Firmicutes

Firmicutes is a major phylum of bacteria that comprises many Gram-positive species characterized by relatively low genomic G+C content. Members are metabolically and ecologically diverse and occupy a wide range of environments, including soil, water, sediments, and the microbiota of animals and plants. The group contains both free-living and host-associated forms, including many important in industry and medicine.

A distinguishing feature of most Firmicutes is a thick peptidoglycan cell wall, which typically yields Gram-positive staining. The phylum encompasses both spore-forming lineages, such as Bacilli and Clostridia, and non-spore-forming lineages, including many lactic acid bacteria. The ability to form endospores among some members enables survival under harsh conditions.

Key genera include Bacillus and Clostridium (often soil dwellers and, in some species, pathogens), Lactobacillus, Lactococcus, and Leuconostoc (lactic acid bacteria used in fermentation), Staphylococcus and Streptococcus (numerous pathogens and commensals), and Listeria (notably L. monocytogenes). Members range from ecological workhorses in fermentation to clinically significant pathogens, as well as producers of enzymes and antibiotics.

From a taxonomic perspective, Firmicutes has been refined through genomic data, but it remains a foundational phylum in bacterial taxonomy, grouping Gram-positive, low-G+C bacteria. Its members contribute to nutrient cycling, food processing, biotechnology, and health and disease.