Bacterial Variation: Formation and Fate of Certain Variant Cells of Bacillus megatherium

Abstract

The formation, development and fate of individual cells living and growing on hanging blocks of nutrient agar were studied. Serial camera-lucida drawings and photo-micrographic records were kept. Variant cells (coccoid, globular and balloon-like, pointed, branched, clubbed, or filamentous with bulbous swellings) did not appear in cultures until growth and reproduction had practically ceased. In the environment responsible for their formation they simply increased in size up to a certain point and then remained dormant until death and autolysis ensued. When transferred to new and wholesome surroundings while still viable, they invariably returned to the "normal" rod form. The 3 prongs of Y-shaped organisms frequently elongated and broke up into ordinary bacilli. Clubbed individuals also increased in size and segmented into rods. The cells which developed from the swollen ends of the clubs were considerably thicker than those which formed from the unswollen ends. The thick rods curled and twisted in an unusual fashion. A peculiar winding-together or braiding of long filaments was observed. The factors responsible for it appeared to be purely mechanical. Cells of unusual shape seemed to form in response to unfavorable environmental conditions, and the stimulation of cellular variation by growth crowding was clearly evident. The aberrant cells studied seemed to be simple adaptive variants. They did not appear to represent stages in an orderly reproductive cycle. No evidence was obtained which suggested the existence of viable granules.