Expert: Walter Hintz - 1/9/2007

Question
Hello, I am a ninth grader and I am doing an experiment on the effect of oregano oil on the growth of E.coli and Bacillus Cereus.  Recent research suggests that oregano oil has antibacterial properties, which is what I am testing.
 The hypothesis for this experiment is that oregano oil will inhibit the growth of E.coli and Bacillus Cereus possibly kill the bacterium.  Oregano oil at larger amounts will kill bacteria.  Oregano oil at smaller amounts will lead to a gradual decline of the growth of E.coli.  If oregano oil is added, more inhibition will be shown in E.coli cultures than Bacillus Cereus cultures because Bacillus Cereus is gram-positive and therefore has thicker cell walls.
   Do you have any suggestions for my experiment? What actually kills the bacteria in the oregano oil? How do I measure the growth of bacteria since I am testing inhibition? I would appreciate it if you would answer these questions. Thank you!  

Answer
Hi Jackie
You are introducing too many variables here.  You are using two different organisms and varying amounts of oil.  You also have at least four hypothesis.. If you want to test the effect of different amounts of oil you should work with only a single bacterium.  I suggest Bacillus since it is easier to grow.
 If you wish to subject the bacteria to varying amounts of oil how do you plan to do this? You can't dilute the oil.
 My suggestion is that you isolate a culture of Bacillus and inoculate agar plates. Leave some as control plates and flood a thin layer of oil over the others.  Your measurements merely involve counting colonies.
 Another possibility is that you can change your hypothesis to" Oregano oil is more likely to inhibit the growth of Gram positive bacteria then Gram negative bacteria"  Now you can use both E.coli and Bacillus

I forgot to answer your question about why oregano oil kills germs.  I cannot answer because we have not established that it does inhibit growth.
 It also occured to me that you may want to try making solutions of powdered oregano in water.  It would be easier to work with then insoluble oil.