Layne, Joseph S. (University of Mississippi School of Medicine, Jackson), and Emmett J. Johnson. Natural factors involved in the induction of cyst formation in Azotobacter. J. Bacteriol. 87:684-689. 1964.-Recent reports have stimulated an interest in the natural factors responsible for the induction of a resistant form in the genus Azotobacter; therefore, an investigation of these factors was undertaken. A. agilis (vinelandii) was grown in Burk's nitrogen-free medium from which one mineral, or a combination of two minerals, was omitted. These media were made with triple-distilled, deionized water and were sterilized by filtration. After 18 hr of growth, up to 75% of the cells present were resistant forms if a single mineral was omitted, and up to 95% if two minerals were omitted. Formation of resistant forms was also induced by reduction of the sucrose content of the medium. When the sucrose content was reduced from the normal 0.5% to 0.1%, up to 60% resistant forms were obtained. Further reduction of the sucrose content to 0.05% brought about the formation of up to 80% resistant forms. Electron micrographs show that cysts grown in liquid media in response to mineral deficiencies lack the characteristic exine and intine of the cysts grown on the more commonly used solid medium containing butanol as the carbon source.