Goodfellow 2012: This taxon is one of the major phyla in the domain Bacteria, as inferred from its branching pattern in the 16S rRNA gene tree (Garrity and Holt, 2001; Ludwig and Klenk, 2005) and taxon-specific 16S rRNA signatures (Zhi et al., 2009). The separation of the phylum from other bacterial taxa is supported by conserved indels in some proteins (e.g., cytochrome-c oxidase subunit 1, CTP synthase and glutamyl-tRNA synthase), by the presence of a large insert in 23S rRNA (Gao and Gupta, 2005; Gao et al., 2006) and by distinctive gene arrangements (Kunisawa, 2007). The nearest phylogenetic neighbor to the phylum is not clear (Ventura et al., 2007) though the Firmicutes are usually considered in this context. In this volume of the Manual, the phylum Actinobacteria encompasses five classes, 19 orders, 50 families, and 221 genera. However, many new taxa continue to be discovered so this listing is inevitably incomplete. The constituent classes are Acidimicrobiia class. nov., Actinobacteria (Stackebrandt et al., 1997), Coriobacteriia class. nov., Rubrobacteria class. nov., and Thermoleophilia class. nov.
Oren and Garrity, 2021: The properties of the taxon are as described by Goodfellow 2012. Correction of the effectively published synonym: Actinobacteria (sic) Goodfellow 2012.