Pseudomonas aeruginosa biofilms grows stronger after 95 % ethanol treatment
@Chris is right. ~70% ethanol is the way to go.

From a discussion here :


  The effectivity of ethanol as e.g. desinfectant or antiseptic agent depends on the concentration of ethanol-water-mixture: An ethanol percentage of 50-80% destroys the cell wall/membrane of bacteria by denaturing their proteins and dissolving their lipids (effective against most bacteria, fungi and some viruses; ineffective against bacterial spores). Therefore, the ethanol has to pass the bacterial membrane/wall to get into the bacteria - if you use 100% ethanol instead, the bacteria get 'sealed' and they will survive... An other mechanism is the high osmotic pressure of ethanol/water-mixtures; and the 70% has the highest one.


By using a high ethanol concentration, you're effectively creating a layer of coagulated protein on the surface of the biofilm, "sealing" the bacteria in below that top layer.

As to why water seemed more effective than alcohol, I would venture a guess that some of the bacteria from the biofilm was flushed off. Maybe check the water for bacterial count to verify? :)