LIFE-CYCLES IN BACTERIA 



131 



Fig. 58 

 LIFE-CYCLE OF AZOTOBACTER 



An extraordinary degree of complexity is found in the life-cycle of the nitrogen-fixing 

 bacterium Azotobacter. Not only does this organism produce spore-like cysts (not illustrated 

 here), but two distinctly different types of gonidia. 



The vegetativ'e cell [A] becomes packed with tiny replicas of itself (C), or with motile 

 gonidia (G, H). In both cases, the cycle is initiated by the production, within the mother-cell 

 of an undifferentiated mass of Gram-positive material [B] ; traces of Gram-positivity may 

 be retained to a later stage of gonidium production. 



The large gonidia (C, D, E) grow up directly into typical vegetative cells, and are retained 

 within the remains of the mother-cell wall. The small, motile gonidia (G, H, I) may reproduce 

 for several generations as small. Gram-negative bacteria. 



Fig. 57 



BACTERIAL GONIDIA 



Bacterial gonidia are also well seen in A:otobactey, which produces more than one kind, 

 and in some spiral organisms. 



(1) Azotobacter mother cells showing large and small gonidia (compare Fig. 58). The 

 large types grow directly into vegetative cells, the small gonidia may reproduce as such for 

 several generations. (Gram's stain, x 3000). 



(2) Small gonidium of Azotobacter. It resembles those of Rhizobium but has more fiagella 

 and is less nearly spherical. (Electron micrograph, gold-shadowed, X 16,000). 



(3) Electron micrograph of Spirilliiin sp. showing attached cysts, from which the gonidia 

 are produced. ( ■ 6000). 



(4) ^Mature cyst with fiagella still attached. ( ■ 12,000). 



(5) Developing gonidia, of Spirillum, each with a single polar flagellum. Note the 

 liU^pharoplasts. ( ■ 12,000). 



