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Firmicutes is a diverse phylum of Gram-positive bacteria characterized by a low GC content in their genomes. This phylum includes organisms with monoderm or diderm cell envelopes, highlighting a complex evolutionary history. Firmicutes comprises several major orders, including Lactobacillales, Clostridiales, and Bacillales, which exhibit remarkable diversity in their morphology, metabolism, and ecological roles.
The order Lactobacillales includes lactic acid bacteria, which are fermentative organisms that produce lactic acid as a primary metabolic product. These non-sporulating rods or cocci rely on fermentative metabolism and substrate-level phosphorylation for energy. Important genera include Lactobacillus and Lactococcus, commonly associated with dairy fermentation. Pathogenic and nonpathogenic Streptococcus species form chains or tetrads, while Enterococcus species are closely associated with humans as part of the normal flora.
The order Bacillales includes both nonsporulating and endospore-forming genera. Nonsporulating genera such as rod-shaped Listeria and cluster-forming coccus Staphylococcus are noteworthy. Endospore-forming genera like Bacillus and Paenibacillus are significant for their ecological versatility and industrial applications. The order Clostridiales encompasses nonsporulating genera like acid-tolerant Sarcina and endospore-forming genera like Clostridium and Sporosarcina. These organisms thrive in anaerobic conditions and contribute to diverse metabolic processes.
A unique feature of Firmicutes is the presence of phototrophic genera such as Heliobacterium and Heliophilum, which perform anoxygenic photosynthesis using bacteriochlorophyll g. These anaerobic species lack intracytoplasmic photosynthetic membranes and utilize pyruvate for photoheterotrophic growth. Another interesting group is the diderm bacteria within the classes Negativicutes and Halanaerobiia, which possess outer membranes similar to Gram-negative bacteria, suggesting a complex evolutionary trajectory.
Firmicutes play critical roles in ecosystems, industries, and human health, demonstrating extraordinary metabolic and structural adaptations that underpin their evolutionary success.
