Microorganisms produce a large variety of secondary metabolites such as polyketides (erythromycin A 8), non-ribosomal peptides (vancomycin 9), or amino sugars (neomycin B 10) (Figure 2). Many of these compounds are very powerful toxins for other microorganisms. This is why secondary metabolites of microorganisms are an invaluable source for bioactive compounds such as antibiotics.
Most microbial toxins either target the bacterial cell wall biosynthesis or are inhibitors of the protein biosynthesis, some secondary metabolites block DNA- or RNA-polymerases, while others interfere with key steps in the primary metabolism of foreign microorganisms such as the folate biosynthesis.
The production of microbial toxins is very often coordinated by quorum-sensing signals. Only upon reaching a certain cell density, antibiotics are produced. Possibly, it is advantageous for single cells to avoid earlier recognition by competitors which might in turn induce their defense reactions if they encounter aggressive behavior of neighboring microorganisms.
A general problem for the investigation of the ecological role of microbial secondary metabolites is that antibiotics such as vancomycin 9 or neomycin B 10 are highly active in very low doses. Therefore, the amount that it is needed to compete with other microorganisms is
HO OH OH
O O OMe
Erythromycin A 8
2N O OH
Neomycin B 10
HO NH3 HOoh H3C O CH2OH
HO NH3 HOoh H3C O CH2OH
Avermectin B1b 12
Figure 2 Secondary metabolites (antibiotics) from microorganisms.
too low to be detected easily in complex natural samples such as soil.
However, for example, cocultivation experiments of Streptomyces coelicolor and Bacillus subtilis clearly indicate the interactions of these two microorganisms at the interface. Streptomyces coelicolor does not form aerial structures at the frontier to B. subtilis. This response is induced by the bio-surfactant surfactin 11 produced by B. subtilis. Since surfactin-non-producing mutants did not provoke the different growth behavior at the border zone whereas surfactin alone did, this experiment provides evidence that microorganisms react to foreign microorganisms by adapting their metabolism. Biosurfactants are produced by many microorganisms and may play a role, not well studied so far, in the interactions of different microbial communities.
Another interesting class of secondary metabolites is avermectins such as avermectin B1b 12, which show strong activity against nematodes. Since avermectins are produced by the soil bacterium Streptomyces avermitilis, it might be speculated that avermectins serve the microorganism to defend its living space against feeding nematodes. Thus microoganisms seem not only to direct their chemical defense arsenal against other microorganisms but also against higher organisms.
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