Impact of Pseudomonas fluorescens strain CHA0 and a derivative with improved biocontrol activity on the culturable resident bacterial community on cucumber roots

Information on the effects of released wild-type or genetically engineered bacteria on resident bacterial communities is important to assess the poten tial risks associated with the introduction of these organisms into agroeco systems. The rifampicin-resistant biocontrol strain Pseudomonas fluorescens CHA0-Rif its derivative CHA0-Rif/pME3424, which has improved biocontrol ac tivity and enhanced production of the antibiotics 2,4-diacetylphloroglucino l (Phl) and pyoluteorin (Plt), were introduced into soil microcosms and the culturable bacterial community developing on cucumber roots was investigat ed 10 and 52 days later. The introduction of either of the two strains led to a transiently enhanced metabolic activity of the bacterial community on glucose dimers and polymers as measured with BIOLOG GN plates, but natural succession between the two sampling dates changed the metabolic activity of the bacterial community more than did the inoculants. The introduced strai ns did not significantly affect the abundance of dominant genotypic groups of culturable bacteria discriminated by restriction analysis of amplified 1 6S rDNA of 2500 individual isolates. About 30-50% of the resident bacteria were very sensitive to Phl and Pit, but neither the wild-type nor CHA0-Rif/ pME3424 changed the proportion of sensitive and resistant bacteria in situ. In microcosms with a synthetic bacterial community, both biocontrol strain s reduced the population of a strain of Pseudomonas but did not affect the abundance of four other bacterial strains including two highly antibiotic-s ensitive isolates. We conclude that detectable perturbations in the metabol ic activity of the resident bacterial community caused by the biocontrol st rain CHA0-Rif are (i) transient, (ii) similar for the genetically improved derivative CHAO-Rif/pME3424 and (iii) less pronounced than changes in the c ommunity structure during plant growth. (C) 1998 Federation of European Mic robiological Societies. Published by Elsevier Science B.V. All rights reser ved.