Teredinibacter waterburyi sp. nov., a marine, cellulolytic endosymbiotic bacterium isolated from the gills of the wood-boring mollusc Bankia setacea (Bivalvia: Teredinidae) and emended description of the genus Teredinibacter

Abstract

A cellulolytic, aerobic, gammaproteobacterium, designated strain Bs02^T, was isolated from the gills of a marine wood-boring mollusc, Bankia setacea (Bivalvia: Teredinidae). The cells are Gram-stain-negative, slightly curved motile rods (2-5×0.4-0.6 µm) that bear a single polar flagellum and are capable of heterotrophic growth in a simple mineral medium supplemented with cellulose as a sole source of carbon and energy. Cellulose, carboxymethylcellulose, xylan, cellobiose and a variety of sugars also support growth. Strain Bs02^T requires combined nitrogen for growth. Temperature, pH and salinity optima (range) for growth were 20 °C (range, 10-30 °C), 8.0 (pH 6.5-8.5) and 0.5 M NaCl (range, 0.0-0.8 M), respectively when grown on 0.5 % (w/v) galactose. Strain Bs02^T does not require magnesium and calcium ion concentrations reflecting the proportions found in seawater. The genome size is approximately 4.03 Mbp and the DNA G+C content of the genome is 47.8 mol%. Phylogenetic analyses based on 16S rRNA gene sequences, and on conserved protein-coding sequences, show that strain Bs02^T forms a well-supported clade with Teredinibacter turnerae. Average nucleotide identity and percentage of conserved proteins differentiate strain Bs02^T from Teredinibacter turnerae at threshold values exceeding those proposed to distinguish bacterial species but not genera. These results indicate that strain Bs02^T represents a novel species in the previously monotypic genus Teredinibacter for which the name Teredinibacter waterburyi sp. nov. is proposed. The strain has been deposited under accession numbers ATCC TSD-120^T and KCTC 62963^T.

Keywords: Bankia setacea; Teredinibacter; Teredinidae; bivalve; cellulolytic symbiont; shipworm.

Fig. 1.

Cells of Teredinibacter waterburyi Bs02^T viewed by transmission electron microscopy. (a) Longitudinal and (b) transverse sections showing the nucleoid (N), outer membrane (OM), inner membrane (IM). (c) Negatively-stained cell showing the presence of a single polar flagellum. (d) Scanning transmission electron microscope (sTEM) image of a whole mounted cell of T. waterburyi Bs02^T, (e, f) the same cell imaged by energy dispersive X-ray spectroscopy (EDS) analysis, showing (e) phosphorus and (f) sodium distribution. P, putative polyphosphate granule within the cell. Na, residual sodium precipitated from the medium. Cells were grown to mid-exponential phase in liquid SBM with 0.5 % galactose and 0.025 % NH₄Cl at 20 °C. Bars, 100 nm (a and b), 500 nm (C), 500 nm (d–f).

Fig. 2.

Phylogram depicting inferred relationships among Teredinibacter waterburyi Bs02^T and related bacteria based on 16S rRNA sequences. The tree presented is a subtree excerpted from the tree shown in Figure S1, inferred using 1373 nucleotide positions employing GTR+I+Γ as the substitution model in MrBayes version 3.2.6. Chain length was set to 4 million, subsampling every 2000 generations and discarding the first 20 % of the analytical results as burn-in. Posterior probability values are indicated for each node. The scale bar represents nucleotide substitution rate per site.

Fig. 3.

Phylogram depicting inferred relationships among Teredinibacter waterburyi Bs02^T and related bacteria based on concatenated nucleotide sequences of 37 conserved proteins. The tree presented is a subtree excerpted from the tree shown in Figure S2. PhyloSift was used to automatically mine, extract and align the a core set of 37 conserved protein-coding nucleotide sequences from whole genome sequences listed in Table S2. The maximum likelihood tree was constructed using RaxML version 7.04. Bootstrap proportions (100 replicates) are indicated for each node. The scale bar represents substitution rate per site.