Firmicutes

The Firmicutes are a division of bacteria, most of which have Gram-positive cell wall structure. A few, the Mollicutes or mycoplasmas, lack cell walls altogether and so do not respond to Gram staining, but still lack the second membrane found in other Gram-negative forms. Others, such as Megasphaera, Pectinatus, Selenomonas, and Zymophilus have a porous pseudo-outer-membrane that causes them to stain Gram-negative. Originally the Firmicutes were taken to include all Gram-positive bacteria, but more recently they tend to be restricted to 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-shaped forms.

Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and some notable pathogens. Those in one family, the heliobacteria, produce energy through photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage.

There is currently no way of detecting a bacteria as belonging to Firmicutes as the phylum is highly diverse in phenotypic characteristics due to promiscusious plasmid exchange across species and genera of this phyla.

The division Firmicutes as part of the gut flora has been shown to be involved in energy resorption and obesity.

Classes
The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are obligate or facultative aerobes, and the Mollicutes. On phylogenetic trees the first two groups show up as paraphyletic or polyphyletic, as do their main genera, Clostridium and Bacillus. It is likely these groups will undergo revision.

Genera
While there are currently more than 274 genera within the Firmicutes phylum, Notable genera of Firmicutes include:

Bacilli, order Bacillales Bacilli, order Lactobacillales Clostridia Mollicutes
 * Bacillus
 * Listeria
 * Staphylococcus
 * Enterococcus
 * Lactobacillus
 * Lactococcus
 * Leuconostoc
 * Pectinatus
 * Pediococcus
 * Streptococcus
 * Acetobacterium
 * Clostridium
 * Eubacterium
 * Heliobacterium
 * Heliospirillum
 * Sporomusa
 * Mycoplasma
 * Spiroplasma
 * Ureaplasma
 * ''Erysipelothrix'