# Congo Red Interactions with Curli-Producing E. coli and Native Curli Amyloid Fibers

## TL;DR

The study explores how Congo red dye interacts with amyloid fibers produced by E. coli, offering insights into quantifying and comparing these fibers across different strains.

## Contribution

The novel contribution is the characterization of Congo red's binding properties with curli fibers and E. coli cells, including pH dependence and lack of histidine residue influence.

## Key findings

- Congo red binds to curliated E. coli cells without inhibiting growth.
- Congo red binding to curli is pH-dependent.
- Histidine residues in CsgA protein do not affect Congo red binding.

## Abstract

Microorganisms produce functional amyloids that can be examined and manipulated in vivo and in vitro. Escherichia coli assemble extracellular adhesive amyloid fibers termed curli that mediate adhesion and promote biofilm formation. We have characterized the dye binding properties of the hallmark amyloid dye, Congo red, with curliated E. coli and with isolated curli fibers. Congo red binds to curliated whole cells, does not inhibit growth, and can be used to comparatively quantify whole-cell curliation. Using Surface Plasmon Resonance, we measured the binding and dissociation kinetics of Congo red to curli. Furthermore, we determined that the binding of Congo red to curli is pH-dependent and that histidine residues in the CsgA protein do not influence Congo red binding. Our results on E. coli strain MC4100, the most commonly employed strain for studies of E. coli amyloid biogenesis, provide a starting point from which to compare the influence of Congo red binding in other E. coli strains and amyloid-producing organisms.