FACTORS INFLUENCING BACTERIA 69 



Poisons. — ^The action of poisons upon bacteria varies with the 

 microbe, the kind of poison, and the conditions under which this 

 poison is used. Extremely weak solutions of most poisons stimu- 

 late bacteria, a stronger concentration prevents growth and mul- 

 tiplication, whereas a still more concentrated solution may kill 

 them. 



A substance which kills bacteria is called a disinfectant; if it 

 only prevents growth and activity it is known as an antiseptic. 

 The application of a disinfectant as a gas is termed fumigation. 

 When a chemical is added to a food with the idea of preventing 

 decay it is called a preservative. In sanitation disinfectants and 

 antiseptics are extensively used in the prevention of disease. In 

 agriculture ihey are often used on plants or in the soil to destroy 

 injurious microbes. The principles involved in both cases are 

 similar and are subject to certain general laws. 



(i) The efficiency of a disinfectant varies with moisture. A 

 dry poison has but slight action upon bacteria. Dry formalde- 

 hyde, or sulfur dioxid, is practically without effect. In a sim- 

 ilar manner absolute alcohol has not nearly the germicidal power 

 of 60 to 70 per cent alcohol. This is probably due to the abso- 

 lute alcohol coagulating the outer membrane of the organism and 

 thus preventing the poison from diffusing into the vital part. 



(2) The temperature of the medium in which the organisms 

 are present affects materially the action of the disinfectant, A 

 5 per cent solution of common soda at normal temperature may 

 have but little germicidal influence, but raise the temperature of 

 the medium in which it is placed to 40^ C. and it becomes quite 

 effective. 



(3) Bacteria are more easily killed by a disinfectant in pure 

 water than in saliva, blood, or other body fluids. 



(4) Emulsions usually have greater germicidal effects than 

 have true solutions. This is due to the emulsion having little 

 globules of the concentrated poison distributed throughout the 

 solution, and if examined under the microscope these, together 

 with the suspended bacteria, will be seen moving about in the 



