Thanks for the additional explanation. I think I understand what you are doing now.
I recommend adding the juice directly to the antibiotic disc and letting it dry for a minute or two before applying it to the bacterial lawn before incubating the plate. Do you have access to a pipettor with sterile pipette tips? If so, I think you could pipette 20-25 uL of each juice onto a disc. If you don’t have pipettes, then use an eye dropper, or something similar, and add one drop of juice to each disc. One drop is equivalent to 20 uL (microliters), so this will give you a consistent volumne.
The bacteria that have been inoculated onto the surface of the agar will grow up overnight and you can see if there is a difference in the zone of inhibition between the control disc and the antibiotic plus juice discs.
Do you have a different antibiotic or bacterial culture to use? Amoxicillin is an antibiotic that inhibits the formation of the cell wall of Gram positive bacteria. E. coli is Gram-negative, so I don’t know how large the zones of inhibition will be with this combination. If you do not have any options, then proceed as you had planned since your project is due on Monday.
E. coli will grow at home at ambient temperature. Be sure to turn the plates upside down to avoid moisture condensation on the surface of the agar. It may take two days for the lawn to appear, since growth will be slower compared to an incubator at 37 degrees Centigrade. Can you set up the plates at school and seal them with parafilm and take them home? This would be the best way to handle this situation. It sounds like you do have your teacher’s permission to work at home, but normally you should do this type of experiment at school to comply with all science fair rules.
The culture from Carolina Biologicals should be transferred to new agar or broth to maintain it. The cells will die eventually, so you will want to transfer it to keep it alive. For your experiment, it would be best to grow up a new culture in broth for a few hours or overnight until the growth medium is very cloudy. This will give you a new culture in late log or early stationary phase that will be optimum for testing antibiotic sensitivity. You should transfer your stock culture about every two weeks and store it in the refrigerator
If you need more information, do check out the information on the Science Buddies website for information on microbiological techniques and storing bacterial cultures. http://www.sciencebuddies.org/science-f ... ques.shtml
Please post again if you have more questions.