162 BACTERIOLOGY. 



to sterilize without materially altering* the physical or chem- 

 ical character of the medium. An exposure of 15 minutes to 

 steam will surely cause the destruction of all the vegetating 

 organisms that may be present. The spores, however, sur- 

 vive. If the medium is now set aside at the ordinary room 

 temperature for 24 hours, all or nearly all of the spores will 

 germinate, and thus give rise to the growing, vegetating 

 type, The highly resistant spore has transformed itself 

 into a feebly resistant organism. An exposure, on the sec- 

 ond day, to steam for 15 minutes will, therefore, result in 

 the destruction of all these young cells. The material is 

 set aside for another 24 hour period, in order to give any 

 remaining spores an opportunity to germinate. A third ex- 

 posure to steam for 15 minutes, on the third day, will ren- 

 der the medium sterile. 



This method is known as fractional sterilization, and is 

 the one commonly employed in rendering cultural media 

 sterile. The exposures of 15 minutes each, as just described, 

 are sufficient in the case of gelatin. When the medium is in 

 bulk, or if, as in the case of agar, it does not melt readily, 

 it will be necessary to lengthen the exposure to at least 30 

 minutes on each day. The gelatin tubes are to be exposed 

 to steam for 15 minutes, as soon as the tubes are filled. 

 Then on the second and on the third day, this exposure to 

 steam for a like interval is repeated. Nutrient gelatin is 

 sterilized, fractionally, by exposure to steam-heat for 15 minutes 

 on each of three consecutive days. 



Sometimes, bacteria develop in tubes which have apparently 

 been subjected to proper sterilization. Spores, if clumped together 

 in a mass, will not all develop at the same time. Moreover, the 

 spores in the interior of such a mass will resist the action of heat 

 for a considerable length of time. Thus, if a growth of the potato 

 bacillus, rich in spores, be gathered into a little mass, the size of a 

 grain of wheat, and placed in a tube of bouillon, it can be exposed to 

 active steam for 8 or 10 hours, or even long-er, without destroying 

 all the organisms present. The same material stirred up so as to 



