Ultrastructural organization and development cycle of soil ultramicrobacteria belonging to the class Alphaproteobacteria

Gram-negative chemoorganotrophic soil ultramicrobacteria (UMB), strains NF1 and NF3, have been isolated. In their development cycle, the strains formed small coccoid cells of 400-800 nm and ultrasmall cells of 200-300 nm. Phylogenetically, the strains NF1 and NF3 belong to Alphaproteobacteria and are close to the type strain of the recently described species Kaistia adipata. The ultrastructure of UMB cells has been studied using ultrathin sections and freeze-fracturing. It has been shown that the structure of UMB cell walls is of the gram-negative type; the outer membrane and peptidoglycan layer are well differentiated. The cell surface has numerous protrusions (prosthecae) of conical or spherical shape filled with the contents of the periplasm. The formation of unusual cellular structures (not occurring in known free-living bacteria) is a feature of UMB; these include the following: (a) piles of rod-like subunits, ca. 30 angstrom in diameter and 150-250 angstrom in length; (b) long bunches (up to 300-400 angstrom) comprised of filamentous subunits; and (c) large electron-dense spherical bodies (up to 200-300 angstrom in diameter) localized in the periplasm. A distinctive feature of UMB is their ability to grow as facultative parasites on living cyanobacterial (CB) cells. In this case, three types of interaction between UMB and CB have been revealed: (1) adsorption of UMB cells on the surface of CB cells; (2) penetration of UMB into polysaccharide sheathes; and (3) penetration of UMB into CB cytoplasm. UMB cells have been shown to reproduce by budding, with buds (up to 2-3) located directly on the mother cell, without formation of intermediate hyphae.