CYSTS OF AZOTOBACTER

Cells of the family Azotobacteriaceae are unusually variable in morphology although the vegetative cells generally appear as large, plump, rod to oval shaped cells at some time during the growth cycle. In several species, the vegetative cells may give rise to specialized spherical resting cells known as cysts. Each cyst is produced from a single vegetative cell. The cysts have a distinctive morphology, appearing as spherical cells with contracted cytoplasm and a double contoured thick wall. During cyst germination, a process somewhat analogous to spore germination, a young, vegetative cell emerges from the cyst structure leaving behind a residual portion of the cyst coat. Relatively little attention has been given to the cyst state even though the ability to form the structure has been established by Jensen (1954) as a prime criterion for taxonomic identification within the azotobacter family. In contrast, Bergey's Manual (Breed, Murray, and Smith, 1957) does not consider cyst formation in development of azotobacter taxonomy. Winogradsky (1938) has previously reported that cyst formation might be induced in some strains when a simple organic compound, such as ethanol or butanol, is supplied as the carbon source. He claimed cyst formation might be permanently suppressed by growth on glucose or sucrose. Subsequently, Pochon, Tchan, and Wang (1948) described the appearance of the nuclear apparatus of the azotobacter cell during growth, division, encystment, and germination. The purpose of this communication is to relate evidence characterizing the cyst not only as a cytological structure but also as a physiological entity.