Scanning electron microscopy of the gut microflora of two earthworms: Lumbricus terrestris and Octolasion cyaneum.

Scanning electron microscopy was used to investigate the presence of microorganisms, probably bacteria, on the gut surface of earthworms. The washed surfaces of the intestines of two earthworms, Lumbricus terrestris and Octolasion cyaneum, were examined. Numerous organisms resembling bacteria were observed throughout the gut, some in situations suggesting attachment. Compared with similar investigations in other invertebrates, there were fewer bacteria, showing less morphological diversity, on the earthworm gut surface. The majority of organisms viewed were coccoid, some were filamentous, and a few rod-shaped cells were observed. Cocci, often in chains, were seen in the foregut of both species. Although cocci were also numerous in the midgut region, particularly in the typhlosole, in O. cyaneum tufts of segmented, filamentous organisms were also seen with some segments resembling spores. Fewer organisms were found in the hindgut, but in L. terrestris there were segmented, filamentous organisms, attached to the epithelium by way of a "socket-like" structure, similar to that by which segmented, filamentous bacteria (SFBs) are attached to the ileum of rats and mice. Transmission electron microscopy of the hindgut of L. terrestris was undertaken to explore the structure and attachment of SFBs to the gut epithelium. However, although a few rod-shaped bacteria were observed, no SFBs were located. The observations reported here provide evidence that earthworms have an attached gut microflora of filamentous microorganisms which are probably indigenous, and as far as we are aware this is the first published report of such findings in these invertebrates.