Nematode community structure as a sensitive indicator of microbial perturbations induced by a genetically modified Pseudomonas fluorescens strain

The effects of seed inoculation with the Pseudomonas fluorescens strains F1 13lacZY [a genetically marked biocontrol agent producing the anti-fungal ag ent 2,4-diacetylphloroglucinol (DAPG)] and F113G22 [a genetically modified (GM) derivative strain of F113lacZY incapable of producing DAPG] on associa ted nematode communities were investigated over 17 days of plant growth. Pl ant growth measurements and colony forming unit counts (CFU) derived from r hizosphere soil indicated only small and transient perturbations as a resul t of introductions of the GM bacteria. Total nematode numbers were increase d significantly in the rhizosphere of inoculated plants compared with the n on-inoculated control treatments. These increases were mainly due to increa ses in bacterial feeding nematodes. This indicates that inoculation with th e GM P fluorescens strains induced high bacterial growth rates in the rhizo sphere of plants inoculated with these strains. No indication of greater ro ot colonisation by fluorescent Pseudomonas spp. could be found using CFU co unts on Pseudomonas-selective media. Numbers of fungal feeding nematodes de creased initially, probably as a result of lack of intact hyphae in the soi l. However, inoculation with the two different GM P fluorescens strains res ulted in a rapid recovery of fungal feeding nematode populations, whereas i n the non-inoculated control populations of fungal feeding nematodes remain ed small. This result is surprising as one of the strains (F113lacZY) produ ces the anti-fungal agent DAPG and it would be expected that this agent wou ld result in a decrease in fungal activity.