Microbes Must Eat 75 



employing oven heat in sterilization, the temperature is raised to 

 170° C. (a 10-degree temperature margin) and is maintained for 

 at least one hour, with two hours' contact being more common in 

 many laboratories. 



Open Flame Sterilization. In transferring growths from one 

 container to another, the usual practice is to handle these organ- 

 isms by means of sterile wires or wires fashioned into loops. The 

 most convenient method of sterilizing these needles and loops is 

 to plunge them into the direct flame of a bunsen burner or an 

 alcohol lamp. Incineration insures sterility. Surgical instruments 

 cannot, however, be subjected to such rough treatment. 



Filtration as a Means of Sterilization. Some liquids cannot 

 be sterilized by heat because the elevated temperature will coagu- 

 late the material or will adversely affect the chemical structure of 

 the compound. Various types of filters have been developed to 

 overcome the objections to heat treatment, and today we employ 

 filters to sterilize blood sera, to separate exotoxins from their parent 

 cells, to separate viruses from bacteria and other organisms, to aid 

 in the isolation of enzymes, and to free thermo-labile liquids from 

 contaminating organisms. It is not the mere physical removal of 

 organisms by the minute pores in the filters that accomplishes 

 sterilization; adsorption phenomena associated with electrical 

 charges are also instrumental in this filtering process. Liquids may 

 be drawn through filters at a rate faster than gravity would normally 

 allow by applying positive pressure as the liquid is introduced into 

 the filter, but a more common practice is to provide negative 

 pressure (suction) to the receiving flask. Too much positive or 

 negative pressure, however, will create an undesirable differential 

 which may nullify the surface action of the filters by drawing the 

 bacteria away from the sides of the pore spaces where thev have 

 been trapped. A differential of 150 to 200 millimeters of mercury 

 is usually recommended to speed up filtration without sacrificing 

 dependability. We shall mention but four of the commonlv- 

 employed filters. Those readers interested in more complete 

 discussions of the subject are referred to advanced textbooks in the 

 field. 



