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Streptomyces are gram positive, spore-forming bacteria found in soil. They are characterized by their tough, leathery, frequently pigmented colonies and their filamentous growth. When first discovered, these organisms were thought to be fungi, but closer examination revealed a lack of a nuclear membrane and the presence of peptidoglycan, demonstrating their prokaryotic origin. Streptomyces are chemoheteroorganotrophs, growing best at 25°C and pH 8-9. They are capable of using complex organic materials as carbon and energy sources and are involved in the breakdown of these products in the soil. This degradative ability makes these bacteria pivotal in the production of fertile soil for agriculture. They also give soil its characteristic smell by the production of a class of volatile low molecular weight compounds called geosmins.

In the laboratory, different isolates were found to produce numerous compounds capable of inhibiting or killing other microorganisms. Many of these Figure 10-3 shows some examples of streptomycetes

In this section, we describe the enrichment of this group of organisms and test several isolates for their antibiotic producing ability. Dried soil, a great source of inoculum for Streptomyces, will be plated directly onto Actinomycete Isolation Agar (AIA). Since an enrichment step is not necessary, what does that tell you about the population of Streptomyces in the soil? AIA contains starch and sodium caseinate as sole carbon and energy sources. Only organisms capable of degrading these complex polymers (mostly molds and streptomycetes) are able to grow. Sodium propionate (a preservative sometimes present in commercial bread) is added to inhibit the growth of molds. Incubation at 30°C for more than five days results in the formation of Streptomyces colonies that are recognized by their characteristic appearance.

Isolation protocol

Period 1

Materials

Soil sample

3 0.85% NaCl dilution blanks (9 ml)

3 plates Actinomycete Isolation Agar (AIA)

1. Weigh out one gram of soil and place it in one of the dilution blanks. Vortex the suspension for 1 min. Serially dilute the suspension to 10^-3 using the other 2 saline dilution blanks.
2. Label the 3 plates of AIA, 10^-1, 10^-2, 10^-3 and 10^-4. Plate 0.1 ml from each of the 3 dilution blanks onto the appropriate plate of AIA and spread the inoculum using a sterile hockey stick.
3. Incubate the plates at 30°C for 5-7 days.

Below is shown the appearance of a Streptomyces isolation plate after 5 days of incubation

Period 2

Materials

5 AIA plates

1. Examine the AIA plates and look for typical Streptomyces colonies, Figure 10-2. They are small, opaque, compact, frequently pigmented (brown, yellow, pink, etc.), often leathery, and appear dry and dull looking. See Figure 10-3 for examples. Typically, a depression in the agar surface will be observed around the colony. Avoid molds. They usually form much softer, fuzzy colonies if present. AIA has propionic acid in it to inhibit most molds, but some species are resistant to this inhibitor.
2. Prepare a wet mount. Good Streptomyces candidates will be difficult to remove from the agar with the inoculating needle or loop and upon observation under the microscope will reveal a multitude of spores with a few filamentous cells, Figure 10-6.
3. Select five potential Streptomyces colonies and streak each for isolation onto a plate of AIA. Incubate the plates at 30°C for 5-7 days.

Period 3

Materials

6 plates of Penassay Agar

Culture of Streptomyces griseus, an active antibiotic producer

1. Examine each Streptomyces isolate and describe its colony morphology. Verify that it is indeed free from mold, Figure 10-7.
2. Pick a well isolated colony from each plate and streak onto Penassay Medium. Place a single streak down the middle of the plate for each isolate; do not streak for isolated colonies (Figure 10-6). The test cultures will be added to the Penassay plate next period. Also streak a plate of the known culture of S. griseus onto the last Penassay plate in a similar manner. Incubate at 30°C for 5-7 days.

Period 4

Materials

Broth cultures of 5 test organisms

1. Examine the plates for growth and freedom from contamination. Check the cultures for the production of geosmins (the smell we associate with soil) by carefully smelling the plates along the edge while leaving the plates closed. Figure 10-9 shows some examples of a good test plates
2. Cross-streak the plates with each of the 5 test cultures. Start each streak near the edge of the plate and streak toward the Streptomyces colony (Figure 10-6). Finish the streak near, but not touching, the Streptomyces streak. Do not reverse the direction of the streak and only streak one time!
3. Incubate at 30°C for 2 days

Period 5

1. Observe the S. griseus plate and look for growth inhibition in the test cultures due to antibiotic production. If a test culture is inhibited, there should be a decrease in the amount of growth as you move in toward the Streptomyces streak. Examine your Streptomyces, do any of the isolates show production of antibiotics. Figure 10-10 shows an example of a Streptomyces isolate that inhibit several strains.
2. Examine the plate above. Are any of the isolates antibiotic producers? What further tests would need to be done to find out?

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