Bacterial phyla

The following is a list of bacterial phyla that have been validly published as of December 2023.

The following is a list of candidate phyla that have not been cultured.

Despite the unclear branching order for most bacterial phyla, several groups of phyla consistently cluster together and are referred to as supergroups or superphyla. In some instances, bacterial clades clearly consistently cluster together but it is unclear what to call the group. For example, the Candidate Phyla Radiation includes the Patescibacteria group which includes Microgenomates group which includes over 11 bacterial phyla. The LPSN recognizes four kingdoms of bacteria as validly published: Bacillati, Fusobacteriati, Pseudomonadati and Thermotogati.^[67]

Given the rich history of the field of bacterial taxonomy and the rapidity of changes therein in modern times, it is often useful to have a historical perspective on how the field has progressed in order to understand references to antiquated definitions or concepts.

When bacterial nomenclature was controlled under the Botanical Code, the term division was used, but now that bacterial nomenclature (with the exception of cyanobacteria) is controlled under the Bacteriological Code, the term phylum is preferred.

In 1987, Carl Woese, regarded as the forerunner of the molecular phylogeny revolution, divided Eubacteria into 11 divisions based on 16S ribosomal RNA (SSU) sequences, listed below.^[76][77]

• Purple Bacteria and their relatives (later renamed Proteobacteria^[78])
  • alpha subdivision (purple non-sulfur bacteria, rhizobacteria, Agrobacterium, Rickettsiae, Nitrobacter)
  • beta subdivision (Rhodocyclus, (some) Thiobacillus, Alcaligenes, Spirillum, Nitrosovibrio)
  • gamma subdivision (enterics, fluorescent pseudomonads, purple sulfur bacteria, Legionella, (some) Beggiatoa)
  • delta subdivision (Sulfur and sulfate reducers (Desulfovibrio), Myxobacteria, Bdellovibrio)
• Gram-positive Eubacteria^{[Note 1]}
  • High-G+C species (later renamed Actinobacteria^[82]) (Actinomyces, Streptomyces, Arthrobacter, Micrococcus, Bifidobacterium)
  • Low-G+C species (later renamed Firmicutes^[82]) (Clostridium, Peptococcus, Bacillus, Mycoplasma)
  • Photosynthetic species (Heliobacteria)
  • Species with Gram-negative walls (Megasphaera, Sporomusa)
• Cyanobacteria and chloroplasts (Aphanocapsa, Oscillatoria, Nostoc, Synechococcus, Gloeobacter, Prochloron)
• Spirochetes and relatives
  • Spirochetes (Spirochaeta, Treponema, Borrelia)
  • Leptospiras (Leptospira, Leptonema)
• Green sulfur bacteria (Chlorobium, Chloroherpeton)
• Bacteroides, Flavobacteria and relatives (later renamed Bacteroidetes
  • Bacteroides (Bacteroides, Fusobacterium)
  • Flavobacterium group (Flavobacterium, Cytophaga, Saprospira, Flexibacter)
• Planctomyces and relatives (later renamed Planctomycetes)
  • Planctomyces group (Planctomyces, Pasteuria [sic]^{[Note 2]})
  • Thermophiles (Isocystis pallida)
• Chlamydiae (Chlamydia psittaci, Chlamydia trachomatis)
• Radioresistant micrococci and relatives (later renamed Deinococcus–Thermus^[83] or Thermi)^{[Note 3]}
  • Deinococcus group (Deinococcus radiodurans)
  • Thermophiles (Thermus aquaticus)
• Green non-sulfur bacteria and relatives (later renamed Chloroflexi^[87])
  • Chloroflexus group (Chloroflexus, Herpetosiphon)
  • Thermomicrobium group (Thermomicrobium roseum)
• Thermotogae (Thermotoga maritima)

Traditionally, phylogeny was inferred and taxonomy established based on studies of morphology. The advent of molecular phylogenetics has allowed for improved elucidation of the evolutionary relationship of species by analyzing their DNA and protein sequences, for example their ribosomal DNA.^[88] The lack of easily accessible morphological features, such as those present in animals and plants, hampered early efforts of classification and resulted in erroneous, distorted and confused classification, an example of which, noted Carl Woese, is Pseudomonas whose etymology ironically matched its taxonomy, namely "false unit".^[76] Many bacterial taxa were re-classified or re-defined using molecular phylogenetics.

The advent of molecular sequencing technologies has allowed for the recovery of genomes directly from environmental samples (i.e. bypassing culturing), leading to rapid expansion of our knowledge of the diversity of bacterial phyla. These techniques are genome-resolved metagenomics and single-cell genomics.

• Bacterial taxonomy#Phyla endings
• International Code of Nomenclature of Bacteria
• Branching order of bacterial phyla (Woese, 1987)
• Branching order of bacterial phyla (Gupta, 2001)
• Branching order of bacterial phyla (Cavalier-Smith, 2002)
• Branching order of bacterial phyla (Rappe and Giovanoni, 2003)
• Branching order of bacterial phyla (Battistuzzi et al., 2004)
• Branching order of bacterial phyla (Ciccarelli et al., 2006)
• Branching order of bacterial phyla after ARB Silva Living Tree
• Branching order of bacterial phyla (Genome Taxonomy Database, 2018)
• List of Bacteria genera
• List of bacterial orders
• List of sequenced bacterial genomes