitating bacterial aggregation/biofilm formation, if this function was regulated and if it could be inhibited.
The SAAT TibA from the highly virulent enterotoxigenic E. coli (ETEC) was known to be glycosylated by the TibC glycosyltransferase. We determined the crystal structures of the glycosylated and unglycosylated forms of TibA and used this to inform further biophysical and phenotypic studies. We found that TibA self-associates similarly to that of Antigen43, but with a more extensive interface, to facilitate bacterial aggregation/biofilm formation. Glycosylation by TibC was found to physically block TibA self-association to reduce bacterial aggregation/biofilm formation. We are now developing specific inhibitors for SAAT mediated aggregation/biofilm formation and have determined the first Antigen43-inhibitor crystal structure.
In summary, we have used our 3 new autotransporter crystal structures to inform the mechanism of SAAT mediated bacterial aggregation/biofilm formation, to elucidate how glycosylation regulates SAAT function and to develop a strategy to block these virulence processes.