The importance of bacteria in the feeding of marine bivalves has been demonstrated by several authors. Some examples of good growth of molluscs were noted on a bacterial diet only. But, while the intense filtration of bacteria by molluscs has been observed, the exact role of bacteria in the nutrition of bivalves was not completely known. The filtered particles were sometimes eliminated as pseudofaeces, without any intestinal transit. On the other hand, live cells could also pass through the gut without being degested. To study in detail the fate of bacterial cells distributed as food to young bivalves, we used a new method which combines histology and scanning electron microscopy. This made it possible to observe, on serial histological sections of whole animals, the gut content and the condition of the ingested cells at the different levels of the intestinal tract. The ingestion and digestion by young mussels (Mytilus edulis) of some marine bacterial strains belonging to different taxonomic groups were studied by this method. Thus, partially digested bacterial cells were observed in the stomach, when the hind gut contained undamaged cells, three hours after food distribution. The results obtained for all the strains we tested are presented and discussed in this paper
