INTERACTION OF A GENETICALLY-MODIFIED PSEUDOMONAS-FLUORESCENS WITH THE SOIL-FEEDING EARTHWORM OCTOLASION CYANEUM (LUMBRICIDAE)

The geophagous earthworm Octolasion cyaneum was maintained in microcos ms for up to 14 d in the presence of a genetically-modified microorgan ism (GEMMO), Pseudomonas fluorescens, escens KTG. The GEMMO contained a marker cassette, which was inserted into the chromosome, consisting of the genes coding for kanamycin and gentamycin resistance and also a cryIVB sequence. Plate counts of P. fluorescens KTG were higher in th e burrow wall on day 2, and lower on days 7 and 14 than those in the u nworked bulk soil. Numbers of P. fluorescens KTG were consistently sig nificantly lower in cast material than in the unworked soil. Counts fo r total bacteria revealed no significant differences between bulk soil , burrow wail and casts. When earthworms were fed on soil containing P . fluorescens KTG, the population size of the GEMMO declined progressi vely on passage from the foregut to the hindgut, then increased slight ly in the casts relative to the hindgut. However counts in fresh casts were still significantly lower than the corresponding uningested soil . Populations of P. fluorescens KTG in casts increased by up to approx imately 10-fold over the first 2 d of the ageing period. Thereafter, p late counts of the GEMMO were slightly less than the corresponding soi l kept under the same conditions, showing a similar rate of decline ov er the 50-d period. Total bacterial plate counts in the aged casts inc reased by approximately 25-fold during the first 2 d of incubation, su bsequently declining whilst remaining significantly higher than the to tal bacterial plate counts in the corresponding soil which remained re latively constant throughout the experiment. Following a single exposu re of the earthworms to the GEMMO, counts of the modified bacterium we re detected in casts for upto 15 d. The interactions between bacteria and earthworms are discussed in relation to the potential for dispersa l of GEMMOs by soil invertebrates.