A representative genomic 16S rRNA sequence of
M. hydrothermalis T1
T was compared using NCBI BLAST [
4,
5] under default settings (e.g., considering only the high-scoring segment pairs (HSPs) from the best 250 hits) with the most recent release of the Greengenes database [
6] and the relative frequencies of taxa and keywords (reduced to their stem [
7]) were determined, weighted by BLAST scores. The most frequently occurring genera were
Thermus (91.0%),
Oceanithermus (4.9%),
Marinithermus (3.3%) and
Thermothrix (0.8%) (118 hits in total). Regarding the two hits to sequences from members of the species, the average identity within HSPs was 100.0%, whereas the average coverage by HSPs was 98.0%. Among all other species, the one yielding the highest score was
O. profundus (NR_027212), which corresponded to an identity of 91.9% and HSP coverage of 93.3%. (Note that the Greengenes database uses the INSDC (= EMBL/NCBI/DDBJ) annotation, which is not an authoritative source for nomenclature or classification.) The highest-scoring environmental sequence was EU555123 [
8] ('Microbial Sulfide Hydrothermal Vent Field Juan de Fuca Ridge Dudley hydrothermal vent clone 4132B16'), which showed an identity of 91.6% and HSP coverage of 92.1%. The most frequently occurring keywords within the labels of all environmental samples which yielded hits were 'spring' (6.9%), 'hot' (5.3%), 'microbi' (3.7%), 'nation, park, yellowston' (3.2%) and 'skin' (3.0%) (132 hits in total). Environmental samples which yielded hits of a higher score than the highest scoring species were not found. These key words are in accordance with the biotope of the strain T1
T in the original description [
1], although 'skin' indicates the possible presence of relatives in a moderate environment.
shows the phylogenetic neighborhood of M. hydrothermalis T1T in a 16S rRNA based tree. The sequences of the three identical 16S rRNA gene copies in the genome differ by two nucleotides from the previously published 16S rRNA sequence (AB079382).
The cells of strain T1
T are Gram-negative, non-motile, straight rods measuring 7.5 - 9.4 µm by 0.9 - 1.0 µm during the exponential growth phase [
1] ( and ). In the stationary growth phase the cells tend to form filaments [
1]. Rotund bodies were not observed from the cells [
1]. Cells of strain T1
T have an envelope which consists of a cytoplasmic membrane with a simple outline and a cell wall with an inner, electron-dense thin layer, which presumably represents the peptidoglycan [
1]. Colonies are whitish and have 2.5 - 3.0 mm of diameter [
1]. The organism is an obligate heterotroph and grows only under strictly aerobic culture conditions [
1]. Growth was not observed in anaerobic or autotrophic culture conditions [
1]. However, it should be noted that according to Mori and colleagues [
32] this was tested only in the presence of sulfide. Steinsbu and colleagues [
3] argue that it is therefore possible that
M. hydrothermalis has the capability of anaerobic growth under unreduced conditions, as has been observed for
Rhabdothermus arcticus,
Vulcanithermus mediatlanticus,
O. profundus and
O. desulfurans [
3,
32-
34]. Unlike members of the genus
Thermus, reactions were negative for catalase- and cytochrome oxidase and hydrolysis of gelatin, starch or casein was negative [
1]. Growth occurs over the temperature range of 50.0 - 72.5°C (optimum 67.5°C), pH range 6.25 - 7.75 (optimum pH 7.0), and at NaCl concentrations in the range 0.5 - 4.5% (optimum 3%) [
1]. The generation time under the above listed optimal condition and in medium MJYPV is about 30 minutes [
1].
M. hydrothermalis T1
T differs from the members of the genera
Oceanithermus by having a higher optimal temperature for growth and a higher oxygen tolerance [
3]. Strain T1
T is able to utilize complex organic substrates such as Casamino acids, tryptone and yeast extract as sole energy and carbon sources [
1].
| Table 1Classification and general features of M. hydrothermalis T1T according to the MIGS recommendations [20] and the NamesforLife database [21]. |
Strain T1T shares with its closest related genome-sequenced neighbors, O. profundus [17], Meiothermus silvanus [18] and Thermus thermophilus [16] (Figure 1), the presence of two linked 5S-23S rRNA gene clusters, with two 16S rRNA genes located separately in the genomes, but has one surplus, third 16S rRNA gene copy.
Chemotaxonomy
The major cellular fatty acids of strain T1
T, when grown at 67.5°C, were
iso-C
15:0 (40.4%),
iso-C
17:0 (28.5%), C
16:0 (12.9%),
anteiso-C
15:0 (6.0%),
anteiso-C
17:0 (5.4%),
iso-C
16:0 (2.8%) and
iso 3-OH C
11:0 (1.0%). Menaquinone-8 was the major respiratory quinone. The fatty acid and respiratory quinone composition were similar to those of members of the genus
Thermus, as described previously [
35,
36]. However, the presence of
iso 3-OH C
11:0 in strain T1
T distinguishes it from
Thermus species [
1].