Ultrastructure adaptations of symbiotic bacteria associated with freshwater fish and their cestode parasites

The ultrastructure of symbiotic bacteria colonizing the mucous membrane of the gut of freshwater fish and the tegument surface of their cestodes was studied. A comparative analysis of published and original data made it possible to define the ultrastructure peculiarities promoting the adaptation of bacteria to the digestive transport surfaces of fish and their intestinal parasites. The studied peculiarities may be split into three groups: (1) the structure and composition of the bacterial cell surface, which allows the colonization of gut epithelia and cestode tegument; (2) the intracellular ultrastructure of the bacteria; and (3) the reorganization of cell types, i.e., the formation of the dormant cells and nanobacteria.