Microorganisms are found across all three domains of life: Archaea, Bacteria, and Eukarya. Microbes within the domains Archaea and Bacteria are exclusively prokaryotes, which means their cells lack a nucleus. Conversely, microbes within the Eukarya domain are eukaryotes, characterized by cells that have a true nucleus. Certain microbes, like viruses, are acellular and therefore do not fit into any of the three domains.

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In modern genetics-based taxonomy, life is categorized into three domains above the kingdom level: Archaea, Bacteria, and Eukarya. Based on the analysis of small subunit ribosomal RNA (rRNA) gene sequences, this phylogenetic model suggests all three evolved from a common ancestral cell type, but is skewed to show that Archaea and Eukarya share a closer evolutionary relationship with each other than either does to Bacteria.

Three Domains of Life

https://openstax.org/details/books/microbiology/

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.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

OpenStax Microbiology Textbook

Bacteria is a very diverse domain, characterized by their prokaryotic cells and lack of ether linkages, with membranes made of phospholipid bilayers.

Bacteria (domain)

Microbial Classification by Domain

The domain Eukarya comprises all eukaryotes, which include both unicellular and multicellular organisms such as protists, fungi, plants, and animals. The primary defining characteristic of these organisms is that their cells contain a true nucleus.

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