Although less obvious, in the microscopic world efficient chemical defense is as crucial as for macroorganisms. Microorganisms have to compete for living space and nutrition, they have to face attack by other organisms, and they have to cope with changing environmental conditions. Consequently, many microorgansisms have evolved a variety of defense strategies including mechanical defenses such as formation of biofilms or resistant spores as well as chemical defense by highly toxic compounds. Indeed microorganisms such as Streptomyces or Bacillus species are well known as producers of complex secondary metabolites, for example, polyketides. Often microbial natural products exhibit attractive pharmaceutical potential as antibiotics or anticancer drugs. But many of such drug candidates turn out be too toxic to make their way into medicinal use. This indicates the ecological role of some of the secondary metabolites as powerful defense compounds.
Compounds such as nonribosomal peptides are synthesized by an enormous enzymatic machinery requiring a huge metabolic investment. Clearly, in their natural environment, microorganisms make use of their secondary metabolites for their own benefit inter alia for defense. Due to the focus on the pharmaceutically relevant properties of microbial natural products, so far relatively little is known about the ecological function of the secondary metabolites for the producing microorganisms. This article tries to illustrate the role of some microbial secondary metabolites as a means of defense of the producing microorganisms or their symbiosis partners as well as to present strategies that help microorganisms survive in a hostile environment.
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