Microorganisms are unicellular organisms but they coordinate their behavior using small organic signal molecules or peptides. They sense their cell density (quorum sensing) by receptor-mediated measurement of the concentration of signal molecules. Using cell-cell communication systems, microorganisms coordinate their morphological and chemical differentiation. Upon reaching a certain cell density, the expression of virulence and pathogenesis factors is induced. Thus their chemical signaling allows microorganisms to behave in many aspects like a multicellular organism or like social insects such as ants. Examples for microbial signaling compounds are W-acylhomoserine lactones, for example, 1 (Gramnegative bacteria), 7-butyrolactones such as 2 and 3 (Streptomyces), 3-hydroxypalmitic acid methyl ester (4, Ralstonia solanacearum), furanosyl borate (5, AI-2 auto-inducer 2), diketopiperazines such as 6, 2-acyl-3-hydroxy-4-quinolones, for example, 7 (Figure 1), or peptides (Gram-positve bacteria). W-Acylhomoserine lactones are widespread among Gram-negative bacteria. Although many Gram-negative microorganisms use W-acylhomoserine lactones with the same core structure, variation of the side-chain length and its substitution pattern guarantees the necessary species specificity. Instead signals such as AI-2 5 seem to serve interspecies communication.
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