Undertanding the mechanisms of regulation of conjugation is crucial for the development of strategies to prevent and limit the dissemination of antibiotic resistances and virulence determinants. The results of our studies on the regulation of conjugal transfer of plasmid pRK100 revealed the role of three important global regulators CRP, H-NS and Lrp. Our results also show that plasmids are important vehicles in the horizontal mobility of pathogenicity isalnds and thus contribute to the appearance of new pathogenic strains as well as new bacterial species. Elucidation of the connection between binding/transport of toxic substances and the simultaneous increased sensitivity of cells for detergents which we have shown for transporters such as Bcr and P-glycoprotein might represent a tool in the battle against cancers resistant against chemotherapuetics. Elucidation of the export mechanism of Bcr is important for future biochemical modifications of bacitracin and similar peptides to obtain more efficient antibiotics with less side effects. The variability of virulence factors is frequent in molecules recognized by the immune system (for example fimbria). Variability of toxin genes is extremely rare and Clostridium diffecile is the best described example at the molecular and at the biochemical level. Understanding variability is important for the development of methods to control and prevent infectious diseases. Because C. difficile is the most important causative agent of hospital acquired gastrointestinal infections in developed countries, the development of a vaccine is a priority. To accomplish this goal data on the prevalence of strain type are needed. In the past it was generally accepted that extreme environments are inhabited by nonculturable procaryotes. Fungi were not known to inhabit such extreme environments as extreme salt waters in salterns or the extremely cold Arctic environment. From the waters of the Sečovlje salterns, which served as a model environment, we were the first to describe the presence of extremophilic fungi. The fungi were isolated using various methods and subsequently we described their population dynamics. Most were identified using classical, biochemical and molecular methods. We determined that the active population consisted of Hortaea werneckii, Phaeotheca triangularis, Trimmatostroma salinum and the halotolerant Aureobasidium pullulans. Fungi which we initially discovered and described in the Sečovlje salterns were later also isolated in extremely salty saltern waters and salt lakes on three continents (Spain, Portugal, Israel, Namibia, Domincan republic, USA) and we thus demonstrated that in such extreme environments they are present on a global scale. Among the described fungal species ten were new while for some rare species the appropriate ecological niches were described. Subsequently, we focused on black halophilic yeast using Hortaea werneckii as a model for studies of adaptation at the physiological and molecular levels.