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UBC Theses and Dissertations

Validation of a microcosm designed for pre-release risk assessment of soil microorganisms using plant growth promoting rhizobacteria Nairn, James D.


Soil microcosms may prove to be a useful tool in the development of standardized methods for evaluating survival and persistence of soil organisms, particularly genetically modified microorganisms, before field release. In this study, a relatively simple soil microcosm, previously shown to be useful for predicting survival of a genetically modified pseudomonad in bulk soil and the rhizosphere of wheat, was tested using two plant growth promoting rhizobacteria (PGPR) strains which were naturally resistant to antibiotics, and spruce seedlings as the test plant. Bacillus polymyxa Pw-2R and Pseudomonas chloroaphis Sw5-RN were each inoculated onto bare soil and into the rhizosphere of spruce seedlings in field plots as well as in intact soil core microcosms that were incubated under controlled environmental conditions. Survival data collected over a two-year period were used to generate polynomial regressions that modeled the persistence of these strains in the field as well as in the soil microcosms. Comparison of the slopes and intercepts of these regressions indicated that the intact soil core microcosm closely predicted the survival of both PGPR strains in bulk soil and in the spruce rhizosphere. These results demonstrate that this small and inexpensive intact soil core microcosm may be appropriate for general use in assessing field persistence of diverse soil microorganisms before environmental release. During this study, a temporary loss of antibiotic resistance was observed in both PGPR strains, as they failed to grow on primary isolation agar media with antibiotics. However, they thrived on agar media with antibiotics if they were first isolated on agar without antibiotics. These results suggest that when using antibiotic resistance as a method to monitor rhizosphere microorganisms, the apparent masking of antibiotic resistance should be evaluated thoroughly. Bacillus polymyxa Pw-2R and Pseudomonas chloroaphis Sw5-RN are both plant growth promoting rhizobacteria. Pw-2R has previously been shown to be capable of colonizing internal root and shoot tissues of hybrid spruce. There were no endophytic Pw-2R detected when attempts were made to isolate Pw-2R from internal tissue in this study. Results further indicated spruce seedling growth was not significantly enhanced by the inoculation with the PGPR strains. These results are consistent with the theory that positive results from PGPR seem to be linked with harsh growing sites and interaction with indigenous microorganisms from the primary site of PGPR isolation.

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