194 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



successive days, in flowing steam in an Arnold sterilizer. After each 

 sterilization the medium is kept at room temperature to permit of the 

 germination of spores. Lower temperatures are occasionally employed, 

 particularly for the sterilization of blood serum or other native pro- 

 teins an exposure of to 70 C. for an hour on each of six successive 

 days usually suffices. Bacteria may be removed from fluid media 

 and from various sera and solutions containing thermolabile toxins 

 or similar products by filtration through sterile porous filters made of 

 unglazed porcelain or diatomaceous earth Pasteur or Berkefeld 

 filters. These filters are made with varying degrees of porosity, 



FIG. 17. Arnold steam sterilizer. Boston Board of Health type. (Park.) 



regulated largely by the thickness of their walls to accommodate vary- 

 ing needs. Usually the fluid is forced through the walls of the filter 

 into the center, which is hollow, by suction. The clear, bacteria-free 

 filtrate passes into a sterile container attached to the filter. The filters 

 and their necessary accessory parts are sterilized in the autoclave for 

 fifteen minutes at fifteen pounds live-steam pressure. Turbid fluids 

 should be passed through several layers of filter paper prior to filtra- 

 tion, to remove the grosser particles which otherwise would soon 

 clog the filter. A time limit, usually not exceeding two hours as a 

 maximum, should be set, beyond which filtration should be stopped 



