Take a pot of natural yoghurt and... use it to make some more. Stir a spoonful into some UHT milk, loosely plug with clean cotton wool, keep it warm overnight, refrigerate when done. Make sure everything is clean: sterilise with Milton sterilising fluid first. Do not eat the product, though. If one "goes off", great, you can see the effect of another species of microbe.
Turn this into an open-ended study. What might matter? How can you assess product without eating it? Cooking the starter first, or adding penicillin discs will stop the yoghurt growing. Why?
Use a data-logger to record pH changes in growing yoghurt (remember a control with killed or absent starter). Incubate the whole lot (culture, pH probe, data-logger) in the incubator overnight. Discuss how increasing acidity preserves the milk. For post-16 pupils, take the pH data from the data-logger, copy it into a spreadsheet and convert it using a simple formula into hydrogen ion concentration.
Plot against time, and you get a wonderful curve for production of a primary metabolite.
Relate to the sigmoid population growth curve. This was first suggested to me by one of my A-level pupils.
You can also take a smear of yoghurt on a slide and do a simple Gram's stain. See the bacteria using a good microscope, preferably with an oil immersion lens.
For a biology teacher, Lactobacillus bulgaricus and Streptococcus thermophilus really are friendly bacteria
Dr Jeremy Airey is professional development leader at the National Science Learning Centre, www.slcs.ac.uknational