Hey Mel

Thanks for you question! I have always heard that many plant extracts possess antibacterial properties but I never really questioned how they accomplished this. I did some research and happened on a great review of this very topic.

In the review the authors mention that the primary antibacterial component of eucalyptus oil is a tannin compound (specifically, 1,8-cineole-terpineol). The specific mechanism used by tannins to injure bacteria is poorly understood. Because tannins are not known to cross the cell wall/plasma membrane of bacteria it is assumed that their effects must happen outside of the bacterial cell. It is thought that tannins could deactivate enzymes in the bacterial cell wall. These enzymes are thought to be necessary for bacteria to flourish in a given environment.

Another potential anti-microbial mechanism is iron-sequestering. Bacteria absolutely require iron to live. Tannins can bind quite tightly to iron in the environment, making it very difficult for bacteria to get to it. Without iron the bacteria cannot grow.

Finally, tannins are also thought to be able to disrupt cell membranes. By damaging the bacterial membranes, the bacteria lose the ability to regulate transport in and out of the cell.

I hope this answers your question!

Billy.

P.S. The following are sources I used in writing the answer to your question. They are all pretty dense, but I do recommend reading the review article by Marjorie Cowan if you can access it.

H Akiyama, K Fujii, O Yamasaki, Y Oono and K Iwatsuki. “Antibacterial action of several tannins against Staphylococcus aureus.” Journal of Antimicrobial Chemotherapy. 48, 487-91. 2001.

A Scalbert. “Antimicrobial properties of tannins.” Phytochemistry. 30(12), 3875-83. 1991.

S Inouye, T Takizawa and H Yamaguchi. “Antibacterial activity of essential oils and their major constituents against respiratory tract pathogens by gaseous contact.” Journal of Antimicrobial Chemotherapy. 47, 565-73. 2001.

M Cowan. “Plant Products as Antimicrobial Agents.” Clinical Microbiology Reviews. 12(4), 564-82. 1999.