Prokaryotic-eukaryotic cell junctions: Attachment of spirochetes and flagellated bacteria to primate large intestinal cells

Abstract

The attachment sites of two types of commensal gram-negative bacteria to surface epithelial cells in human rectum and monkey colon were studied by electron microscopy of thin sections and freeze-fracture replicas. Spirochetes and flagellates are known to coexist in the brush border region of primate colonic epithelial cells, but their mode of attachment has not been described in detail. Both elongated, spiral organisms adhere by one end to columnar cell apical surfaces, oriented parallel to host cell microvilli. The attached end of the flagellate scarcely indents the host cell membrane and does not adhere to adjacent microvilli. The attachment site involves a circumferential cuff of bacterial outer membrane which is separated from host cell membrane by a uniform 6-nm gap filled with fibrillar cell coat material. Peroxidase passed freely through this gap. In freeze-fracture replicas, the zone of adherence is marked by a loose band of heterogeneous particles on the inner leaflet of the outer bacterial membrane. The spirochete, in contrast, pushes a long sleeve of host cell membrane into the terminal web cytoplasm and makes close contact with the invaginated columnar cell membrane and with adjacent microvilli. Freeze-fracture reveals that at the point where the spirochete body passes into the columnar cell glycocalyx, a series of irregularly spaced circumferential particle arrays appears on the inner leaflet of the bacterial cell outer membrane. These arrays extend over the entire embedded portion of the organism, but are replaced at the embedded tip by a distinct, closely packed series of circular ridges and grooves. Columnar cell membranes show no intra or submembrane specialization at the attachment sites of either organism. Autoradiographs reveal that radiolabeled glucosamine and N-acetylmannosamine (but not sulfate or fucose) may be incorporated by the bacteria under organ culture conditions, and that maturation of the columnar cell glycocalyx is a crucial factor in bacterial attachment.