BACTERIAL GENERA 321 



Azotobacter. — Relatively large rods, or even cocci, sometimes almost yeast-like in 

 appearance, dependent primarily for growth energy upon the oxidation of carbohydrates. 

 Motile or non-motile ; motile forms possess a tuft of polar flagella. Obligate aerobes ; 

 usually growing in a film upon the surface of the culture medium. Capable of fixing 

 atmospheric nitrogen when grown in solutions containing carbohydrates and deficient 

 in combined nitrogen. 



Type species. Azotobacter chroococcum. 



Rhizobium. — Minute rods, motile when young. Speciahzed forms abundant and char- 

 acteristic when grown under suitable conditions. Obligate aerobes, capable of fixing 

 atmospheric nitrogen when grown in the presence of carbohydrates and in the absence 

 of compounds of nitrogen. Produce nodules upon the roots of leguminous plants. 



Type species. Rhizobium leguminosarum. 



Pseudomonas. — Rod-shaped organisms, usually motile, by means of polar flagella. 

 Generally Gram-negative. Non-sporing. Aerobic ; some species are facultative anae- 

 robes. Frequently produce a water-soluble pigment, which is yellow, green, blue, purple, 

 or brown in colour, and which diffuses through the medium. Some species form a non- 

 diffusible yellow pigment, and some species are photogenic. Fermentation of carbo- 

 hydrates as a rule not active. Frequently gelatin-Uquefiers, and active ammonifiers. 

 Common in soil and water. Many yellow species are plant parasites. 



Type species. Pseudomonas 'pyocyanea. 



(On grounds of priority the American Committee recommend that this organism should 

 be called Ps. aeruginosa.) 



Vibrio. — Short, curved, rigid rods, arranged singly or united into S-forms or spirals. 

 Motile by a single polar flagellum, which is usually relatively short. (Some species may 

 have two or three polar flagella.) Non-sporing. Usually Gram-negative. Aerobic and 

 facultatively anaerobic. Many species liquefy gelatin and are active ammonifiers. Com- 

 monly found in water. Most species are saprophytic ; a few are pathogenic to man. 



Type species. Vibrio choleroe. 



Spirillum. — Rigid rods of spiral form, varying considerably in the number, length 

 and breadth of the spirals. Usually motile by means of a tuft of polar flagella (5 to 20)- 

 The flagella occur at one or both poles ; their number varies greatly, and is difficult to 

 determine, since in stained preparations several are often united into a common strand. 

 Generally Gram-positive. Some species form a reddish-yellow or greenish-yellow pigment. 

 Found in water or putrid infusions. 



Type species. Spirillum undula. 



Neisseria. — Gram-negative cocci, usually arranged in pairs. Strict parasites, often 

 growing poorly on ordinary media, but growing well on serum media. Frequently patho- 

 genic. 



Type species. Neisseria gonorrhoecB. 



Leuconostoc. — Spherical or ovoid cells, arranged in pairs and chains ; the cocci are 

 surrounded by a gelatinous envelope, which unites them into zooglceal masses. Usually 

 Gram-positive, but decolorize easily. Saprophytes, usually growing in cane-sugar solutions. 



Type species. Leuconostoc mesenteroides. 



Streptococcus. — Spherical or ovoid cells, arranged in short or long chains, or in pairs. 

 Non-sporing, usually non-motile. Most species Gram-positive. Some species form 

 capsules. Growth tends to be relatively shght on artificial media, and some species grow 

 poorly in the absence of added native protein. Several species produce characteristic 

 changes in media containing blood. Various carbohydrates are fermented with the pro- 

 duction of acid. Most species fail to liquefy gelatin. Most species are aerobic and 

 facultatively anaerobic ; some are anaerobic. Many species are normally parasitic on 

 man or animals. Some species are highly pathogenic, and some produce soluble toxins. 



Type species. Streptococcus pyogenes. 



P.B. M 



