Firmicutes
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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 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.
The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are obligate or facultative aerobes, and the Mollicutes. On molecular 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.
Notable genera of Firmicutes include:
Bacilli, order Bacillales
Bacilli, order Lactobacillales
Clostridia, order Clostridiales