Fate of soil bacteria and fungi in the gut of earthworms

The responses of soil microorganisms to the action of gut fluids of three earthworm species, Aporrectodea caliginosa, Lumbricus terrestris and Eisenia fetida were investigated. It was found that the midgut fluid taken form anterior part of the digestive tract could suppress formation of colonies of bacteria, inhibit the germination of spores and reduce the radial growth rate of some fungal colonies. Heating the midgut fluid at 98 degrees C for 10 min did not eliminate its suppressive activity. This suggests that a non-protein compound (s) is involved in this action of the midgut fluid. The suppressing effects were shown to be selective towards soil bacteria. However, the responses of microbial cells to the midgut fluid did not correspond to the taxonomic affiliation of the microorganisms tested. The mechanism of the suppressing effect might be a destruction of microbial cell membranes, as demonstrated for the gut of soil millipedes. The selective activity of the gut fluid of earthworms could be a significant factor for the animal's nutrition as well as for regulating the steady state of the intestinal microbial community, and modification of microbial communities in soil. (c) 2007 Elsevier Masson SAS. All rights reserved.