THE FUNGI 367 
faculative parasites, while many others, like the majority of the 
common soil and water bacteria, are restricted to dead organic 
matter and so are obligate saprophytes. Most of the parasitic 
bacteria are faculative saprophytes, since they can be grown upon 
artificial culture media. The leprosy bacillus is one of the few 
examples of parasitic bacteria which has resisted attempts to 
grow it upon artificial culture media and so is an obligate parasite. 
NitTRIFICATION.—There occur a group of soil bacteria called 
nitrifying bacteria which thrive in the presence of simple inorganic 
salts and in the entire absence of organic matter. Some of these 
(Nitrosomonas and Nitrosococcus) oxidize the ammonia of the soil 
to nitrous acid while others (Nitrobacter) convert the nitrous acid 
to nitric acid. The nitric acids reacts with alkaline soil con- 
stituents to form nitrates which pass into solution in the soil 
water and are utilized by higher plants in their growth. This 
process is called nitrification. 
‘DENITRIFICATION.—Denitrification is a process carried on by 
denitrifying bacteria including Pseudomonas fluorescens, Ps. pyocyanea, 
and the colon and typhoid bacilli. Many of the denitrifying 
organisms flourish in “‘sour”’ soil, in the absence of air, and obtain 
oxygen for their respiration by reducing the nitrates and nitrites 
to gaseous oxides of nitrogen or to free nitrogen, liberating the 
gas into the atmosphere. 
GrowtH Propucts.—The products resulting from bacterial 
growth include pigments, enzymes, acid and alkali, putrefactive 
substances, ptomaines and toxins. 
REPRODUCTION, —Bacteria multiply and reproduce them- 
selves by ¢ by cleavage or fission. A young individual increases in 
size up to the limits of the adult form, when by simple cleavage at 
right angles to the long axis, the cell divides into two individuals. 
MorpHo.ocy DuE To CLEAVAGE.—According to limitations 
imposed by cleavage directors, the spherical forms called micro- 
cocci or cocci assume a chain appearance, or a grape cluster- 
like appearance, or an arrangement in packets or cubes having 
three diameters. These aggregations are the basis for the follow- 
ing genera of spherical-shaped bacteria: 
Diplococcus (plural, diplococci), occurring in twos, as organisms 
producing meningitis, pneumonia and gonorrhea. 
