|MadSci Network: Microbiology|
Hydrogen peroxide is formed as an inadvertent by-product during the growth of bacteria. Hydrogen peroxide is toxic to the bacterial cell. Within a bacterial cell there are many proteins which are essential for the cell to live. Hydrogen peroxide can destroy these proteins.
If you place hydrogen peroxide on certain bacterial colonies bubbles will form. The bubbles result from an enzyme called catalase which can convert hydrogen peroxide to water and oxygen gas. The bubbles you see are the oxygen gas being released. The bacteria use catalase to degrade the toxic compound to chemicals which are not toxic to the cell.
Hope this helps
Try the links in the MadSci Library for more information on Microbiology.