7p BACTERIOLOGY 



siderable interval elapses before the agar becomes liquifipd, es- 

 pecially if it be in large flasks, and it is well to allow 30 to 35 

 minutes at 110° C, for its sterilization. Closely packed surgical 

 dressings serve to protect the interior, and considerable time may 

 be required for penetration of a sterilizing temperature into such 

 packages. In such instances it is unwise to rely upon the gauge 

 as an indicator of the temperature throughout the materials 

 being sterilized.^ It is well to test the efficiency of the steriliza- 

 tion from time to time by enclosing test objects in the center of 

 several packages. A convenient test object for surgical auto- 

 claves may be made by spreading spores of B. suhtilis or B. vulga- 

 tus on a sterile cover-glass and placing it in a sterile test-tube 

 plugged with cotton, and then drying the preparation thor- 

 oughly in the incubator for 24 hours. A number of these may be 

 prepared and subsequently kept in the refrigerator until used. 

 After the test object has been exposed in the autoclave, sterile 

 broth is added to the tube by means of a capillary pipette. The 

 development of culture from the spores indicates lack of effi- 

 ciency in the process of sterilization. 



Discontinuous or fractional sterilization by moist heat is em- 

 ployed to sterilize certain kinds of culture media, more especially 

 blood serum and gelatin, which are likely to be injured by heat- 

 ing above 100" C, or by prolonged heating. In this method the 

 medium is exposed to a temperature deemed sufiicient to kill the 

 vegetative forms of bacteria but not the spores. An interval is 

 then allowed for the generation of these spores, whereupon the 

 heat is again applied. This sequence is repeated until, according 

 to past experience, sterilization may be regarded as almost cer- 

 tainly accomplished. In the case of gelatin, steaming (100° C.) 

 for 15 to 20 minutes on three consecutive days is the usual 

 practice; with inspissated serum, exposure for i hour at 60° to 

 70° C. on six successive days is usually sufficient. These methods 

 are applicable only to media in which spores may germinate and 

 they may fail to sterilize even in case of such materials, especially 

 in the presence of rapidly growing spore-producing bacteria 



