Limits to progressive reduction of resident skin bacteria by disinfection.

Antiseptic preparations used repeatedly to disinfect the skin caused a reduction in yield of resident flora to a low equilibrium level beyond which further reduction did not occur. This equilibrium varied with the antiseptic preparation used. In a comparison of three preparations, the lowest equilibrium level was obtained with 95% ethyl alcohol. The further reduction in yield of skin bacteria on using alcohol after repeated disinfection with an antiseptic detergent preparation ('two-phase' disinfection) was not paralleled by a further reduction when the preparations used in the two phases were reversed, the antiseptic detergent being used after repeated disinfection with ethyl alcohol. 'Two-phase' disinfection was therefore seen to be due to further reduction in skin bacteria from the low equilibrium obtained with the antiseptic detergent to the lower level obtainable by alcohol disinfection. When repeated disinfection to equilibrium with alcohol was followed by a 'second-phase' handwash with a non-antiseptic detergent and water, there was a large increase in the yield of skin bacteria. This finding supports the view that a low equilibrium level occurs when the reduction in density of accessible bacteria through disinfection is balanced by the emergence of bacteria drawn from deeper layers of the skin.