Here's a list of some microbiology ideas. You will need to speak to your teacher as to whether any of these are appropriate/can be done with the resources available at your school. Also consider that you will not be able to get ideas from your classmates and discuss your progress if you are doing something dissimilar to them.
Factors affecting relative activity of immobilised enzymes..eg bead size or flow rate or temperature or substrate concentration or vessel shape (use one only!)
Comparing antibacterial action of (eg) Strepsils vs Fishermens Friends
Comparing antibacterial action of (eg) TCP vs Aspirin/disprin or similar
Which is the cheapst bug killer? Cost effectiveness comparison of antibacterials (eg those in the previous two suggestions.
Factors affecting the production of simple sugars through the malting of barley grains.
Factors maximising alcohol yield from (eg) fruit juices or grains or potato.. could use a single yeast strain OR more than one(but don't overdo it!).
Are there differences in water potential between fruit at different ripenesses? Or in potatoes as they sprout? (Many do between Feb-April)
Effect of fruit ripening on concentration of reducing sugars(a variation on the previous suggestion)This one is a very tricky one to carry out!
Effect of fruit ripening on the concentration of proteolytic enzyme in pineapples.(This digests gelatin)
Factors affecting tomato seed germination. Does washing, treatment/removal of testa affect it? (You'll need to research and suggest reasons!).
The rate of fermentation of various carbohydrate(e.g. lactose/glucose/starch/sugar etc. etc.) by yeast(or their growth). Could relate to several themes including respiration.
Effects of alcohol on yeast respiration
Effects of different alcohols on yeast respiration
Action of inhibitors on the activity of the enzyme succinic dehydrogenase obtained from plant tissue. Possible inhibitors include silver nitrate and malonic acid.
Effect of sodium chloride on amylase
Effects of temperature or pH on enzyme activity.Careful! Needs great refinement and originality to be at all worthwhile!!
Effects of calcium chloride on (different) milk clotting enzymes
Effects of substrate concentration on enzyme activity. Needs great care to be worthwhile!
Root tip squashes - but beware! You need to make these quantitative.
Effects of different pH solutions on membrane permeability
Effects of alcohols on membrane permeability
Effects of heat(cooking)/light(storage)/cold(freezing or refrigeration) on vitamin C content in named fruit(ie optimal conditions are...?)
Is vitamin C denaturation related to the cooking method (ie boiling, frying, baking). Why?
Marcus