The following article summarizes several studies conducted to evaluate essential oil (Oregano, Cinnamon Bark, and Tea Tree) efficacy on bacteria and the ability of bacteria to form a resistance to them over time (like they do with pharmaceutical antibacterials).

The author of the article summarizes as follows, "The reason for research in reversal of antibiotic resistance, broadly, is to preserve a healthy microbial ecosystem surrounding humans. The emergence of antibiotic resistance clearly demonstrates the dynamic evolution of microorganisms’ response to the hostile environment of antibiotics."

Read this article for yourself to find out the results of these studies:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3950955/