|Bacillus subtilis, Gram stained|
The Firmicutes (Latin: firmus, strong, and cutis, skin, referring to the cell wall) are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative. Scientists once classified the Firmicutes to include all Gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus).
Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)  and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16S rRNA-based LTP release 111 by The All-Species Living Tree Project 
♠ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature LPSN
While there are currently more than 274 genera within the Firmicutes phylum, notable genera of Firmicutes include:
Bacilli, order Bacillales
Bacilli, order Lactobacillales
Firmicutes make up the largest portion of the mouse and human gut microbiome. The division Firmicutes as part of the gut flora has been shown to be involved in energy resorption and obesity.
It was once impossible to categorically define a given bacterium as belonging to Firmicutes, as the phylum is highly diverse in phenotypic characteristics due to its members' promiscuous plasmid exchange across species and genera, but the presence of Firmicutes can now be detected by real-time polymerase chain reaction (PCR) techniques.
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- Phylum "Firmicutes" - J.P. Euzéby: List of Prokaryotic names with Standing in Nomenclature