METHODS OF STERILIZING. 199 



It has recently been shown that many bacteria under circumstances 

 otherwise favorable are killed by exposure to sunlight. 



Related Forms. According to our present ideas of classification the 

 bacteria form a somewhat isolated group, their nearest relatives being the 

 slitne-moulds {Myxomycetes) and especially the Mijxobacteria of Thaxter, on 

 the one hand, and the CyanopJiycece the "blue-green" or "fission" algae 

 on the other. Neither of these, however, need be considered here. 



Why Bacteria are Considered to be Plants. The bacteria were 

 formerlj regarded as infusorial aninialcules (because they abound 

 in infusions, and many have the power of active m(jveiiient). 

 They are still regarded by some as animals. Most ])iologists, 

 however, regard them as plants, because they can live without 

 proteid food (which no animal, so far as known, can do), and 

 because in their method of reproduction and in their growtli- 

 forrns they more nearly resemble the Cydnojjhycece than they do 

 any animal. There is also reason to think that their cell- wall is 

 composed of cellulose. 



Bacteria and their Environment. The relations of organisms to tem- 

 perature and moisture have been more thoroughly studied for the bacteria 

 than for any other unicellular organisms on account of tlieir bearing upon 

 modern theories of infectious disease. In general, temperatures above 

 70° C. are fatal to ordinary bacteria. In general, as is shown by common 

 experience with the "keeping" of foods in cold storage, bacteria are be- 

 numbed but not killed by moderate cold. But in special cases, particu- 

 larly when they are dried slowly, bacteria may withstand even prolonged 

 boiling or freezing or the action of poisons, so that the removal or destruc- 

 tion of the last traces of bacterial life is often very difficult. 



Sterilization and Pasteurizing. The removal of all traces of living 

 matter from any substance, and in particular the destruction of all bac- 

 terial life, is known as sterilization. To free organic substances from the 

 larger forms of life is a comparatively easy matter; but bacteria are so 

 minute and so ubiquitous that scarcely anything is normally free from 

 them, and they are so hardy that it is exceedingly difficult to destroy them 

 without at the same time destroying the substances which it is desired to 

 sterilize. They are not normally present in the living tissues of plants or 

 animals which are sealed against their entrance by skins or epitiielia ; but 

 after these are broken or cut open (as in wounds) bacteria speedily invade 

 the tissues. Ordinary earth, as has been said above, teems with bacteria, 

 which are easily dried and disseminated in dust driven by the wind. What- 

 ever is in contact, therefore, with the air or exposed to dust or dirt is never 

 free from bacteria, and meat or milk which in the living animal are nor- 

 mally sterile, if exposed to the air soon become contaminated with bacteria. 

 Sterilization (such as is required to preserve canned goods, for example) 



