200 Microbes and You 



the very second a disinfectant is applied to microbes, the organ- 

 isms are killed. This is not true. Time and temperature are very 

 difficult to separate, as was previously pointed out with respect to 

 thermal death time and thermal death point. 



3. Moisture. Moisture plays an important role in the coagu- 

 lation of protein, and since coagulation at least partially explains 

 some disinfection reactions, available moisture influences the 

 ultimate disinfection reaction. Moisture also serves as a carrier 

 for heat and for transporting chemicals into the cells. We know 

 that 100% alcohol is practically worthless as far as killing bacteria 

 is concerned, but when the alcoholic strength is cut to between 

 50 and 70% by the addition of water, the added moisture aids in 

 the coagulation of protoplasm and in the eventual destruction of 

 microbes. 



4. Concentration of Reacting Substances. With chemical re- 

 actions in general, the higher the concentration of reacting sub- 

 stances—within limits— the more chemical end product will be 

 formed. We do know, however, that the most concentrated solu- 

 tions of disinfectants are not always the most effective for killing 

 bacteria. There is an optimum strength, above and below which 

 there is a diminution of activity. 



5. Presence of Extraneous Matter. Chemicals vary in their 

 disinfecting ability in the presence of organic matter. Some 

 chemicals, like chlorine, are rapidly neutralized if extraneous mat- 

 ter is present. Other chemicals have less affinity for organic mat- 

 ter and kill microorganisms quite promptly even when "dirt" is 

 present. 



6. Sm^ace Tension. This consideration was touched upon in 

 the discussion of penetrating power as a characteristic of an ideal 

 disinfectant. It has been demonstrated that marked changes in 

 some organisms, especially surface changes of the cells, can be in- 

 duced by lowering the surface tension. Cells are easier to "get at" 

 when the chemical agent is in a solution of low surface tension. 



It goes almost without saying that the tvpe of organism, its 

 age and history, and many other factors will influence the disin- 

 fection process. 



