The Thermomicrobia is a group of thermophilic green non-sulfur bacteria. Based on species Thermomicrobium roseum (type species) and Sphaerobacter thermophilus, this bacteria class has the following description:

The class Thermomicrobia subdivides into two orders with validly published names: Thermomicrobiales <small>Garrity and Holt 2001</small> and Sphaerobacterales <small>Stackebrandt,</small> <small>Rainey and Ward-Rainey 1997</small>. Gram negative. Pleomorphic, non-motile, non-spore-forming rods. Non-sporulating. No diamino acid present. No peptidoglycan in significant amount. Atypical proteinaceous cell walls. Hyper-thermophilic, optimum growth temperature at 70-75&nbsp;°C. Obligatory aerobic and chemoorganotrophic. At that time the genus was categorized under family Achromobacteraceae, but it became a distinct phylum by 2001. In the same year, another strain of rose-pink thermophilic bacteria was isolated from Yellowstone National Park, which was named Thermobaculum terrenum. Later analysis based on genome put this species under Thermomicrobia class. However, the current standing of Thermobaculum terrenum is disputed. While it has nitrite-oxidizing activity, which is unique in the Thermomicrobia class, it is placed under the Thermomicrobia class based on 16s rRNA phylogeny.

In 2014, two thermophilic, Gram-positive, rod-shaped, non-spore-forming bacteria (strains KI3<sup>T</sup> and KI4<sup>T</sup>) isolated from geothermally heated biofilms growing on a tumulus in the Kilauea Iki pit crater on the flank of Kilauea Volcano (Hawaiʻi) were proposed as representatives of new species based on 16s rRNA phylogeny. The KI3<sup>T</sup> strain, later named as Thermomicrobium carboxidum, is closely related to Thermomicrobium roseum. The KI4<sup>T</sup> strain, later named as Thermorudis peleae, was proposed as a type strain of new genus Thermorudis.

In 2015, a thermophilic bacteria strain WKT50.2 isolated from geothermal soil in Waitike (New Zealand) was proposed to be a novel species, later named Thermorudis pharmacophila. Phylogenic analysis based on 16s rRNA place it within Thermomicrobia class, as close relative to Thermorudis peleae. In addition, Sphaerobacter thermophilus was found in sewage sludge that went through thermophilic treatment. However, species of this class do share some features, as listed below:

  • All members except Thermobaculum terrenum have inability to utilize some common monosaccharides (e.g. glucose, fructose, etc.) as sole carbon source. Thermomicrobium carboxidum and Thermorudis peleae show resistance against both of those antibiotics, while Sphaerobacter thermophilus shows resistance against only metronidazole. The same feature was found in Sphaerobacter thermophilus and Thermorudis pharmacophila.

Meanwhile, other members of Thermomicrobia class are reported to be Gram-positive and have typical monoderm cell envelope.

! colspan=1 | 120 marker proteins based GTDB 10-RS226

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Taxonomy

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI).

  • Order Sphaerobacterales <small>Stackebrandt, Rainey & Ward-Rainey 1997</small>
  • Family Sphaerobacteraceae <small>Stackebrandt, Rainey & Ward-Rainey 1997</small>
  • Genus Sphaerobacter <small>Demharter et al. 1989</small>

References