220 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
flowing steam before the pressure within the chamber is raised. It is 
important to recall that Dreger and Walker^ fomid that bacterial spores 
suspended in glycerol or oil were not killed by an exposure of more than 
thirty minutes to steam pressure at 119° C.^ Media containing gelatin 
or carbohydrates^ are sterilized at a lower temperature by discontinuous 
sterilization — one-half hour on three 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 steriliza- 
STERILIZING CHAMBEB 
1L_ J ^^ 
Fig. 15. — Arnold steam sterilizer. 
(Abbott.) 
Fig. 16.— Arnold steam sterilizer. 
Ordinary type. 
tion of blood serum or other native proteins— an exposure of from 65° to 
70° C. for one 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 filtra- 
tion through sterile porous filters made of unglazed porcelain or diato- 
maceous earth— Pasteur or Berkefeld filters. These filters are made 
with varying degrees of porosity, regulated largely by the thickness of 
, 1912, 17, 142 
Actual Temperatures in Autoclave Sterilization, Jour. 
' Jour. Pathol, and Bacteriol. 
- See Benton and Leighton: 
Infec. Dis., 1925, 37, 353. 
3 Carbohydrates are best sterilized as concentrated aqueous solutions. Appropriate 
amounts are subsequently added to the protein constituents of the completed medium. 
* Theobald Smith: The Conditions under which Discontinuous Sterilization may 
be Ineffective, Jour. Exper. Med., 1898, 3, 647. 
