Capsulation of in vitro and in vivo grown Bacteroides species.

By centrifugation on a four step Percoll density gradient cells of Bacteroides species could be separated according to the size of extracellular structure. The difference in size was visible by both light and electron microscopy. Two structures were observed on Bacteroides fragilis by electron microscopy, namely a fibrous network and an electron dense layer. An electron dense layer was visible on Bacteroides ovatus only when stained with ruthenium red. B. fragilis cells grown in the mouse peritoneal cavity did not produce a large fibrous network. An electron dense layer was observed on some cells in the presence of ruthenium red stain and cells possessing this layer were phagocytosed in vivo.