Kingdom Mychota [ 23 



have prevented the formulation of a theory, once entertained, that all bacteria 

 require neutral media, and are accordingly incapable of causing diseases of plants. 

 The species of Pectobacterium, as P. carotovorum, cause rots. Those of Agro- 

 bacterium cause galls; A. tumefaciens causes crown gall of many plants. 



Rhizobium includes the species which produce little galls ("nodules") on the 

 roots of plants and which benefit their hosts by fixing nitrogen. The best known 

 hosts of Rhizobium are plants of the family Leguminosae; the relationship between 

 Leguminosae and Rhizobium is a classic example of symbiosis. There are several or 

 many species of Rhizobium, scarcely distinguishable morphologically, but living on 

 different groups of legumes. The race which was first recognized and isolated, R. 

 Leguminosarum Frank 1890 [Schinzia Leguminosarum Frank 1879; Bacillus Radicic- 

 ola Beijerinck 1888) is that which attacks plants of the pea tribe. Bewley and Hutch- 

 inson (1920) accounted for the variety of forms which Rhizobium can assume. In 

 the roots of plants it occurs as involution forms. In culture, it is a peritrichous rod, 

 but the flagella are often reduced to one, and it has been confused with the mono- 

 trichous bacteria (Conn and Wolfe, 1938). 



Family 10. Azotobacteriacea [Azotobacteriaceae] Bergey, Breed, and Murray in 

 Bergey's Manual 5th ed., preprint, v and 71 (1938). These are the organisms which 

 were originally isolated by Beijerinck (1901) by inoculating with garden soil shallow 

 layers of a nitrogen-free nutrient solution containing mannite. The commonest species, 

 Azotobacter Chroococcum, is usually seen as ellipsoid cells, as much as \\x thick and 

 7[J. long, solitary, with peritrichous flagella, or forming non-motile clusters imbedded 

 in a heavy capsule. Beijerinck observed the occurrence of globular involution forms 

 as much as 15^ in diameter. Lohnis and Smith (1916) made a thorough study of 

 variations in form, and reported a remarkable variety of other stages, including the 

 symplasm. 



The Pasteurellacea and Rhizobiacea are apparently reasonably close allies of 

 the Achromobacteriacea. The Azotobacteriacea stand somewhat apart. The remain- 

 ing families of the present order are more definitely distinct, being marked by mono- 

 trichous or lophotrichous flagella. 



Family 11. Spirillacea [Spirillaceae] Migula 1894. Family Pseudomonadaceae 

 Winslow et al. in Jour. Bact. 2: 555 (1917). Rods and spirals, Gram negative, mono- 

 trichous or lophotrichous; not producing much acetic acid, and mostly heterotrophic. 



Pseudomonas is a numerous genus of rods which may or may not produce a fluores- 

 cent pigment soluble in water; they do not produce a yellow pigment which is in- 

 soluble in water. The original species, P. aeruginosa, was isolated from pus, in which 

 it produces a blue-green discoloration; it is by itself weakly if at all pathogenic. 

 Other species have been isolated from fresh and salt waters and brines; the bacteria 

 which produce phosphorescence on salt fish are of this genus. Many further species 

 arc: pathogenic to plants, producing chiefly leaf spots. 



Phytomonas Bergey et al. 1923 {Xanthomonas Dowson 1948) includes numerous 

 plant pathogens which in culture produce an insoluble yellow pigment; among them 

 are the causes of cabbage rot, walnut blight, and leaf spots on many plants. 



Pacinia Trevisan 1885 includes monotrichous curved rods. The type species P. 

 cholerae-asiaticae is the cause of Asiatic cholera. Among numerous other species 

 the majority are harmless saprophytes in waters. Recent authorities have treated the 

 cholera organism as the type of the genus Vibrio Miiller (1773); their action is an in- 

 tolerable falsification of the usage of a full century preceding the discovery of the 

 cholera organism. 



