Binding of avidin to bacteria and to the outer membrane porin of Escherichia coli

The binding of avidin to different bacteria was studied using tetramethylrhodamine isothiocyanate (TRITC)-conjugated avidin in fluorescence microscopy, enzyme immunoassay and [14C]biotin-avidin complex. We observed binding of avidin to all Gram-negative bacteria tested and to some Gram-positive bacteria. The binding was dose-dependent, reached the maximum in 10 min, was optimal at physiological pH and occurred at 4°C, 22°C, and 37°C. This binding of avidin was independent of the saturation of its biotin-binding sites. The avidin receptor of the cell envelope of Escherichia coli K-12 was shown to be the major porin protein (OmpF/OmpC) of the outer membrane.