338 Journal of Agrictdiural Research voi. xvi, no. 12 



and unable to convert nitrate to free nitrogen. (3) A Gram-negative, 

 rapidly liquefying denitrifier, such as described by Ivchmann and Neu- 

 mann as Bacterium pyocyaneum. Whether these authors worked with 

 the true blue-pus organism or not, there seems to be an organism of this 

 description that is different from the true Ps. aeruginosa. Several such 

 cultures have been isolated in this laboratory, all of vv^hich fail to show 

 pyocyanin even when grown in nitrite broth (the method described by 

 Eisenberg {15, p. 470) as showing the production of this pigment to 

 advantage) and extracted with chloroform. These cultures have dif- 

 fered among themselves and may represent several varieties. Un- 

 doubtedly the B. pyocyaneus of Severin (45, 46), Jensen {24), and others, 

 who studied denitrifiers from manure and soil, was an organism (or 

 organisms) of this type rather than of the true pyocyaneus type. (4) 

 A nonliquefying denitrifier described by Van Iterson (25) as B. deni- 

 trofluorescens, which is probably distinct from the above and from the 

 following, although it has not been studied here. (5) Ps. putida, a 

 nonliquefying organism unable to denitrify. Although some writers 

 seem to think liquefaction an unsatisfactory basis for the separation of 

 these species, there seems no chance for reasonable doubt that an organ- 

 ism unable to liquefy after six months is different from the very rapid 

 Hquefiers studied in the present work. The difficulty in making this 

 distinction may perhaps be due to the failure to distinguish between 

 true liquefaction by the living cells and slow digestion of the gelatin 

 by enzyms liberated from the cells after death. 



Further investigation is necessary before it can be decided whether 

 these five types represent different varieties of the same species, five 

 separate species, or even five different type species about which distinct 

 groups of species (perhaps genera) should be gathered. 



CHARA.CTERISTICS OF TYPICAL FLUORESCENT ORGANISMS 



Morphology. — Small, short rods, not much over 0.5 /x, 01 perhaps 

 somewhat smaller; no spores; a few flagella in a tuft at one pole; Gram- 

 negative. (A few fluorescent spore formers have been described, and 

 Edson and Carpenter (13) mention a weakly fluorescent peritrichic rod; 

 but these are apparently unrelated organisms. The pyocyaneus type 

 has been described as Gram-positive.) 



Cultural characteristics. — Growth on agar smooth, soft to slimy; 

 on potato smooth, brownish, medium discolored. Nearly all other 

 cultural characteristics vary. 



Greenish fluorescence is the most striking characteristic of the entire 

 group, but it is not a constant characteristic. It is produced in some 

 media and not in others. Gessard {21, 22), Lepiere (ji), and Jordan 

 (25) have studied the ability of these organisms to cause fluorescence 

 with rather discordant results. They differ considerably in their con- 

 clusions as to the composition of the medium necessary for the produc- 



