Microorganisms (i.e. bacteria, yeasts and fungi) are organisms that are normally invisible to the naked eye. However, when placed at excessive growth conditions their effects become noticeable for instance in acidified milk and molded bread. In this way we may encounter micro-organisms on a daily basis. Less often we acknowledge the beneficial micro-organisms of which we use their products or effects. Our daily bread or fermentation products as wine require the function of the yeast Saccharomyces cerevisiae, also termed bakers yeast. The table mushroom. component of various dishes is actually the fruiting body of a fungus (Agaricus bisporus). Another important microorganism for mankind is the filamentous fungus Penicillium chrysogenum. This fungus is able to produce a compound that inhibits growth of certain bacteria. Such compounds are called antibiotics (anti (against) and bios (life)). This antibacterial activity was first discovered in 1928 by Sir Arthur Fleming, who termed this compound penicillin. During the invasion of Normandy for the first time in history wounded soldiers were effectively treated with penicillin to prevent and cure infections of (gramnegative) bacteria. In the following years the demand for penicillin increased dramatically. This triggered a large body of research devoted to understand the molecular principles of the production of penicillin by P. chrysogenum. The major building blocks of the D-lactam penicillin are three amino acids, valine, cysteine and aminoadipate. Of these, valine and cysteine are synthesized by the fungus whereas amino adipate is a limiting compound during industrial fermentations and hence is supplemented to the cultivation media.