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Streptomyces – the smell of life

The Mall in Central Park, New York City in late autumn on rainy day
The Mall in Central Park, New York City in late autumn on a rainy day

 

Did you know that humans can detect the smell of wet soil 200,000 times better than sharks sense blood? [1] It appears our olfactory abilities are not that bad after all, at least when it comes to finding potential sources of food. Petrichor, the term to describe the scent was coined in 1964, by scientists I. Bear and R.G. Thomas, meaning “petros” – stone and “ichor” – the blood of the gods [2] in Greek.

Divine or not, Streptomyces is a genus of over 800 bacterial species and subspecies responsible for the earthy smell of Autumn we know and love. But could it be that our innate senses are drawn to wet dirt for more reasons than farming? 

 

Could eating dirt cure the plague?

That is yet to be tested, but Streptomyces are certainly fit for more purposes than poetic walks after the rain. They are the most important source of antibiotics [3].

What is an antibiotic? By definition, it is a substance produced by one organism that is capable of inhibiting the growth or destroying other organisms  – a direct translation from Greek would be ‘anti-life’. In nature, this yields Streptomyces a competitive advantage. Astonishingly, they are responsible for nearly two-thirds of natural antibiotics [4].

For instance, Streptomyces griseus produces Streptomycin, the first antibiotic against tuberculosis, and a drug of choice against the agent of Plague Yersinia pestis, along with other 28 pathogenic bacteria species [5].  Streptomyces avermitilis helps keep parasites in check with its potent avermectins, and Chloramphenicol – a drug effective against 92 pathogens, is produced by Streptomyces venezuelae.

Streptomyces glaucescens under a microscope
Image of Streptomyces glaucescens. Courtesy of Tobias Kieser, John Innes Centre

 

Mavericks of the Streptomyces family

Although these species are not considered to be important agents of infection, it is worth noting that not all Streptomyces bacteria are friends of humanity, however. Two rebels, Streptomyces somaliensis and Streptomyces sudanensis go against the grain by infecting people’s feet with actinomycotic mycetoma.

First described as ‘Madura foot’ in 1842, the disease is thought to date back to the Byzantine period and typically presents as cutaneous and subcutaneous tissue swelling, thickening, or painless nodule involving feet 80% of the time [6].

Mycetoma global distribution map
Mycetoma global distribution map, GIDEON. Dark blue color indicates recent reports of autochthonous cases.

 

Although bacterial mycetoma is endemic throughout the tropical world, S. somaliensis and S. sudanensis are only found in Eastern Africa, as names indicate. In other parts of the world, S.albus has been known to occasionally rear its head [7] in skin infections, although such occurrences are very rare. 

And here’s the most fascinating fact – a distant ‘cousin’ of these pathogens, Streptomyces cattleya is an effective carbapenem used to treat Streptomyces spp. infections. A family feud in all its glory!

 

Why are bacteria named as fungi?

Streptomyces are about 450 million years old. Despite being a genus of bacteria, they are misleadingly suffixed with ‘-myces’, which stands for ‘fungi’ in Greek. This is because the first known example of the species contained branching filaments [8], a characteristic common to fungi.

Other actinobacteria, such as Mycobacteria and Actinomyces bear similar morphological features and thus carry a badge of mushroom in their names. Mycoplasmata – elusive gram-unidentifiable bacteria called ‘fungus form’, were named so by A. B. Frank in 1889. He thought ‘the “infection threads” of the organism were hyphae, and he knew of no hyphal-forming bacteria’ [9].

 

Want to learn more about Streptomyces? Try GIDEON ebook Guide to Medically Important Bacteria, 20% off on our website.

Interested in Mycetoma? Take a look at Mycetoma: Global Status

References:

[1] H. Campbell, “Geosmin: Why We Like The Smell Of Air After A Storm”, American Council on Science and Health, 2018. [Online]. Available: https://www.acsh.org/news/2018/07/28/geosmin-why-we-smell-air-after-storm-13240. [Accessed: 11- Sep- 2020].

[2] I. BEAR and R. THOMAS, “Nature of Argillaceous Odour”, Nature, vol. 201, no. 4923, pp. 993-995, 1964. Available: 10.1038/201993a0 [Accessed 11 September 2020].

[3] K. Chater, “Streptomyces”, Brenner’s Encyclopedia of Genetics, pp. 565-567, 2013. Available: 10.1016/b978-0-12-374984-0.01483-2 [Accessed 11 September 2020].

[4] A. Hasani, A. Kariminik and K. Issazadeh, “Streptomycetes: Characteristics and Their Antimicrobial Activities”, International Journal of Advanced Biological and Biomedical Research, vol. 2, no. 1, pp. 63-75, 2014. Available: http://www.ijabbr.com/article_7033_7733c8235876d7ba635f6c831a916648.pdf. [Accessed 11 September 2020].

[5] “Streptomycin”, App.gideononline.com, 2020. [Online]. Available: https://app.gideononline.com/explore/drugs/20910. [Accessed: 11- Sep- 2020].

[6] V. Lichon and A. Khachemoune, “Mycetoma”, American Journal of Clinical Dermatology, vol. 7, no. 5, pp. 315-321, 2006. Available: 10.2165/00128071-200607050-00005 [Accessed 11 September 2020].

[7] M. Martin, A. Manteca, M. Castillo, F. Vazquez and F. Mendez, “Streptomyces albus Isolated from a Human Actinomycetoma and Characterized by Molecular Techniques”, Journal of Clinical Microbiology, vol. 42, no. 12, pp. 5957-5960, 2004. Available: 10.1128/jcm.42.12.5957-5960.2004 [Accessed 11 September 2020].

[8] K. Chater, “Streptomycesinside-out: a new perspective on the bacteria that provide us with antibiotics”, Philosophical Transactions of the Royal Society B: Biological Sciences, vol. 361, no. 1469, pp. 761-768, 2006. Available: 10.1098/rstb.2005.1758 [Accessed 12 September 2020].

[9] C. Krass and M. Gardner, “Etymology of the term Mycoplasma”, INTERNATIONAL JOURNAL of SYSTEMATIC BACTERIOLOGY, vol. 23, no. 1, pp. 62-64, 1973. Available: https://www.microbiologyresearch.org/docserver/fulltext/ijsem/23/1/ijs-23-1-62.pdf. [Accessed 12 September 2020].

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