Like other actinomycetes, the Streptomyces group of microbes straddles the somewhat artificial boundary between bacteria and fungi. Growth on culture plates looks ‘fungal’ and the organism produces spores and germtubes that look like mycelia. Nonetheless these soil organisms are bacteria and of great interest to microbiologists as the producers of many antibiotics in use today.
Starting with streptomycin in the 40ies, the list of antimicrobials produced by Streptomyces species is long; it includes aminoglycosides, tetracyclines, chloramphenicol, fosfomycin, and even daptomycin. Ivermectin, an antiparasitic agent of incredible importance for the treatment of onchocerciasis (river blindness), is another product of the Streptomyces family. Merck has developed ivermectin for Third World countries without much prospect of making a profit, a truly remarkable contribution to mankind only matched by the similarly generous work done by Novartis with their antimalarial, Coartem (artemisinin/lumefantrine). And the polyene group of antifungals (nystatin, amphotericin B) are derived from Streptomyces as well.
Sir David Alan Hopwood has written a book that describes his life’s work with the organism, from the pre-genomic era until today [1]. He has chosen to study a bacterium that turned out to be more multifunctional than most, with a very large genome and many more mechanisms of survival built into its DNA than, say, E. coli or M. tuberculosis possess. As an example, take the number of ABC transporters, of which S. coelicolor has 140, while E.coli has 80 and MTB a mere 32.
Its unique ability to produce antibiotics may just be a clever mode of scavenging for nutrients.
Hopwood’s book details why and how Streptomyces turns on its antibiotic machinery at certain times of its lifecycle to support growth requirements. He discovered how antibiotic synthetic genes come in clusters on the genome, and the benefits of having embedded their own resistance gene, a fascinating discovery in the long path of dissecting this microbe’s physiology. How many more antimicrobial substances this organism is producing is hard to say, there are still a few that have not yet been fully exploited.
Not to forget, Steptomyces sp. are also producers of anticancer agents of which bleomycin is probably the best known example, of products used in crop protection and for the prevention of infection in fisheries. The beta-lactamase inhibitor clavulanic acid is yet another Streptomyces product.
This is a book I enjoyed perusing; hence I wanted to share it here for those who do not only read articles, or worse: just abstracts. Much can be learned from those who stick with a certain research topic for a long, long time. Actinorhodin, the attractive blue color produced by S. coelicolor got Hopwood started in the field, so here a few color pictures of the ‘celestial blue’ producer.
While you are at it, have a look at the article referenced below in which Sir Hogwood explains how and why these bacterial ‘weapons-producers’ survive their own killing machinery [2]. Have fun reading…
References
[1] D. Hopwood: Streptomyces in Nature and Medicine: The Antibiotic Makers. Cary, NC, USA: Oxford University Press, 2007. Oxford University Press
[2] D. Hogwood: How do antibiotic-producing bacteria ensure their self-resistance before antibiotic biosynthesis incapacitates them? Molecular Microbiol (2007) 63, 937
