234 ESSENTIALS OF BACTERIOLOGY 



Preservatives are substances which prevent fermentation, 

 but they are not always germicides. 



In considering the value of a germicide, the strength in 

 which it acts is the main consideration. Some very weak 

 chemicals will inhibit and destroy the growth of bacteria if 

 used in sufficiently concentrated solutions. Some bacteria will 

 die in an acid media; others are destroyed by too much alkali. 

 Some bacteria are very readily destroyed in pure cultures, but 

 are resistant to a considerable degree in the body tissues. 

 Again, a germicide may be ideal in laboratory experiments, 

 but wholly impractical at the clinic. 



Germicides are tested by action in various dilutions or in 

 gaseous form on threads impregnated with virulent and spore- 

 forming organisms. The length of time is noted that it takes 

 to destroy anthrax bacilli or pyogenic organisms. 



The infected material is subjected to the solution and then 

 inoculated on media and compared with control, or tested for 

 virulence on animals. Spore-forming organisms are very 

 resistant to the most potent agents. 



Heat is perhaps the best general germicide. For all articles 

 that can be subjected to boiling or the direct flame there is 

 no safer agent. 



Superheated steam, or steam under pressure, is now in 

 general use in sterilizing surgical dressings and instruments, 

 and requires less time than ordinary steam. 



The salts of metals of high atomic weights come next in order. 

 Bichlorid of mercury and cyanid of mercury are the most 

 powerful of chemical germicides, but in the human body they 

 can be used in dilute solutions only, and in contact with highly 

 albuminous solutions, insoluble and inert albuminates are 

 liable to form, lessening the germicidal value. A i : 200 

 solution combined with an acid will destroy the spores of 

 anthrax in one hour, but much weaker solutions will destroy 

 the anthrax bacilli in the blood, and for all practical purposes 

 a i : 2000 solution is sufficient, destroying bacterial life in a 

 few minutes. 



