The fine structure of Arthrobacter pascens and the development of mesosomes during the growth cycle

Actively growing cells of Arthrobacter pascens were sectioned and examined by electron microscopy. The cell wall, plasma membrane, mesosomes, and mode of division are typical of Gram-positive organisms. Mature mesosomes appear as a series of uniform, convoluted tubules (280–325 Å in diameter) bounded by a single unit membrane. The development of the mesosomes was followed throughout the growth cycle by thin section and negative staining techniques. Coccoid stationary phase cells (48 hours) contained a large number of simple membranous inclusions. On transfer to fresh media the cells undergo a 4-hour predivisional lag during which time the cells enlarge to pleomorphic rods. During this period only one to three developing mesosomes are noted per cell. By 4 to 5 hours when active division has been initiated, only mature vesiculotubular mesosomes are present, primarily at or near the site of septum formation. Rudimentary membrane inclusions begin to reappear at the end of active growth (7 to 8 hours) corresponding to the time of medium limitation. During the reversion of the pleomorphic rods to the coccoid stage, mature mesosomes disappear and only rudimentary profiles remain. The fact that the period required for the development of mesosomes coincides with the predivisional lag period of A. pascens suggests a definite role of these membranous intracellular organelles in the division process.