The ability of pathogenic bacteria to infect lichens is a poorly studied phenomenon. The present thesis focused on this topic by analysing to what extent established biofilm like communities on lichens are affected by by introducing allochthonous bacteria on the lichens. PCR techniques were applied in the experiments to amplify partial rRNA genes of the bacteria, including single strand conformation polymorphism (SSCP) fingerprinting. Fluorescence in situ hybridization (FISH) and observations with confocal laser scanning microscopy (CLSM) were used to visualize the colonization of the sprayed bacteria. Experiments were primarily conducted with the large foliose lichen Peltigera membranacea. Natural samples were transferred into a glass box that was prepared to simulate the natural habitat. The Stenotrophomonas maltophilia strain was sprayed on the lichen thalli at different concentrations and samples were taken at different time points within four weeks for the detection of the sprayed bacteria. For comparison other samples of these lichens were also maintained in a closed Petri dish under lab conditions. In samples exposed to simulated habitat conditions S. maltophilia could neither be detected by FISH nor SSCP, but the S. maltophilia strain could be detected on Peltigera membranacea in those samples that were kept under constant conditions in closed Petri dishes under lab conditions. The ecosystem function pathogen defense of lichen appears to be stable under natural conditions and does not allow the establishment of non-specific strains but in a different habitat without the natural influences the bacterial strain was able to maintain on the surface and the bacterial community got disturbed. Yet, disturbance of genuine natural habitat conditions leads to possible infestation by S.maltophilia and possibly other bacteria.