Formation of cystlike structures by iron-limited Azotobacter vinelandii strain UW during prolonged storage

Abstract

Slant or liquid cultures of nonencapsulated Azotobacter vinelandii strain UW survived at least 2 years at 20 °C, whereas parallel cultures died after 6 months storage at 4 °C. Survival was decreased at 20 °C when the growth medium was acidic and it was the least at 4 °C when the growth medium was iron limited. Although strain UW was capable of forming cyclopropyl fatty acids and alkyl resorcinols, these indicators of encystment were neither abundant nor present, respectively, in 1.7-year-old cells. Only lauric acid was more abundant in lipid extracts from old cells than from vegetative cells or mature cysts. The old cells were very fragile and rapidly lost viability upon dilution or dehydration. Electron microscopy revealed the production of fragile cystlike structures in iron-limited medium after 1.7 years at 20 °C. These structures had a distinct exine layer composed of membranelike plates, a poorly defined intine, and an extremely dense central body lacking storage granules or internal membranes. Iron-sufficient cultures produced similar but free central bodies, although cystlike structures were rarely seen.