Ultrastructure of Resting Cells of Some Non-Spore-Forming Bacteria

Abstract

Using electron microscopy (ultrathin sections and freeze-fractures), we investigated the ultrastructure of the resting cells formed in the cultures of Micrococcus luteus, Arthrobacter globiformis, and Pseudomonas aurantiaca under conditions of prolonged incubation (up to 9 months). These resting cells included cyst-like forms that were characterized by complex cell structure and the following ultrastructural properties: (i) a thickened or multiprofiled cell wall (CW), typically made up of a layer of the preexisting CW and one to three de novo synthesized murein layers; (ii) a thick, structurally differentiated capsule; (iii) presence of large intramembrane particles (d = 180-270 A), occurring both on the PF and EF sides of the membrane fractures of M. luteus and A. globiformis; (iv) a peculiar structure of the cytoplasm, which was either fine-grained or lumpy (coarse-grained) in different parts of the cell population; and (v) a condensed nucleoid. Intense formation of cyst-like cells occurred in aged (2- to 9-month-old) bacterial cultures grown on diluted complex media or on nitrogen-, carbon-, and phosphorus-limited synthetic media, as well as in suspensions of cells incubated in media with sodium silicate. The general morphological properties, ultrastructural organization, and physiological features of cyst-like cells formed during the developmental cycle suggest that constitutive dormancy is characteristic of non-spore-forming bacteria.