Revised Road Map to the Phylum Firmicutes

The phylum Firmicutes contains three classes, “ Bacilli ”, “ Clostridia ” and “ Erysipelotrichia ”. The Mollicutes were removed from the phylum given the general low support by alternative markers and its unique phenotypic properties, in particular the lack of rigid cell walls. The family Erysipelotrichaceae , which includes wall‐forming Gram‐positive organisms previously classified with the Mollicutes , was retained in the Firmicutes as a novel class, “ Erysipelotrichia ”, and order, “ Erysipelotrichales ”. While the bipartition of the classes “ Clostridia ” and “ Bacilli ” is corroborated by the new analyses, some of the taxa previously assigned to the “ Clostridia ” tend to root outside the Firmicutes and may represent separate phyla. These include taxa previously classified within the “ Thermoanaerobacterales ” and Syntrophomonadaceae , which may contain a number of phylogenetic clades that are distinct at the phylum level. However, given the absence of corroboration by other phylogenetic markers for many of these assignments and a clear consensus on the definition of a phylum, these taxa were retained within the Firmicutes for the present.