A new lineage of halophilic, wall-less, contractile bacteria from a brine-filled deep of the Red Sea

Abstract

A novel strictly anaerobic bacterium designated strain SSD-17B(T) was isolated from the hypersaline brine-sediment interface of the Shaban Deep, Red Sea. Cells were pleomorphic but usually consisted of a central coccoid body with one or two "tentacle-like" protrusions. These protrusions actively alternated between a straight, relaxed form and a contracted, corkscrew-like one. A peptidoglycan layer was not detected by electron microscopy. The organism forms "fried-egg"-like colonies on MM-X medium. The organism is strictly anaerobic and halophilic and has an optimum temperature for growth of about 30 to 37 degrees C and an optimum pH of about 7. Nitrate and nitrite are reduced; lactate is a fermentation product. The fatty acid profile is dominated by straight saturated and unsaturated chain compounds. Menaquinone 4 is the major respiratory quinone. Phylogenetic analysis demonstrated strain SSD-17B(T) represents a novel and distinct lineage within the radiation of the domain Bacteria. The branching position of strain SSD-17B(T) was equidistant to the taxa considered to be representative lineages of the phyla Firmicutes and Tenericutes (with its sole class Mollicutes). The phenotypic and phylogenetic data clearly show the distinctiveness of this unusual bacterium, and we therefore propose that strain SSD-17B(T) (= DSM 18853 = JCM 14575) represents a new genus and a new species, for which we recommend the name Haloplasma contractile gen. nov., sp. nov. We are also of the opinion that the organism represents a new order-level taxon, for which we propose the name Haloplasmatales.

FIG. 1.

Series of consecutive light micrographs (phase contrast) of strain SSD-17B^T, illustrating the contraction of the “tentacle-like” protrusions. The time span between the first and last micrographs was about 8 s.

FIG. 2.

Scanning electron micrographs showing examples of morphological variants of the central body and protrusions of strain SSD-17B^T. (A) Central body with two protrusions (relaxed state). (B) Central body with two coiled protrusions (contracted stage). (C) Detail of coiled protrusions showing helical winding; occasionally thin filaments are visible (arrows). (D) Protrusions exhibiting lateral polyp-like protuberances. (E) Central body exhibiting protrusions with chains of microspheres. (F) Terminal bacterial body with a helical protrusion (one coil) and several extended polyp-like secondary protuberances, frequently with terminal bulbous structures of various diameters.

FIG. 3.

Transmission electron micrographs of ultrathin sections of morphological variants of protrusions from strain SSD-17B^T. (A) Central body with one “tentacle-like” protrusion (relaxed state); the surface of the cell is coated by a diffuse electron-dense layer (arrows). (B) Detail of “tentacle-like” protrusions (relaxed state); the surface of the cell is coated at both sides by a diffuse electron-dense layer (arrows). (C) Longitudinal section of a helical protrusion (contracted stage); several windings are cross sectioned. (D to F) Cross sections of helical protrusions. The outer side of the rings (C), lateral protuberances (D), and helixes (E and F) are “rough” (black arrows) compared to the inner surface (open arrows). The cytoplasmic membrane can be recognized locally (C, framed area; F, circle).

FIG. 4.

Phylogenetic tree based on a comparison of the 16S rRNA gene sequences of strains SSD-17B^T and representatives of the main lineages within the domain Bacteria. Additional species of the phyla Firmicutes and Tenericutes were included to establish the relationship of this novel lineage to these previously described phyla. The tree was reconstructed from distance values using the neighbor-joining method. Scale bar, 5 inferred nucleotide substitutions per 100 nucleotides.