CHANGES IN DEVELOPING PLANT MICROBIAL COMMUNITY STRUCTURE AS AFFECTED BY CONTAMINATED IRRIGATION WATER

Authors: Ibekwe, Abasiofiok - Mark; Grieve, Catherine

Submitted to: Journal of Microbiological Methods
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2004
Publication Date: 3/1/2004
Citation: Ibekwe, A.M., Grieve, C.M. 2004. Changes in developing plant microbial community structure as affected by contaminated irrigation water. Journal of Microbiological Methods. 48:239-248.

Interpretive Summary: There is an increasing concern about on-farm contamination of fresh produce by pathogens from contaminated water or manure used for fertilization. During environmental contamination of fresh produce, the pathogenic strain will compete with other bacteria in the environment for nutrients and this will determine their rate of survival in that environment. Little information is available on the interaction of Escherichia coli O157:H7 with total microbial community of fresh produce grown in different soils that may provide different nutrient levels for bacterial survival. This study showed that with increase in different bacterial groups with plant age, there was a corresponding decrease in the concentration of pathogens. We documented clearly the increase in microbial diversity with plant age during the experimental period. The microbial community was shown to be stable after 9 days from the beginning of the experiment and remained unchanged throughout the rest of the study. This showed that most plant tissues harbor few bacteria shortly after emergence from seeds, but population can increase rapidly thereafter. Therefore, irrigation of food crops with contaminated water should be avoided especially during the early stages of growth.

Technical Abstract: The effects of two contrasting soils and contaminated irrigation water 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 Escherichia coli O157:H7 at planting and 15 days after planting. PCR was used to amplify 16S ribosomal DNA (rDNA) 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 to 12 days and this may be the most likely period for maximum pathogen contamination in plants.