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Introduction: Soil micro-organisms have evolved functions that allow them to withstand the strong competition for survival that characterizes most of their habitats. The production of antibiotic or antifungal compounds is one of these mechanisms. The relevant molecules often exhibit valuable therapeutic properties., Exegesis: Chromobacterium violaceum is a soil-borne bacterium producing a characteristic antibiotic termed violacein. It is part of a series of compounds released by C. violaceum to oppose competitors and predators in the soil and water environments. Violacein, and one of these compounds, i.e. structure FR901228, exhibit antiparasitic and antitumoral activities of potential medical interest. Genes involved in the synthesis of these compounds are available, the genome sequence of C. violaceum (strain ATCC 12472) being published., Conclusions: The above example, involving Chromobacterium, is not an exception: soil constitutes a reservoir of molecules, enzymatic activities and micro-organisms of biological interest, the study of which will undoubtedly lead to developments in fields as diverse as agronomy or animal and human therapeutics.

Many drugs are derived from active molecules isolated from plants or micro-organisms. Nevertheless, a lot of species have not yet been studied; they constitute an important source of new structures that could possess interesting pharmacological activities and/or original modes of action, for example in the field of anticancer or antiparasitic drugs (taxol, artemisinin, swainsonine, camptothecin,...). Among all the bioactive natural products, we studied more particularly indole alkaloids extracted from different tropical Strychnos species. Several new alkaloids have been isolated from cytotoxic crude extracts. Some of them possess interesting cytotoxic, antitumor or antiparasitic properties. We also realised molecular modelling studies in order to explain the activity of usambarensine.

The analyses of the different possible conformations of usambarensine, tubulosine and emetine, which possess interesting cytotoxic and amoebicide properties showed us that they cannot take the planar conformation proposed to explain the activity of this type of molecules on protein synthesis. Nevertheless, a common (non planar) conformation exists where the angles between the two plans containing the aromatic rings are nearly the same for the three compounds. This fact could confirm the hypothesis of a common receptor which would be slightly different in amoebae because of its higher sensitivity to usambarensine.