The natural functions of secondary metabolites

Secondary metabolites, including antibiotics, are produced in nature and serve survival functions for the organisms producing them. The antibiotics are a heterogeneous group, the functions of some being related to and others being unrelated to their antimicrobial activities. Secondary metabolites serve: (i) as competitive weapons used against other bacteria, fungi, amoebae, plants, insects, and large animals; (ii) as metal transporting agents; (iii) as agents of symbiosis between microbes and plants, nematodes, insects, and higher animals; (iv) as sexual hormones; and (v) as differentiation effectors. Although antibiotics are not obligatory for sporulation, some secondary metabolites (including antibiotics) stimulate spore formation and inhibit or stimulate germination. Formation of secondary metabolites and spores are regulated by similar factors. This similarity could insure secondary metabolite production during sporulation. Thus the secondary metabolite can: (i) slow down germination of spores until a less competitive environment and more favorable conditions for growth exist; (ii) protect the dormant or initiated spore from consumption by amoebae; or (iii) cleanse the immediate environment of competing microorganisms during germination.

The nonribosomal peptide biosynthetic system--on the origins of structural diversity of peptides, cyclopeptides and related compounds

A variety of peptides have been detected in microorganisms. Some have found applications in various fields, for example the classical beta-lactam antibiotics, immunosuppressors like cyclosporin, promising new antibacterials like teichoplanin or daptomycin and antifungals like echinocandin. For none of these has it been established how their complicated biosynthetic pathways have evolved or what functions they fulfill within or for their producers. So it is unclear what selection processes limit the range of their structural analogues within various groups of microorganisms. We here consider recent data in the field of biosynthesis and how they may suggest mechanisms of genetic diversity. These may illustrate the complexity of genetic and intracellular organization of biosynthetic pathways and indicate the cellular context of some metabolites related to the complex background of the production of each metabolite. Research focusing on various targets like the increase of productivity of fermentations or the spread of resistances to antibacterials is slowly being understood.

[Secondary metabolites as endogenous effectors of microbial cytodifferentiation]

The present survey covers the regulatory role of microbial secondary metabolites and related compounds as endogenous signals of cell differentiation of the producing organisms. Several antibiotics have been shown to exert autoregulatory effects on differentiation-associated functions. The mechanisms of self-protection of the producing cells against the autotoxicity of secondary metabolites are discussed in terms of an integral part of the modulation of signal strength. As a further topic, the review deals with the hormone-like interference of particular metabolites with differentiating cells. Conclusive discussion concerns the potential use of microbial signal molecules either as tools for directed manipulations of product syntheses or as pharmaceutics.