30 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



E. When motile, with polar fla- 

 gella, frequently producing a 



fluorescent or a yellow pigment. IV. Pseudomonadaceae. 

 EE. Not as E. Fla.gella when pre- 

 sent, peritrichous V. Bacteriaceae. 



DD. Cells bent or spiral VI. Spirillaceae. 



I. FAMILY NITROBACTERIACEAE 



Bacteria belonging to this family are usually rod-shaped, 

 occasionally spherical. In some genera the cells are motile 

 and in others nonmotile. In a few genera there is a de- 

 cided tendency toward the formation of threadlike or 

 branched involution forms. Endospores ;are never pro- 

 duced. These bacteria are grouped together primarily 

 because they must grow in the presence of free oxygen, and 

 secure their growth energy by the direct oxidation of car- 

 bon, or hydrogen, or simple compounds of these elements 

 and of nitrogen. None produce disease, and with the ex- 

 ception of the genus which lives in the nodules on the roots 

 of leguminous plants, none are parasitic. 



The genera of sufficient agricultural interest to be dis- 

 cussed are Acetobacter, Nitrosomonas, Nitrobacter, Azoto- 

 bacter and Rhizobium. In addition to these genera three 

 others 1 have been described. 



Acetobacter. In this genus the organisms are rod- 

 shaped, frequently in chains, and usually nonmotile. The 

 bacteria grow on the surface of alcoholic solutions, securing 

 the energy for their growth usually by the oxidation of 

 alcohol to acetic acid, or occasionally by the oxidation of 

 sugars to simpler compounds. Involution forms are not 

 infrequent, occurring as filamentous, club-shaped, and even 

 branched cells. Organisms belonging to this genus con- 

 stitute the so-called mother of vinegar which forms over 



1 The genus Hydrogenomonas secures its energy by the oxidation 

 of free hydrogen, Methanomonas by the oxidation of methane, 

 Carboxydomonas by the oxidation of carbon monoxide. 



