Lichtgedanken 03

Rubrik 18 As humans, we are changing habitats —even in a complete isolation, we are never left alone. Humans and trillions of microorganisms form a highly com- plex ecosystem. Carrying hundreds of various species of bacteria, fungi and other microbes around with us, we sha- re nutrition and protect each other. This means that it is not only our digestion which benefits from our intestine micro- biota. Increasingly, studies suggests that the intestine microbiota also participate in a variety of life processes, for instance from our immune response and circula- tory system to our mental state. One of the most prominent »inhab- itants« of the human intestine is the yeast Candida albicans . Despite the fact that every human being has an indivi- dual composition of microbiota, Candida albicans can be found in the majority of the population. Having a size of a few micrometres only, these unicellular or- ganisms usually live in balance with other microorganisms as commensal organisms not only in the intestine but also in the oral cavity or on the skin. »Under certain circumstances, however, this rather unspectacular fungus may become a truly life-endangering patho- gen,« says Prof. Ilse Jacobsen. As the Professor of Microbial Immunology at the Leibniz Institute for Natural Product Research and Infection Biology, Hans Knöll Institute (HKI), she and her team do research into the reasons for such a serious change. Usually, the yeast cells live isolated and have a round form. However, the fungus is also able to grow into long, multicellular filaments or hyphae. Using its »tentacles«, which may grow up to 100 micrometres, Candi- da albicans can penetrate the cells of the intestinal epithelium and bore holes into the barrier. »Consequently, it is not only the fungus itself that may reach the surrounding tissue but also other microorganisms as well. Both may infect internal organs and, under circumstances, spread to the blood circulation resulting in infec- tions throughout the body,« explains Ilse Jacobsen. In a worst-case scenario, patients suffer from sepsis—a systemic inflammatory response affecting the whole body. Approximately a quarter of the infected persons do not survive such an infection (see p. 20). What might lead to such a dramatic transformation? How is it possible that Candida albicans abandons the peaceful community of intestine population and turns against its host organism in such terrific manner? Paradoxically, one of the risk factors is the progress of the modern medicine. For systemic fungal infections affect already seriously ill pa- tients in particular. »Patients in intensi- ve care often receive antibiotic therapy,« points out Jacobsen. However, this does not only mean a targeted reduction of germs, it may also lead to a particular illness. The microbiologist further explains: »Taking antibiotics may seriously influ- ence the microbial life communities of the patient and disrupt their natural ba- lance.« Numerous studies prove that the changed constellation in the »ecosystem human« is particularly advantageous to Candida albicans and to a series of bacte- ria. They gain an advantage in terms of growth and force other organisms to the margins. »Dr Jekyll and Mr Hyde« in the intestine Prof. Dr Ilse Jacobsen and her team examine how harmless inhabitants may become dangerous intruders. The team researches into the pathogenesis of systemic fungal infections. Together with other microorga- nisms, facultative pathogenic intestine inhabitants, for example Candida albicans and Proteus mirabilis , assist us in suppressing other intestine pathogens and train our immune system. Nevertheless, they may also cau- se life-endangering infections, including sepsis. A decisive factor for the transformation of »Dr Jekyll« into »Mr Hyde« in the intestine microbiota is the interplay of microbes in the »ecosystem human«. BY UTE SCHÖNFELDER Prof. Dr Ilse Jacobsen is head of the research group Microbial Immunology at the Leibniz Institute for Natural Product Research and Infection Biology, Hans Knöll Institute (HKI) and Professor for Microbial Immunology at the University Jena. F E AT U R E