Effects of Physical Forces on Bacteria 157 



waste products, many cells cannot efficiently metabolize, and the 

 weaker members of the species give up the struggle. 



Lipman exposed organisms to a temperature of minus 270° C. 

 without causing death of all the cells. Absolute zero is only three 

 degrees colder than this, and at — 273° C. all chemical activity is 

 believed to cease. 



While extremely cold temperatures do not ordinarily destroy 

 bacterial cultures in short periods of time, the combination of re- 

 peated freezing and thawing is very harmful to microbes. Experi- 

 ments with Sahnonella typhosa bear out these statements. 



ALTERNATE FREEZING AND 

 FROZEN SOLID THAWING 



Before freezing 40,896 Before freezing 40,896 



Frozen 24 hours 29 . 780 Frozen 3 times 90 



Frozen 3 days 1 , 800 Frozen 5 times 



Frozen 4 days 950 Frozen 6 times 



Lyophilization. If an aqueous suspension of bacteria is frozen 

 rapidly by such materials as solid carbon dioxide (dry ice), and 

 the frozen product is dried by high vacuum sublimation, the dried 

 bacteria kept sealed in this vacuum remain viable for years. This 

 is called the lyophil process, or lyophilization. Lyophilic is a 

 designation given to colloidal substances, such as proteins, which 

 have an affinity for water. Lyo comes from the Greek and means 

 to dissolve, while phihis means loving. When literally translated 

 the word means to dissolve readily. One of the earliest devices 

 for drying bacteria was the Chryochem Process of Flosdorf and 

 Mudd. 



Lyophilization has many practical applications. The American 

 Type Culture Collection, the central agency from which pure 

 cultures of organisms may be purchased, maintains its stock culture 

 collection in part by lyophilizing the organisms. It is possible to 

 keep many bacteria for a number of years in their original state 

 when they are so treated. We should consider this technic as a 

 combination of freezing and drying, rather than either process 

 alone. 



