L6hnis and Smith (1923) pointed out that not less than seven different morphological types can be developed and stabilized from Azotobacter cultures. This extreme transmutation of cells has been questioned by subsequent investigators (Lewis, 1937, 1941). Nevertheless, peculiar morphological phases do occur in Azotobacter cultures and no satisfactory explanation has been given as to their function. Several new techniques, as well as some new concepts, have been developed since these earlier workers debated the significance of the morphological cell types in Azotobacter species. The purpose of the investigation now being reported was to see whether phase microscopy, electron microscopy, and nuclear staining might add information concerning the cytology of Azotobacter. Also, recent discussions of sexuality and peculiar reproductive processes in bacteria (Dienes, 1946; Lederberg, 1948) encouraged the authors of this paper to search for nuclear arrangements among large bacteria, such as in the genus Azotobacter. MATERIALS AND METHODS About 20 strains of organisms representing all species of the genus Azotobacter were observed. One culture, however, was chosen for this report so that results would be comparable. The organism selected for this study was Azotobacter agile (University of Illinois strain ZN 350). This species was grown on several different media, but the results in this report are from cells grown on Ashby's nitrogen-free medium. Cultures ranging in age from 3 hours to 60 days were examined. Each culture was observed with phase microscopy after light nigrosin background staining, according to a method previously described (Eisenstark and McMahon, 1949), to determine morphology and to view internal structures at different ages. Electron microscopy was done with the R.C.A. type EMC instrument. Cultures were prepared by suspension of cells in distilled water followed by centrifugation and resuspension. With full recognition that this method may produce slightly distorted cells,

No abstract