146 BACTERIOLOGY. 



2. Saprophytic, those subsisting upon dead organic matter. 



3. Aerobic, those requiring oxygen. 



4. Anaerobic, those subsisting without oxygen. 



5. Facultative, those that are anaerobic, but may become 

 aerobic; and those saprophytic that may become parasitic. 



6. Pathogenic, those producing diseases in man and animals. 



7. Chromogenic, those producing pigments. 



8. Zymogenic, those producing fermentations. 



9. Motile, those having independent motion. 



10. Liquefying, those that produce a ferment which liquefies 

 solid nutrient gelatin. 



PRODUCTS. 



Bacteria are powerful agents in breaking up complex chemical 

 substances into simple ones. They attack the tissues of living 

 organisms and produce the toxins and ptomaines which cause the 

 symptoms of destructive diseases. They assist animals in destroy- 

 ing dead organic matter, else the earth would be covered with 

 its dead. Thus, by their cooperation with other humble or- 

 ganisms, they make life upon the earth possible. They assist in 

 manufacturing articles of economic value, such as butter, cheese, 

 etc. They convert starch into sugar, dissolve cellulose, change al- 

 bumin from an insoluble to a soluble form, convert urea into am- 

 monium carbonate, produce lactic acid in milk, produce acetic acid 

 from alcohol, and manufacture bright pigments. 



The pigment formed by Badllis prodigiosus was once supposed 

 by the superstitious to be blood formed by supernatuTal power. Cer- 

 tain gaseous products are sometimes evolved by the action of bac- 

 teria. The foul odors from putrefying bodies may be ascribed to 

 their action. Among the common gases which are liberated from or- 

 ganic substances by the agency of bacteria may be named nitrogen, 

 carbon di-oxide, and sulphuretted hydrogen. The demonstration of 

 gaseous products may be made by the growth of certain species in 

 nutrient media in a saccharometer. Indol is produced by some spe- 

 cies. The test for this substance is made by adding to the peptone 

 solution in which the bacteria have been vegetating for twenty-four 

 hours about ten drops of c. p. sulphuric acid. The development of a 



