CHAPTEE XV. 



PRINCIPLES OF DISINFECTION DISINFECTANTS. 



WHEREVER there is disease due to pathogenic microorganisms the 

 latter will, through exhalation, secretions, and excretions, direct or 

 indirect contact, transport by insects or otherwise, soil or contaminate 

 objects in the neighborhood of the sick animal. When stables, barns, 

 harness, feed, water supply, or any other objects are soiled or con- 

 taminated with pathogenic bacteria they are said to be infected. The 

 removal or destruction of such infecting bacteria is called disin- 

 fection. Disinfection may sometimes take place without destruction 

 of the bacteria, as in the filtration of water through filters so dense as 

 to prevent the passage of the microorganisms. As a rule, however, 

 the object of disinfection is to destroy the pathogenic bacteria. 

 Physical and chemical means may be employed for this purpose. 

 Among the former, heat and sunlight are particularly important; 

 among the latter such chemicals as accomplish the object in com- 

 paratively weak concentrations. 



Effectiveness of Disinfectants. The effect of every disinfectant must 

 be studied separately for each type of pathogenic organism and also 

 for the vegetative forms and spores of sporulating bacteria. The 

 spores are, as has been previously pointed out, much more resistant 

 to all disinfecting procedures than the vegetative forms. In the 

 theoretical laboratory study of disinfection it is necessary to ascertain 

 not merely the circumstances under which physical or chemical 

 means kill all bacteria, but also to find out at what degree or con- 

 centration they have a marked hindering or inhibiting influence upon 

 bacterial growth and multiplication. In practice the absolute killing 

 of all infecting bacteria is seldom possible. Diluting, diminishing, 

 and damaging them in such a way that their growth is inhibited and 

 they are robbed of much of their virulency must generally be con- 

 sidered as sufficient. The simplest and most potent physical agent 

 used for disinfection is heat. Bacteria grow best at a certain temper- 

 ature, called their optimum temperature; any degree above it retards 

 their growth, and any degree above their maximum temperature 

 inhibits growth entirely and generally damages them seriously. When 

 working with chemical agents the least concentration which will 

 entirely inhibit growth must first be ascertained, and then the con- 

 centration which will destroy bacteria and the necessary time to 

 accomplish it. Absolutely harmless, non-pathogenic bacteria, some 

 of which form spores more resistant than the spores of any pathogenic 



