Functionality of autoinducer systems in complex environments.

Cell-to-cell signalling via small diffusible molecules, usually termed quorum sensing (QS), represents a common behaviour in bacteria. This signalling regulates life style switches in many, if not most symbiotic microbial species either beneficial or pathogenic for their eukaryotic hosts, but is also involved in controlling environmental processes such as biofouling, degradation processes in sewage plants or environmental pollutions and N cycling [1–4]. Biochemically, the core of a generic system comprises a cytoplasmatic signal synthase (or several involved enzymes), a small, diffusible signal which is released into the environment, and a signal receptor located in the cell membrane or in the cytoplasma. The signal-receptor complex directly or indirectly controls the expression of target genes (Fig. 5.1). The signal was termed autoinducer (AI), because the same cells produce and react on the signal molecules. For an overview of the various chemical realizations of AI systems see, e.g. Atkinson and Williams [5]. Originally, three main types of AI molecules have been described: (a) Mainly gram-negative proteobacteria, but also some cyanobacteria and archaebacteria employ molecules of the acylhomoserine lactone (AHL) group as AIs, (b) oligopeptide AIs occur in gram-positive bacteria, and (c) AI2 has been described as a universal signal for interspecies communication. Recently, a still increasing number of AIs belonging to various chemical classes have been discovered