In reconstructed phylogenetic trees T. aggregans and representatives of the genera Sulfophobococcus, Desulfurococcus and Staphylothermus form a relatively stable distinct branch within the family Desulfurococcaceae, order Desulfurococcales. Most members of this clade thrive in terrestrial habitats and are characterized by having a coccoid morphology and a strictly anaerobic, heterotrophic metabolism.
shows the phylogenetic neighborhood of T. aggregans strain M11TLT in a 16S rRNA based tree. The genome of strain M11TLT contains only a single 16S rRNA gene that differs by one nucleotide from the previously published 16S rRNA gene sequence generated from the same strain (X99556), which contains nine ambiguous base calls. The difference between the genome data and the here reported 16S rRNA gene sequence is most likely due to sequencing errors in the previously reported sequence (NAS).
Figure 1 Phylogenetic tree highlighting the position of T. aggregans relative to the other type strains of the other genera within the family Desulfurococcaceae. The tree was inferred from 1,307 aligned characters [8,9] of the 16S rRNA gene sequence under the (more ...)
Cells of T. aggregans
are regular cocci that preferentially grow in grape-like aggregates consisting of five to several hundred individuals [1
]. They have normally dimensions between 0.2 and 0.8 µm ( and ), but under suboptimal growth conditions a swelling of cells was observed leading to dimensions of up to 3.5 µm. Flagella-like appendages are formed but motility was not described [1
]. The cell envelope consists of a cytoplasmic membrane that is covered by an amorphous layer of unknown composition. A regularly arrayed surface-layer protein was not detected by transmission electron microscopy of freeze-etched specimen, i.e. under experimental conditions which allow instant visualization of S-layers in cells of related genera.
Transmission electron micrographs of cells of T. aggregans strain M11TLT. A: cells were shadowed with platinum; B: cells were freeze-etched. Scale bars, 1 µm
Table 1 Classification and general features of T. aggregans strain M11TLT according to the MIGS recommendations 
is hyperthermophilic and grows optimally at 85°C, the temperature range for growth is 67 to 90°C. The pH range for growth is 5.0 - 7.0 with an optimum at pH 6.5. The strain grows optimally in the absence of exogenous NaCl, but can be adapted to salt concentrations of up to 0.7%. The doubling time under optimal growth conditions is 110 min [1
is strictly anaerobic and grows heterotrophically on yeast extract, peptone, gelatin, amino acids, heat-treated xylan, and glucose. Upon growth on yeast extract and peptone, the fermentation products acetate, isovalerate, CO2
were identified. No growth on meat extract, amylose, glycogen, cellulose, cellobiose, maltose, raffinose, pyruvate and acetate was found. Growth of strain M11TLT
is inhibited by sulfur and H2
]. It has been reported that addition of sulfur (0.05% w/v) to growing cultures leads to complete inhibition of growth, production of H2
S and finally lysis of cells. A growth-inhibiting effect of sulfur was also reported for Sulfophobococcus zilligii
], but is absent in the closely related genera Desulfurococcus
. In contrast, in both of the latter genera sulfur has either a stimulatory effect [21
] or is even required for growth [22
] and reduced to H2
S. Interestingly, an inhibiting effect in cultures of T. aggregans
and S. zilligii
was not observed, if growth media were supplemented with the sulfur compounds sulfide, sulfite or thiosulfate [1
], so that this effect seems to be restricted to elemental sulfur. The inhibiting effect of H2
on growth is reversible and can be explained by a product inhibition of sensitive hydrogenases, which may be required for the disposal of reducing equivalents as hydrogen during fermentation.
The lipid composition of T. aggregans
was analyzed by thin-layer chromatography. Core lipids were mainly composed of acyclic and cyclic dibiphytanyl glycerol tetraethers with one to four pentacyclic rings. In addition, traces of diphytanyl glycerol diethers were also detected [1
]. The presence of cyclic tetraether lipids in this species seems to be a diagnostic trait, because thus far these lipids were not detected in the related genera Sulfophobococcus
] or Desulfurococcus
]. Unfortunately, no data about the polyamine, quinone or cytochrome composition in T. aggregans
are currently available. However, respiratory lipoquinones could not be detected in Sulfophobococcus zilligii
, Desulfurococcus mucosus
and Desulfurococcus mobilis
], whereas homospermidine was identified as principal polyamine in several species closely related to T. aggregans