Hi Tessa,
Welcome to Science Buddies! This sounds like a great project and you have an excellent question.
For any of the items that you are testing, there is probably a concentration that will stimulate growth and a concentration that would inhibit the growth of E. coli. When doing a science project, it’s best to try a range of concentrations.
Since, E coli is an intestinal microbe, you might try to estimate the concentration of an item that might be present in the intestinal tract if someone were to eat a certain quantity at one time. Or, you could do a literature search and find out if anyone had published a similar experiment, and start with the concentration that others have used.
For example, here is a link for a research paper along with a couple of paragraphs from the materials and method section detailed method that was used to prepare garlic extracts for testing a different microorganism. Please note that the authors tested 8 concentrations of a standardized garlic extract; you might want to decrease this to 3 to 5 concentrations.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3147487/
Fresh garlic was obtained from Global Farms Enterprises, Inc. (Los Angeles, CA), stored at room temperature (ca. 22°C), and used within 2 weeks. The peeled cloves were put into a juice extractor (Waring Pro) to produce fresh garlic juice under aseptic conditions. The raw garlic juice was immediately put into 50-ml conical sterile centrifuge tubes and centrifuged at 7,000 rpm (Fisher accuSpin model 400 benchtop centrifuge, Pittsburgh, PA) for 10 min at 22°C. The supernatant was recovered and filtered through a 10.0-μm-pore-size polycarbonate membrane (K99CP04700; GE Water & Process Technologies, Trevose, PA) and then through a 1-μm-pore-size polycarbonate membrane (K10CP04700; GE Water & Process Technologies, Trevose, PA) and, finally, through a 0.4-μm-pore-size polycarbonate membrane (K04CP04700; GE Water & Process Technologies, Trevose, PA) under vacuum to remove potential microbial contamination, generating the garlic concentrate. The whole process to make the garlic concentrate was completed within 20 min. Then, the garlic concentrate was stored at 4°C and protected to avoid light exposure. The concentrate was added into 0.85% (wt/vol) sterile saline water and/or nutrient broth no. 2 within 30 min to avoid loss of volatile organosulfur compounds.
Garlic concentrate was prepared at various concentrations (0, 6.25, 12.5, 25, 37.5, 50, 75, and 100 μl/ml) in 100 ml of sterilized 0.85% (wt/vol) saline water to study its bactericidal effect with limited nutrients and in 100 ml of nutrient broth no. 2 to investigate the inhibitory or suppressive effect of garlic concentrate. Each concentration of garlic extract was added to both saline water and broth and then inoculated with 1 ml of 7 log CFU/ml of a C. jejuni cocktail to achieve an initial inoculum level of approximately 5 log CFU/ml. Each inoculate was mixed well by vortexing and then incubated at 4, 22, and 35°C for 0, 1, 3, 5, 7, 10, and 24 h (saline water samples) and 0, 1, and 3 days (broth samples) microaerobically. At each sampling time, the samples were serially diluted with 2% (wt/vol) sterile buffer peptone water, and the appropriate dilution was spiral plated. After incubation at 42°C for 48 h, numbers of viable cells were determined
If testing multiple concentrations of each item exceeds your time and resources, then I recommend cutting down on the list of test items. Testing just one item would be a complete project.
I have a question for you. How are you going to measure your results?
Donna
