Most Firmicutes have cell walls, and these bacteria can be found in a great variety of habitats. They are grouped in the Class Bacilli or Class Clostridia. Diverse Firmicutes include Staphylococcus, Micrococcus, Streptococcus and Lactobacillus. Some staphylococci and micrococci are commonly found on human skin and mucosal surfaces. Streptococcus is most famous for causing ·strep throat· but many benign streptococci are normally found in the mouth and throat. Lactobacillus is common in the making of yogurt and cheese products. Some Lactobacillus species are associated with mucosal surfaces of humans. These resident Lactobacillus species help maintain our health by preventing colonization by disease-associated bacteria.

Some Firmicutes can form an endospore, a resistant differentiated cell produced under special, usually stressful, conditions. Endospore-forming bacteria such as Bacillus and Clostridium species can be classified by their aerotolerance. Many anaerobic organisms fall under the Clostridium banner. These organisms have very diverse ways of getting energy without using oxygen, but almost all are fermenters. Some Clostridium species are used by industry to produce solvents, an end product of their fermentation activity. Others produce toxins. One famous application of a Clostridium toxin is the use of Clostridium botulinum toxin, also known as BoTox, to paralyze muscles of the face to reduce skin wrinkles. Epulopiscium is closely related to Clostridium species.