Bacilli prefer to live in oxygen-rich environments but some are capable of survival without it. Members of this group are commonly found in soil. Some are responsible for the disease anthrax while others produce antibiotics or insecticides. Bacillus subtilis is one of the primary model organisms used by researchers to understand topics ranging from cell differentiation to iron storage and DNA replication.

The organisms described above represent only a tiny part of the diversity found within the group of Firmicutes. Their huge impact on fields as diverse as agriculture, medicine, food production and ecology make them a vital subject of inquiry.