The Congo red derivative FSB binds to curli amyloid fibers and specifically stains curliated E. coli.

The Congo red derivative (E,E)-1-fluoro-2,5-bis(3-hydroxycarbonyl-4-hydroxy) styrylbenzene (FSB) specifically stains the functional amyloid curli in Escherichia coli biofilms. FSB binds to curli with similar affinity as Congo red, yet exhibits much greater fluorescence upon binding to curli as compared to Congo red and does not exhibit undesired binding to the cellulosic component of the biofilm. Thus, FSB presents a powerful tool to identify and visualize curli in E. coli biofilms and also enables new biophysical investigations of curli.