Application of cationic hydrous thorium dioxide colloids for tracing acidic groups of bacterial slime matrices of whole mount E. coli cells. (a) Bright-field image of mid-log E. coli cells which have been labelled with cationic 1.5 nm thorium dioxide colloids. Distinct irregular electron dense clusters of slime polymers are found associated with the cell surface (dashed arrow) or the cell periphery. Occasionally, slime is present as individual clusters in the background (triple arrows). Squared box indicates an area of high mass thickness, revealing low thorium intensities. (b) Detailled view of homogenous monodispersed particles of thorium dioxide; individual particles of 1.0 to 1.5 nm in diameter are circled, double arrows indicate oligoparticulate clusters. (c) High resolution elemental distribution map of thorium, corresponding to the bright-field image (a). (d) PEELS from an individual slime cluster, circled in (a) and (c). The boxed area in the spectrum record indicates the characteristic energy range of Th-O4,5, corresponding to the slit-width setting.