Changes in developing plant microbial community structure as affected by contaminated water.

The effects of sand and clay soils and water contaminated by Escherichia coli O157:H7 on the development of rhizosphere and phyllosphere microbial communities were analyzed to determine the influence of plant age on microbial community structure and composition. Community bacterial nucleic acids were extracted from lettuce rhizosphere and phyllosphere samples at different stages of plant development after the soils were irrigated with water contaminated with E. coli O157:H7 at planting and 15 days after planting. PCR was used to amplify 16S ribosomal RNA (rRNA) for total bacterial community composition and the products were subjected to denaturing gradient gel electrophoresis (DGGE). Prominent DGGE bands were excised and sequenced to gain insight into the identities of predominant bacterial populations. The majority of DGGE band sequences were related to bacterial genera previously associated with the rhizosphere and phyllosphere, such as Pseudomonas, Acidobacterium, Bacillus and Agrobacterium. The PCR-DGGE patterns observed for rhizosphere samples were more complex than those obtained from the bulk soil and the phyllosphere. The Shannon index of diversity (H) was used to determine the complexity of the DGGE bands from the phyllosphere, rhizosphere and the bulk soils at different growth stages. A higher diversity was observed in the clay soil than sandy soil during the first week. Few changes in diversity were observed after the first week. The results show that microbial community development in lettuce may take about 7-12 days and this may be the most likely period for maximum pathogen contamination in plants.