Cysts of Azospirilla Under Various Cultural Conditions

Summary

Cyst formation and germination in A. brasilense ATCC 29145 and A. lipoferum ATCC 29707 was observed by using routine laboratory staining and phase contrast and electron microscopy. Different cultural conditions and carbon sources were tested for the induction of cyst formation. Various morphological forms were found to be resistant to desiccation for a period of more than six months. Even though morphologically and ultrastructurally, A. brasilense cysts obtained under nitrogen limiting conditions resemble Azotobacter vinelandii cysts in some respects, the exact sequence of events associated with encystation and germination is yet to be understood. Unlike A. vinelandii, A. brasilense is found to synthesize and excrete brown-black melanin-like granular pigments during encystation under nitrogen limiting conditions. An attempt to separate the different cyst forms by ultracentrifugation in a sucrose density gradient, yielded three different bands. Most of the dense cyst forms with aggregated pigmented granules were pelleted at the bottom of the 70% gradient.

A desiccation resistant form in A. lipoferum with abundant PHB granules, was obtained in a liquid medium containing fructose and KNO₃, but the formation of a definite cyst-like structure and melanin-like granules were not observed in either solid or semisolid medium. Thus the formation of cysts and melanin pigments appear to be controlled by genetic and envirmental factors.