*Methanobrevibacter oralis* is a species of methanogenic archaea that inhabits the human oral cavity, specifically the gums. While research in 2004 initially suggested a causal relationship between the activity of these methanogens and periodontal disease, subsequent studies demonstrated that no such causal link exists. Instead, periodontal disease likely causes an enlargement of anaerobic regions in the mouth, which are subsequently colonized by *M. oralis*.

Methanobrevibacter oralis

Although archaea inhabit human and animal microbiomes—such as certain methanogens in the human gastrointestinal tract—they are not known to cause infectious diseases in humans, animals, plants, bacteria, or other archaea. Despite ongoing speculation, scientists have yet to find definitive evidence explaining why no members of the domain Archaea appear to be pathogenic.

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Archaea is a domain of unicellular prokaryotes that exhibit a diversity comparable to bacteria and are found across all habitats. While some archaea are mesophiles that inhabit moderate environments, many are extremophiles that thrive in harsh conditions such as extreme temperature or salinity. Structurally, they differ from bacteria in their cell wall and membrane composition, and their genomes are typically larger and more complex. Unlike bacteria and eukaryotes, some archaea are capable of performing methanogenesis.

Archaea

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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- Crenarchaeota
- Euryarchaeota
- Korarchaeota
- Nanoarchaeota
- Thaumarchaeota

Major Phylum of Archaea

The cell membrane of organisms in the domain Archaea differs from that of bacteria and eukaryotes in three key ways:
1. The membrane phospholipids contain ether linkages, rather than ester linkages.
2. The lipid chains are branched isoprene chains, rather than straight-chained fatty acids.
3. The membrane can exist as a lipid monolayer instead of a lipid bilayer.

Archaeal Plasma Membrane Structure

Archaeal cell wall structure differs from bacterial cell walls primarily because archaea do not contain peptidoglycan. Instead, they often contain a similar polymer called pseudopeptidoglycan (or pseudomurein). Alternatively, some archaea possess a layer of glycoproteins or polysaccharides that serves as the cell wall, and a few archaeal species appear to lack cell walls entirely.

Archaeal Cell Walls vs. Bacterial Cell Walls

Eukaryotic cells contain multiple, linear chromosomes that are haploid or diploid and are wrapped around histone proteins. In contrast, prokaryotes typically possess a single, circular chromosome that is haploid, with bacteria lacking histones and archaea containing histones.

Comparison of Eukaryotic and Prokaryotic Chromosomes

The genomes of organisms in the domain Archaea are typically larger and more complex than those of bacteria, which presents difficulties in their taxonomic classification.

Archaeal Genomes

Methanogenesis is a unique metabolic process in which methane is produced. It is performed exclusively by certain members of the domain Archaea and is absent in both bacteria and eukaryotes.

Methanogenesis

Archaea in Human Microbiota and Disease

Fossil evidence indicates that some members of the domain Archaea are among the oldest organisms to have existed on Earth.

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