| Thermomicrobia | |
|---|---|
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Pseudomonadati |
| Phylum: | Thermomicrobiota Garrity and Holt 2021[2] |
| Class: | Thermomicrobia Garrity and & Holt 2002[1] |
| Orders | |
| Synonyms | |
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TheThermomicrobia is a group ofthermophilic green non-sulfurbacteria. Based on speciesThermomicrobium roseum (type species) andSphaerobacter thermophilus, this bacteria class has the following description:[3][4]
The class Thermomicrobia subdivides into two orders with validly published names:ThermomicrobialesGarrity and Holt 2001 andSphaerobacteralesStackebrandt,Rainey and Ward-Rainey 1997. Gram negative. Pleomorphic, non-motile, non-spore-forming rods. Non-sporulating. Nodiamino acid present. No peptidoglycan in significant amount. Atypical proteinaceous cell walls. Hyper-thermophilic, optimum growth temperature at 70-75 °C. Obligatory aerobic and chemoorganotrophic.[note 1]
As thermophilic bacteria, members of this class are usually found in environments which are distant from human activity.[5] However, they have features like improved growth in antibiotics and CO oxidizing activity, making them interesting topics of research (e.g. for biotechnology application).
In 1973, a strain of rose-pink thermophilic bacteria was isolated from Toadstool Spring in Yellowstone National Park, which was later namedThermomicrobium roseum and proposed as a novel species of the novel genusThermomicrobium.[6] At that time the genus was categorized under family Achromobacteraceae, but it became a distinct phylum by 2001.[3]
In 2004, it was proposed, on the basis of an analysis of genetic affiliations, that the Thermomicrobia should more properly be reclassified as aclass belonging to the phylumChloroflexota (formerly Chloroflexi). The bacteriaSphaerobacter thermophilus originally described as an Actinobacteria is now considered a Thermomicrobia.[4][7] In the same year, another strain of rose-pink thermophilic bacteria was isolated from Yellowstone National Park, which was namedThermobaculum terrenum.[8] Later analysis based on genome put this species under Thermomicrobia class.[9] However, the current standing ofThermobaculum terrenum is disputed.[10]
In 2012, a thermo-tolerant nitrite-oxidizing bacterium was isolated from a bioreactor, which was namedNitrolancetus hollandica and proposed as a novel species later in 2014.[11] 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.[12]
In 2014, two thermophilic, Gram-positive, rod-shaped, non-spore-forming bacteria (strains KI3T and KI4T) 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 KI3T strain, later named asThermomicrobium carboxidum, is closely related toThermomicrobium roseum. The KI4T strain, later named asThermorudis peleae, was proposed as a type strain of new genusThermorudis.[13]
In 2015, a thermophilic bacteria strain WKT50.2 isolated from geothermal soil in Waitike (New Zealand) was proposed to be a novel species, later namedThermorudis pharmacophila. Phylogenic analysis based on 16s rRNA place it within Thermomicrobia class, as close relative toThermorudis peleae.[5]
Members of the class Thermomicrobia are broadly distributed across a wide range of both aquatic and terrestrial habitats.Thermomicrobium roseum was found in geothermally heated hot springs,Thermorudis pharmacophila andThermobaculum terrenum from heated soils, andThermomicrobium carboxidum andThermorudis peleae from heated sediments[13][5][14] In addition,Sphaerobacter thermophilus was found in sewage sludge that went through thermophilic treatment.[15] The common features of their habitats include temperature ranging from around 65~75 °C and a pH around 6.0~8.0 (except for Nitrolancea hollandica which grow around 40 °C[11]).
Members of Thermomicrobia class have variation in their basic metabolism.Nitrolancetus hollandica has nitrifying activity that utilize NO2− as energy source, which is unique in the wholeChloroflexota phylum.[12]Thermomicrobium spp. andSphaerobacter thermophilus have constitutive CO oxidizing not found in other species in this class.[13][16] However, species of this class do share some features, as listed below:
Members of Thermomicrobia class exhibit certain level of resistance againstmetronidazole and/ortrimethoprim, which are clinically relevant for humans.[17][18]Thermomicrobium carboxidum andThermorudis peleae show resistance against both of those antibiotics, whileSphaerobacter thermophilus shows resistance against onlymetronidazole.[5] Interestingly,Thermomicrobium roseum andThermorudis pharmacophila have an increased growth in bothmetronidazole andtrimethoprim, a rare trait even within antibiotic resistant bacteria.[5] The mechanisms behind are currently undocumented, and further study is required on this topic.
Members of Thermomicrobia class have variousGram-staining results.Thermomicrobium roseum,Sphaerobacter thermophilus andThermorudis pharmacophila are reported to be Gram-negative and have a typical layereddidermcell envelope structure.[3][4][5] However, their cell envelope composition are atypical compared to typical Gram-negative bacteria. Cell envelope ofThermomicrobium roseum lacks significant amount of peptidoglycan, which is fundamental for typical Gram-negative bacteria, while being rich in protein.[3] Membrane lipids ofThermomicrobium roseum are mostly long chaindiols instead of glycerol-based lipids commonly found in bacteria.[19] The same feature was found inSphaerobacter thermophilus andThermorudis pharmacophila.[5] It was suggested that the high-protein and diol-based lipid composition are responsible for heat resistance of these bacteria.[4][20]
Meanwhile, other members of Thermomicrobia class are reported to be Gram-positive and have typical monoderm cell envelope.[8][12][13] There are some possible explanations of the inconsistency of Gram-staining result within the class. ForThermorudis pharmacophila, a possible explanation suggested by Houghton et al. is that it is actually an atypicalmonoderm bacterium, because its cell envelope contains amino acids usually associated with Gram-positive bacteria, have reaction to KOH,vancomycin andampicillin, and lacks genes responsible for diderm formation.[5] It is also suggested that further study is required to resolve this problem, since the inconsistent reports of cell envelope structure are found for the whole Chloroflexota phylum.
| 16S rRNA basedLTP_10_2024[21][22][23] | 120 marker proteins basedGTDB 09-RS220[24][25][26] | |||
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The currently accepted taxonomy is based on theList of Prokaryotic names with Standing in Nomenclature (LPSN)[27] andNational Center for Biotechnology Information (NCBI).[28]