SURVIVAL OF BACTERIA INTRODUCED INTO SOIL BY MEANS OF TRANSPORT BY LUMBRICUS-RUBELLUS

Four strains of bacteria, Rhizobium leguminosarum biovar trifolii, Pse udomonas fluorescens, Pseudomonas cepacia, and Flavobacterium sp., wer e introduced into loamy sand and then transported by earthworms of the species Lumbricus rubellus to uninoculated soil. Cell densities recov ered from the earthworm gut and casts (both expressed per gram dry mat erial) were significantly lower (up to 3 log units) than cell densitie s recovered from the inoculated soil. Total bacterial counts in casts were similar to those in the inoculated soil. In casts excreted into a sterile environment numbers of colony-forming units (CFU) increased, suggesting a favourable environment for growth. In casts excreted in a non-sterile environment, cell densities of introduced strains decreas ed. Casts therefore did not offer the introduced bacteria a protective microenvironment for survival in the bulk soil. Transport by worms of R. leguminosarum biovar trifolii and of P. fluorescens appeared to oc cur mostly by means of cast production; with the Flavobacterium sp. an d P. cepacia a large proportion of the cells was possibly transported on the skin of earthworms.