Spot the antibiotics producer: STREPTOMYCES



The human gets ill, goes downhill, forgets the dollar bill, takes a pill, regains will, re-feels the thrill.

It’s simply the recuperation cycle.

But, do you ever think about the fundamentals of having the chance of recuperation from infections?


It’s thanks to the pill - the antibiotic.

Good job!

Now, zoom in deeper…

Do you actually know where this savior comes from?

Think no further than the soil.



1st step: Air dry the soil samples for 3-4 h at 45° for the selection of Actinomyces spores, considering that many other bacteria die during this pre-treatment

2nd step:Dilute 1-3g of soil in 10 ml saline solution (NaCl 0,9%) and let the solution stand for 15 min

3rd step: Prepare 3 dilutions (1:10, 1:100, 1:1000) out of it

4th step: Prepare the Mannitol Soy Agar (MSA) petri dishes


In a 250 mL conical flask mix:

  • 2g Agar
  • 2g Mannitol
  • 2g Soya flour (Holland and Barrett)
  • 10 mM CaCl2
  • 100 mL Deionised water

Medium alternative: Inorganic Salt Starch Agar (ISSA)

5th step: Plate an aliquot of 0.1ml of each dilution on the medium (MSA)

The loop used for spreading the solution on the medium (shown on the 2nd photo) is a DIY loop, created from paper clips

6th step: Incubate at 30° for 5-7 days

Important: Use parafilm to fortify the covers of each petri dish, in order to avoid evaporation, as it can lead to contamination.


Optional: Supplementation with cyclohexamide (50 μg/ml) and nystatin (50 μg/ml) to further remove competing bacteria


Observations: The no-dilution petri dish shows a tendency towards contamination. Other petri dishes seem on the right track by far. The small white powdery looks like a good sign.


Observations: There has been overgrowing a hybrid of organisms, but assumingly the pinky/white powdery dots are streptomyces colonies.Such colonies are mostly to be found on the 1:100 and 1:1000 dilution plates. In this case, creating subcultures is the best solution, in order to get a unified-type of biotope in the petri dish!

Only one sample has been contaminated, as it was the first one to get done during the isolation process.




Streptomyces species are the most abundant source of antibiotics. More specifically, they produce the two-thirds of all existent antibiotics. Except that, this genus produces a rife scope of bioactive compounds. It usually inhabits the soil and plays a significant role in decomposing.

So, next time when you get ill, decide to take another pill, in order to re-feel the thrill...

...take a minute to think and praise the genus of good welfare.

Inspiration: - Anna Dumitriu - Dr. Simon Park