Streptomyces: they're twisted!
I'm back from vacation, and trying to catch up. Perhaps this is a good moment for a brief, personal overview of Streptomyces biology, summarizing some important aspects.
Although they may look like molds, Streptomyces organisms are bacteria (eubacteria). There are essential differences at the cell and molecular levels between fungi (which are eukaryotes) and bacteria (which are prokaryotes). The similarities found between streptomycetes and fungi are the result of convergent evolution, adapting to similar environments as saprophytic soil microorganisms.
Streptomyces has a complex life cycle that includes formation of spores and other cell types. Typically, a spore germinates under the right conditions to generate a vegetative or substrate mycelium. This consists of a net of branching hyphae that grow and "dig" into the substrate to reach nutrients. Remarkably, there are few partition walls in the substrate mycelium: as a result, several copies of the genome are contained in every "cell". When nutrients are scarce (or in response to other signals), some hyphae start growing away from the substrate and into the air. In the new kind of hyphae (or aerial mycelium), partition walls are more frequently formed. At the same time, the substrate mycelium suffers a process of programmed cell death and its content is reused by the growing aerial mycelium. Finally, on the distal parts of aerial hyphae, the partition process is complete and yields beautiful chains of spores. Each spore contains a single copy of the genome.
Streptomyces and their close relatives became famous thanks to their ability to produce (among other stuff):
The biosynthesis of these nasty compounds is carefully co-regulated with the processes of cell differentiation, starting during the transition to aerial mycelium (on agar plates) or in late exponential phase (in liquid cultures).
However, "under standard laboratory conditions", the production of these metabolites is not essential for Streptomyces: mutants lacking the ability to produce the compounds are viable and not impaired in growth. This criterion distinguishes secondary metabolism ("reactions are not essential for viability") from primary metabolism ("reactions are essential"). That's why the mentioned compounds are called secondary metabolites.
But, if these funny bugs can live without secondary metabolites, why do they produce them? What's the use for a soil bacterium to produce an anticancer drug (for instance)? Are they spending valuable resources just to make something they don't need? Sure they're not. Of course, the microorganisms have not evolved "under standard laboratory conditions". But discussing about putative functions of secondary metabolites deserves a new post.
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Images:
(a) Several Streptomyces isolates growing on agar plates. (b) A close look at the colonies of Streptomyces coelicolor. Both images by Tobias Kieser, Celia Bruton and Jennifer Tenor, reproduced from Genome Biology 2002, 3:reviews1020.1-1020.4.
Life cycle of Streptomyces coelicolor, reprinted by permission from Macmillan Publishers Ltd:
Esther R. Angert. Alternatives to binary fission in bacteria. Nature Reviews Microbiology 3, 214-224 (copyright 2005).
