The Firmicutes are a group of bacteria, most of which have Gram-positive stains. 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. 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 dessication and can survive extreme conditions. They are found in various environments, and some notable pathogens. Members may be aerobic or anaerobic and those in one family, the heliobacteria, produce food through photosynthesis.

Notable genera of Firmicutes include the following:

Bacilli, order Bacillales

• Bacillus
• Listeria
• Staphylococcus

Bacilli, order Lactobacillales

• Enterococcus
• Lactobacillus
• Lactococcus
• Leuconostoc
• Streptococcus

Clostridia, order Clostridiales

• Acetobacterium
• Clostridium
• Eubacterium
• Heliobacterium
• Heliospirillum
• Sporomusa

Mollicutes

• Mycoplasma
• Spiroplasma
• Ureaplasma
• Erysipelothrix

The Firmicutes are customarily divided into the Bacilli, Clostridia, and Mollicutes. However only the last group is likely to be monophyletic.