Use of tannic acid for the ultrastructural visualization of periplasm in gram-negative bacteria

Tannic acid mordanting reveals the periplasm, the area between the outer membrane and the inner membrane of gram-negative bacteria, Rhizobium sp., Escherichia coli and Enterobacter aerogenes, as an electron-dense layer continuous with the inner leaflet of the outer membrane. The method involves 18 hr of tannic acid treatment after fixation in aldehyde prior to osmium tetroxide postfixation, followed by conventional electron microscopy.