Microbial Ecology - Biological Safety / Soil Ecology

For safety assessment of released genetically modified organisms (GMO) into the environment, the central issue is a putative transfer of the recombinant DNA (rDNA) into indigenous microorganisms (MO) of the respective habitat. Conjugation is considered to be the most efficient mechanism of gene transfer between bacteria. For the probability assessment of conjugal gene transfer subsequent to the release of GMO it is necessary to determine the rDNA mobilizing potential of a bacterial population. Hence, the aim of our project is to analyze the gene mobilizing potential of microorganisms in the nutrient-rich habitat activated sludge. To this end, conjugative plasmids of endogenous microorganisms are currently isolated and genetically characterized. Furthermore, gene transfer is being studied in the laboratory in model sewage plant.

The soil compartment that is directly influenced by plant roots, the rhizosphere, is a place of enhanced microbial activity. Microbial populations of the rhizosphere may have a neutral, negative (e.g. for pathogens), or positive (e.g. for associated or symbiotic nitrogen fixers) impact on plant growth. When transgenic plants are released for agricultural purposes, e.g. plants that are resistent to pathogenic microorganisms, the question has to be addressed concerning the influence of the transgene expression on plant growth promoting microbial populations in the important habitat "rhizosphere". The aim of our project is the competitive analysis of the microbial rhizosphere population of alfalfa and rape seed and their genetically modified derivatives expressing chitinase, lysozyme and defensins. Probable differences of the diversity of microorganisms will be studied with molecular typing methods such as ARDRA (amplified ribosomal DNA restriction analysis).

Bacterial insertion elements (IS elements) are mobile genetic elements that are able to change their position within the genome of their host organism. If IS elements transpose onto a conjugative plasmid they can be transfered together with this plasmid to a new host organism. The rhizosphere of a plant is a favourable place for genetic interactions between rhizosphere bacteria. The aim of this project is to analyze the distribution of selected Sinorhizobium meliloti IS elements in bacterial rhizosphere populations and hence to gain information on the potential gene flow in this habitat.

Soil bacteria in there natural habitat have to stand mainly famine conditions. Short periods of nutrient excess are followed by longer periods of nutrient shortage. Non-sporulating bacteria can manage such changing conditions by entering the stationary phase. To this end, a genetic program is started that enables the cells to survive these harsh conditions. In particular for soil bacterial of the genus Rhizobium the underlying mechanisms for famine adaptation are more or less unknown. We therefore aim to identify and characterize genes important for the survival of S. meliloti under stationary phase conditions.