Demonstrate the growth of leaf yeast on malt/starch agar (LC 2026) (Leaving Cert Biology): Revision Notes

Demonstrate the growth of leaf yeast on malt/starch agar

Background:

Yeast, a member of the fungi kingdom, grows on a nutrient medium containing starch or malt. This medium provides all the necessary food for microorganisms to grow into colonies. Nutrient agar is a solid, jelly-like form of this medium. Yeast present on the lower surface of leaves grows best at 30°C and appears as shiny pink colonies. The presence of leaf yeast indicates good air quality, as they do not grow on trees in heavily polluted areas.

Materials needed:

• Leaf from an ash tree
• Disinfectant
• Petroleum jelly (Vaseline)
• Starch agar plates (2)
• Forceps
• Incubator (set at 30°C)
• Labels

Method:

1. Cut a leaf from an ash tree.
2. In the lab, disinfect all work areas and equipment with disinfectant to create a sterile environment, free from all microorganisms, to prevent contamination.
3. Smear the top surface of the leaf with petroleum jelly (Vaseline). No fungi grow on the top surface due to its waterproof waxy cuticle.
4. Using flamed forceps, transfer the leaf to the starch agar plate, attaching the top surface of the leaf to the lid. This allows fungi on the lower surface to fall onto the nutrient agar.
5. Leave the second plate empty as a control.
6. Seal and label both agar plates, indicating which is the experiment and which is the control.
7. Store the plates in an incubator at 30°C for 24 hours to allow yeast from the underside of the leaf to fall onto the agar. Turn the plates upside down and leave in the incubator for an additional 3 days to reduce condensation formation.
8. Record results.
9. Soak plates in disinfectant before disposal.

Results:

Control

No evidence of any colonies

Experiment

Pink shiny colonies