16S rDNA fingerprinting of rhizosphere bacterial communities associated with healthy and Phytophthora infected avocado roots

Molecular techniques employing 16S rDNA profiles generated by PCR-DGGE were used to detect changes in bacterial community structures of the rhizospher e of avocado trees during infection by Phytophthora cinnamomi and during re peated bioaugmentation with a disease suppressive fluorescent pseudomonad. When the 16S rDNA profiles were analyzed by multivariate analysis procedure s, distinct microbial communities were shown to occur on healthy and infect ed roots. Bacterial communities from healthy roots were represented by simp le DNA banding profiles, suggestive of colonization by a few predominant sp ecies, and were approximately 80% similar in structure. In contrast, roots that were infected with Phytophthora, but which did not yet show visible sy mptoms of disease, were colonized by much more variable bacterial communiti es that had significantly different community structures from those of heal thy roots. Root samples from trees receiving repeated applications of the d isease suppressive bacterium Pseudomonas fluorscens St. 513 were free of Ph ytophthora infection, and had bacterial community structures that were simi lar to those of nontreated healthy roots. Sequence analysis of clones gener ated from four predominant bands cut from the DGGE gels revealed the presen ce of pseudomonads, as well as several previously unidentified bacteria. Di fferentiation of 16S rDNA profiles for healthy and infected roots suggests that rhizosphere bacterial community structure may serve as an integrative indicator of changes in chemical and biological conditions in the plant rhi zosphere during the infection process. (C) 2001 Federarion of European Micr obiological Societies. Published by Elsevier Science B.V. All rights reserv ed.