460 DENITRIFYING BACTERIA 



more broken up and mingled with air the soil the better the process 

 of oxidation of the ammonia salts. 



The chemical action of these bacteria in the soil has been known 

 for a long time, but the greater part of more accurate knowledge is due 

 to Winogradsky, who was the first to devise methods of obtaining 

 them in pure cultures. Winogradsky's first method consisted in the 

 use of a silicon-jelly (water-glass jelly), difficult to prepare. The 

 formulae for his later fluid culture media, which are easier to prepare, 

 were given in Chapter X. The pure cultures obtained enabled 

 Winogradsky to show that the nitrifying bacteria consist of two 

 groups, one oxidizes ammonia compounds into nitrites, the other 

 group changes nitrites into nitrates. 



Nitrosomonas and Nitrosococcus. These are the bacteria of the first 

 group. Their oxidizing action takes place according to the chemical 

 formula (NH 4 ) 2 O -f 3O 2 = N 2 O 3 + 4H 2 O. Nitrosomonas Europea 

 is found in soil in Europe. It is a short rod 1.2 to 1.8 micra long, 

 provided with a short flagellum, and lively motile. Nitrosomonas 

 Javanica was isolated from soil in Batavia, it is apparently round 

 and coccus-like, has a diameter of 0.5 to 0.6 micron, and has a very 

 long flagellum (up to 30 micra). Nitrosomonas Japonica and N. 

 Africana are like the European variety. Nitrosococcus has been 

 found in South America and Australia. They are large cocci, 1.5 to 1.7 

 micra in diameter, not motile, and possess no flagella. The organisms 

 of this group, according to Winogradsky, are easily perishable when 

 desiccated. The flagellate nitrosomonas in young cultures swarm 

 around in a lively manner and impart to the medium an opalescent 

 character; later they sink to the bottom, unite in zoogleal masses, and 

 form a grayish, gelatinous, cloudy sediment. The best method to recog- 

 nize the finer details of the structure of these organisms consists in treat- 

 ing cover-glass preparations with Gram's iodine solution. All varieties 

 of nitrosomonas do not show the swarming stages, some form zoogleal 

 masses from the start, and remain in this stage permanently. 



Nitrobacteria. These form nitrates from nitrites, according to the 

 formula N 2 O 3 + 2O = N 2 O 5 . They are small, oval, or pear-shaped 

 bodies, 0.5 micron long, 0.15 to 0.25 micron wide. They form in 

 fluid media a thin, shiny pellicle, firmly adherent to the vessel wall. 



The two groups of nitrifying bacteria in soil act with such harmony 

 that it is, as a rule, impossible to find any nitrites; nitrates alone can 

 be discovered. 



DENITRIFYING BACTERIA. 



While the nitrifying bacteria have the power to oxidize nitrogen 

 compounds like ammonia to lesser or higher stages of oxidization, 

 there are other bacteria which have -the property of reducing oxygen 

 containing nitrogen compounds into lower stages of oxidation or even 

 ,to take up all their oxygen and set the nitrogen free. Such organisms 

 are called denitrifying bacteria in a general sense. Properly, however, 



