APPENDIX A 

 STERILIZATION 



Many of the student's difficulties begin and end with the sterili- 

 zation of the culture media. Wherever accurate work is to be done, 

 much attention must be given to the process of sterilization. Modern 

 bacteriological work rests upon the pure culture. So long as con- 

 tamination is suspected or known, uncertainty and doubt concerning 

 the results will prevail. The first requisite for the vigorous growth 

 of a pure culture is, therefore, successful sterilization. 



The principal agents of sterilization are three chemicals, heat, 

 and filtration. Each finds its application for special purposes. 



STERILIZATION WITH CHEMICALS 



The efficiency of various chemicals as germicides has been investi- 

 gated by numerous biologists. One class of germicides are used in 

 surgery and contact disinfection work, while another class of more 

 or less volatile compounds are used in laboratories, where toxin and 

 enzyme work is carried on. In choosing a germicide for surgical 

 work it is desirable to have one which shall be highly toxic to the 

 bacteria and shall not at the same time impair its efficiency by form- 

 ing insoluble compounds with the body fluids which may be present 

 in the wounds. 



Certain salts of the heavy metals have shown strong germicidal 

 properties ; among them mercuric bichloride (corrosive sublimate) 

 stands first in efficiency and applicability. The concentration of 

 bichloride generally used for disinfection and surgical work is one 

 part to a thousand of water. Since this substance is slowly soluble in 

 water, a hydrochloric acid solution may be first prepared and further 

 dilutions made as required. Thus, 200 g. of mercuric bichloride dis- 

 solved in a liter of hydrochloric acid makes a 20 per cent stock solu- 

 tion. If 5 cc. of this solution be diluted to 1000 cc., it makes a 

 1 : 1000 solution of bichloride. In the laboratory the stock solution 

 may be conveniently kept in siphon bottles upon the stock table. 



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