Includes a revised taxonomic outline for the phyla Bacteroidetes, Planctomycetes, Chlamydiae, Spirochetes, Fibrobacteres, Fusobacteria, Acidobacteria, Verrucomicrobia, Dictyoglomi, and Gemmatimonadetes based upon the SILVA project as well as a description of more than 153 genera in 29 families. Includes many medically important taxa.

Volume 4 covers completely different phyla of bacteria than the previous 3 volumes. Volume 4 includes 12 phyla, including the Mollicutes, Spirochaetes and Chlamydiae. Volume 3 contained only 1 phylum, the Firmicutes (low DNA G+C content Gram-positive bacteria).

Road map of the phyla Bacteroidetes, Spirochaetes, Tenericutes (Mollicutes), Acidobacteria, Fibrobacteres, Fusobacteria, Dictyoglomi, Gemmatimonadetes, Lentisphaerae, Verrucomicrobia, Chlamydiae, and Planctomycetes.- Taxonomic outlines of the phyla Bacteroidetes, Spirochaetes, Tenericutes (Mollicutes), Acidobacteria, Fibrobacteres, Fusobacteria, Dictyoglomi, Gemmatimonadetes, Lentisphaerae, Verrucomicrobia, Chlamydiae, and Planctomycetes.- Phylum XIV. Bacteroidetes phyl. nov..- Phylum XV. Spirochaetes Garrity and Holt 2001.- Phylum XVI. Tenericutes Murray 1984a, 356VP (Effective publication: Murray 1984b, 33.).- Phylum XVII. Acidobacteria phyl. nov..- Phylum XVIII. Fibrobacteres Garrity and Holt 2001.- Phylum XIX. Fusobacteria Garrity and Holt 2001, 140.- Phylum XX. Dictyoglomi phyl. nov..- Phylum XXI. Gemmatimonadetes Zhang, Sekiguchi, Hanada, Hugenholtz, Kim, Kamagata and Nakamura 2003, 1161VP.- Phylum XXII. Lentisphaerae Cho, Vergin, Morris and Giovannoni 2004a, 1005VP (Effective publication: Cho, Vergin, Morris and Giovannoni 2004b, 617.).- Phylum XXIII. Verrucomicrobia phyl. nov..- Phylum XXIV. Chlamydiae Garrity and Holt 2001.- Phylum XXV. Planctomycetes Garrity and Holt 2001, 137 emend. Ward (this volume).