Spore Prep - Inoculating & Harvesting
A spore prep is a method of preserving a sporulating strain of Streptomyces. The stock is stored in 20% sterile glycerol at -20°C. It will last for many years if it is prepared in a sterile environment and afterwards, treated carefully.
|~5 days 1 hour 35 minutes
The stock of a strain can be used for many further experiments such as: new stocks; phenotypic analysis; conjugations; spore titre, etc.
You should have an idea whether or not you strain sporulates. If it does not, then you may only be able to create a cell suspension glycerol of colony forming units (CFUs). If the strain contains a non-integrative plasmid then antibiotic selection is needed to prevent loss of the plasmid. Other additives may also be needed if the strain is auxotrophic.
General laboratory & molecular microbiology safety rules apply.
A single colony of the Streptomyces strain or some other spore stock, SFM, Petri dishes, ice, sterile 20% glycerol, sdH2O, sterile cotton wool pieces, sterile cotton buds, sterile toothpicks, micro-centrifuge tube(s) (MCT), 2mL screw cap tube(s), 50mL falcon tube(s), sterile 10mL syringe, filtered tips, forceps (tweezers), laminar flow hood, vortexer, 30°C incubator, microwave, falcon centrifuge.
Ensure the equipment and sterile reagents are available. Prepare the laminar flow hood by wiping with 70% ethanol and/or sterilising by UV irradiation. Set an incubator to 30°C.
All parts are to be performed under sterile conditions, i.e. in a laminar flow hood. All equipment and reagents should be sterile. Once the spores have been harvested they need to be kept on ice, especially if another strain is harvested at the same time.
Inoculation Preparation (Sterile!):
- Pour 2 plates of SFM (Soya Flour Mannitol) media, ~30ml per plate, adding any necessary antibiotics / supplements.
- Dry for approximately 20 minutes.
- Pick a single colony using a toothpick, or take 5μl of a previous spore stock; re-suspend / dilute in 200μl sterile dH2O, vortex the MCT well.
- Pipette 100μl of the spore suspension onto each plate.
- Take a sterile cotton bud and prime by dipping in sdH2O.
- Use the primed cotton bud to streak the spore suspension for confluent growth. For best results, use the pattern shown in Figure 1.
- Incubate for ~4-5 days at 30°C.
- Setup the filter by placing a 10mL syringe into a 50mL Falcon and remove the plunger. Using sterile forceps push a piece of cotton wool into the syringe, see Figure 2.
- Add ~5ml sdH2O to the confluent plate.
- With a sterile cotton bud, gently rub the spores off the surface of both plates.
- Remove all the liquid from the plates and pipette into the syringe using a filtered tip.
- Expel the suspension into the falcon tube by replacing the syringes’ plunger.
- Spin the falcon @ ~5000g for 5 minutes.
- Re-suspend the pellet in minimum volume necessary of sterile 20% glycerol (~500μl dependent on the pellet size).
- Transfer the spore stock to a 2mL screw cap tube and store at -20°C.
Slightly thicker plates are needed when inoculating Streptomyces as they take longer to grow than other general laboratory bacteria, ~4-5 days. This is partially due to it having a more complex life cycle (spore → vegetative hyphae → aerial hyphae → spores) and its preferred incubation temperature is 28-30°C. If the stock is being made from a single colony, ensure it is well mixed and carefully streaked. Smoothly poured plates without bubbles and confluent growth will aid the harvesting process.
Whilst harvesting, be careful not to dig into the agar. Use light pressure to begin, gradually increasing; removing the spores without breaking the surface. Agar pieces mean less spores will be collected and pipetting is difficult. When done properly the majority of the surface of the agar should be visible. To rub the spores from the edges of the plate, use a circular motion.
The cotton wool in the syringe helps to filter out any agar pieces and longer hyphal fragments. Filtered tips are needed to help prevent cross-contamination, as an aerosol of spores can enter the pipette chamber.
Centrifuging at a lower-than-normal speed (~5000g) will keep the spores intact ready for further experimentation; it may also help to prevent hyphae from pelleting.
Spore stocks are a precious material. Not only do they take time to prepare, but if the strain has been modified from the wild type a lot of effort has been put into achieving this. Spore stocks should be kept at least -20°C and when needed, thawed on ice; helping to prevent germination. Always use the stocks in a sterile environment, i.e. in a laminar flow hood or by using good technique, under a Bunsen flame. Decontamination of a spore stock is a laborious and unnecessary process.