32 BACTERIA. 



In sterilization, disinfection, and antisepsis, means are employed 

 by which the Bacteria are killed, or checked in their development, 

 for instance, by heat (ignition, cooking, hot vapours, hot air, etc.), 

 or poisons (acids, corrosive sublimate). The process of preserv- 

 ing articles of food, in which they are boiled and then hermeti- 

 cally sealed, aims at destroying the Bacteria, or the spores of 

 those which already may be present in them, and excluding 

 all others. 



As the Bacteria are unable to assimilate carbon from the car- 

 bonic acid of the air, but must obtain it from the carbon- com- 

 pounds already in existence in the organic world, they are either 

 saprophytes or parasites. Some are exclusively either the one or 

 the other, obligate saprophytes or parasites. But there are 

 transitional forms among them, some of which are at ordinary 

 times saprophytes, but may, when occasion offers, complete their 

 development wholly or partly as parasites facultative parasites ; 

 others are generally parasitic, but may also pass certain stages of 

 development as saprophytes facultative saprophytes. 



All chlorophyll-free organisms act in a transforming and dis- 

 turbing manner on the organic compounds from which they obtain 

 their nourishment, and while they themselves grow and multiply, 

 they produce, each after its kind, compounds of a less degree of 

 complexity, i.e. they produce fermentation, putrefaction, sometimes 

 the formation of poisons, and in living beings often disease. 



Those organisms which produce fermentation are called ferments 

 this word, however, is also employed for similar transformations 

 in purely chemical materials (inorganic ferments or enzymes). 

 Many organic (" living ") ferments, among which are Yeast- 

 cells and Bacteria, give off during their development certain 

 inorganic and soluble ferments (enzymes) which may produce 

 other transformations without themselves being changed. Dif- 

 ferent organisms may produce in the same substratum different 

 kinds of transformation ; alcoholic fermentation may for instance 

 be produced by different species of Fungi, but in different pro- 

 portions, and the same species produces in different substrata, 

 different transformations (e.g. the Vinegar-bacteria oxydize diluted 

 alcohol to vinegar, and eventually to carbonic acid and water). 



In the study of Bacteria it is absolutely necessary to sterilize the vessels 

 employed in cultivation, the apparatus, and nutrient solutions, i.e. to free them 

 from Bacteria germs and also to preserve the cultures from the intrusion of any 

 foreign germs ("pure-cultures"). A firm, transparent, nutritive medium is 



