Light and electron microscopic study of the bacterial adhesion to termite flagellates applying lectin cytochemistry

Many of the flagellates inhabiting the hindgut of lower termites are associated with ectobiotic, rod-like bacteria or spirochetes. Different types of attachment sites are present. Electron dense material underlies, e.g., the plasma membrane of Joenia annectens at the contact site, whereas other attachment sites do not show any visible specializations. The host cell's glycocalyx may, however, be reduced at the attachment sites as it is the case in Devescovina glabra. The thick glycocalyx of Stephanonympha nelumbium is not changed at the sites where bacterial rods attach, but spirochetes penetrate to a certain extent. Bacteria which colonize the extracellular surface structures of Microrhopalodina multinucleata express their own glycocalyx to mediate a contact. In this study we focussed on the examination of one common mode of interaction between bacteria and their host cells, i.e., adhesion via lectins and sugars. The sugar composition was analysed by light and electron microscopic labelling experiments using the lectins Con A, WGA and SBA. In general, only the posterior body surface of Joenia which is colonized with bacteria is labelled. The demonstrated sugars are found in fibrous glycocalyx portions surrounding the attachment sites of the bacteria. Such glycocalyx fibres in combination with the electron dense material supporting the attachment sites seem to be the prerequisites for bacterial attachment. In D. glabra, however, a role for sugars in mediating the attachment could not be demonstrated. Removal of the ectobiotes using antibiotics revealed that the specialized contact sites of Joenia are present in the absence of bacteria and thus possibly serve to attract bacteria. Nothing, however, remains of the former attachment sites in bacteria-free Devescovina cells. Attachment sites in this case could be induced by bacterial contact. There is not one general mechanism for bacterial attachment to termite flagellates; rather, adhesion seems to follow different strategies.