Escherichia coli strains with nonimmune immunoglobulin-binding activity

Area of Science:

• Microbiology
• Immunology
• Bacterial Pathogenesis

Background:

• Escherichia coli (E. coli) is a common bacterium with diverse strains.
• Immunoglobulin-binding proteins on bacterial surfaces can play roles in host-pathogen interactions.

Purpose of the Study:

• To identify and characterize strains of E. coli possessing cell surface immunoglobulin-binding activity.
• To investigate the nature and properties of these immunoglobulin-binding factors.

Main Methods:

• Screening of the ECOR reference collection of E. coli for immunoglobulin binding.
• Utilizing affinity-purified antibodies and immunoglobulin G (IgG) Fc fragments for binding assays.
• Fluorescence microscopy to visualize binding on intact cells.
• Western blotting to analyze the molecular weight of binding proteins.
• Enzymatic digestion (proteinase K, trypsin) to assess protein sensitivity and cell surface localization.

Main Results:

• Six out of 72 E. coli strains displayed significant immunoglobulin-binding activity.
• Binding occurred with both affinity-purified antibodies and Fc fragments of human and sheep IgG, indicating a non-immune mechanism.
• Fluorescence microscopy confirmed IgG Fc binding to the surface of unfixed cells.
• Western blots revealed multiple immunoglobulin-binding protein bands, predominantly >100 kDa, with strain-specific patterns.
• Binding activity was sensitive to proteinase K and trypsin, confirming its proteinaceous nature and cell surface exposure.
• Optimal expression of binding activity occurred at 37°C in stationary phase cultures.

Conclusions:

• Specific E. coli strains possess cell surface proteins with non-immune immunoglobulin-binding capabilities.
• These bacterial immunoglobulin-binding proteins are likely surface-exposed and proteinaceous.
• The observed strain-specific patterns suggest diverse molecular mechanisms for immunoglobulin binding in E. coli.