Strain VI-R2^T (= DSM 9946) was first described as ‘Thermus silvanus’ by Tenreiro et al. in 1995 [1]. One year later it was formally named and transferred from the genus Thermus into the then novel genus Meiothermus by Nobre et al. [2]. Currently, there are nine species within the genus Meiothermus [3,4]. The genus name derives from the Greek words ‘meion’ and ‘thermos’ meaning ‘lesser’ and ‘hot’ to indicate an organism in a less hot place [2,3]. The species name was given in honor of Manuel T. Silva, a Portuguese microbiologist and immunologist [1]. Strain VI-R2^T was isolated from the hot spring (vent temperature, 56°C; pH 8.9) located at the end of a 450 m tunnel and from thermal water (temperature 33°C; pH 8.8) piped to a spa at Vizela in northern Portugal [1].

Members of the genus Meiothermus have been isolated from natural hot springs and artificial thermal environments [2,5] in Russia [6], Central France [7], Northern and Central Portugal [1,8], North-Eastern China [9], Northern Taiwan [10], Iceland [11] and the Azores [4]. Interestingly, the genus Meiothermus is heterogeneous with respect to pigmentation. The yellow pigmented species also form a distinct group on the basis of the 16S rRNA gene sequence similarity, whereas the red/orange pigmented strains form two groups, one comprising M. silvanus and the other the remaining species [8,9]. Like all members of the class Deinococci, the lipid composition of the cell membrane of members of the genus Meiothermus contains unusual and characteristic structures.

M. silvanus is well known to form colored biofilms in the paper industry, which makes this species an economic threat [12,13]. M. silvanus uses thread-like organelles for adhesion and biofilm formation to grow on stainless steel [14]. However, coating of stainless steel with diamond-like carbon or certain fluoropolymers reduced or almost eliminated adhesion and biofilm growth of M. silvanus [14]. Other strategies to combat M. silvanus in the paper industry include electrochemical inactivation (oxidation) using different levels of chloride concentration [15]. Here, the inactivation was mainly due to the electrochemically generated chlorine/hypochlorite [15]. A patent based on different natural plant extracts inhibiting biofilm formation of thermophilic species in paper or board machines, amongst them M. silvanus, has been recently issued [16].

The 16S rRNA genes of the seven other type strains in the genus Meiothermus share between 88.5% (Meiothermus chliarophilus [1]) and 89.8% (Meiothermus cerbereus [11]) sequence identity with strain VI-R2^T, whereas the other type strains from the family Thermaceae share 85.8 to 87.8% sequence identity [17]. In addition to being found on paper and board machines [12] uncultured clone 16S rRNA gene sequences very similar to M. silvanus RI-V2^T (X84211) have also been detected in the gut of an invasive wood-boring beetle (98% identity, EU148672) [18] and in seawater adjacent to a Pacillopora meandrina coral colony at Palmyra Atoll (99% identity, EU249942). Environmental samples and metagenomic surveys do not surpass 84% sequence similarity to the 16S rRNA gene sequence of strain RI-V2^T (status May 2010). Here we present a summary classification and a set of features for M. silvanus RI-V2^T, together with the description of the complete genomic sequencing and annotation.

A physiological description based on five strains of the species is given by Tenreiro et al. [1]. The cells are described as Gram-negative nonmotile rods with variable lengths and 0.5 to 0.8 µm in width (Table 1 and Figure 1). On Thermus medium colonies are orange-red pigmented and 0.5 to 1.2 µm in diameter after 72 h of growth [1]. The optimum pH is between 8.0 and 8.5; growth does not occur at pH 5.0 or 10.0. Yeast extract is required for growth [1]. All strains are oxidase positive and catalase negative. Nitrate is reduced to nitrite. Strain VI-R2^T is negative for the enzyme α-galactosidase but positive for β-galactosidase. Casein, elastin, gelatin, hide powder azure, and starch are degraded. The hydrolysis of fibrin is weak or negative. Strain VI-R2^T utilizes D-glucose, D-fructose, D-mannose, D-galactose, D-xylose, maltose, lactose, D-melibiose, glycerol, D-mannitol, D-sorbitol, ribitol, pyruvate, L-glutamate, L-asparagine, L-serine, L-glutamine, and L-proline [1]. Strain VI-R2^T does not utilize L-arabinose, L-rhamnose, sucrose, D-cellobiose, D-trehalose, D-raffinose, meso-erythritol, galactitol, myo-inositol, acetate, succinate, citrate, salicin, or acetamide [1]. Further metabolic traits are listed elsewhere [7]. Also, strain VI-R2^T produces polysaccharide inclusions [1].

Figure 1 Scanning electron micrograph of M. silvanus VI-R2T.

Figure 2 shows the phylogenetic neighborhood of M. silvanus VI-R2^T in a 16S rRNA-based tree. The sequences of the two 16S rRNA gene copies in the genome of M. silvanus VI-R2^T do not differ from each other, but differ by six nucleotides from the previously published 16S rRNA sequence from DSM 9946 (X84211).

Figure 2 Phylogenetic tree highlighting the position of M. silvanus VI-R2T relative to the type strains of the other species within the genus and to the other type strains within the family Thermaceae. The tree was inferred from 1,442 aligned characters [30,31] of the 16S rRNA gene sequence under the maximum likelihood criterion [32] and rooted in accordance with the current taxonomy [33]. The branches are scaled in terms of the expected number of substitutions per site. Numbers above branches are support values from 900 bootstrap replicates [34] if larger than 60%. Lineages with type strain genome sequencing projects registered in GOLD [35] are shown in blue, published genomes in bold, i.e. Thermus thermophilus (AP008226) and the type species of the genus, M. ruber [36].

The genome consists of a 3,249,394 bp long chromosome, and two plasmids of 347,854 bp and 124,421 bp lengths, respectively, with a total G+C content of 62.7% (Table 3 and Figure 3). Of the 3,722 genes predicted, 3,667 were protein-coding genes, and 55 RNAs; 158 pseudogenes were also identified. The majority of the protein-coding genes (64.5%) were assigned a putative function while the remaining ones were annotated as hypothetical proteins. The distribution of genes into COGs functional categories is presented in Table 4.

Figure 3 Graphical circular map of the genome and the larger of the two plasmids (not drawn to scale). From outside to the center: Genes on forward strand (color by COG categories), Genes on reverse strand (color by COG categories), RNA genes (tRNAs green, rRNAs red, other RNAs black), GC content, GC skew.