Bacteria are prokaryotic microorganisms found in nearly every habitat on Earth, including within and on the human body. As prokaryotes, their genetic material (DNA) is not housed within a true nucleus. Most bacterial cells feature cell walls containing peptidoglycan. While many bacteria are harmless or beneficial, some act as pathogens and cause diseases in humans and other animals.

Bacteria

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

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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

A prokaryotic cell is a major category of cell that lacks a nucleus surrounded by a complex nuclear membrane. Instead, its genetic material is generally a single, circular chromosome located in a nucleoid region. While prokaryotic cells generally lack complex membrane-bound organelles, they often contain inclusions that compartmentalize their cytoplasm. Microorganisms composed of these cells are classified within the domains Archaea and Bacteria.

Prokaryotic Cell

Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proc Natl Acad Sci USA. 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744. Retrieved 11 February 2010.

Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya

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 (domain)

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.

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.

Eukarya

Photosynthesis is the process of using the sun's light as energy and converting it to chemical energy to fix gaseous carbon dioxide and create sugars such as glucose. The first organisms with the capability to photosynthesize stemmed from prokaryotes 3.8 billion years ago. What is required for photosynthesis include a light source, carbon, electron donor, and pigments to harvest energy.

Photosynthesis

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

The cell envelope is a collective term for the structural layers that enclose the cytoplasm and internal components of a cell. In prokaryotic organisms, the precise composition of the cell envelope varies depending on the species and cell type, but it universally includes a plasma membrane and, in the vast majority of cases, a surrounding cell wall.

Cell Envelope

Prokaryotes are found virtually everywhere on Earth, demonstrating extreme adaptability. They reside in diverse and harsh environments, including boiling hot springs, the Antarctic ice shield, and deep-sea regions under intense hydrostatic pressure.

Ubiquity of Prokaryotes

Prokaryotes are highly abundant in the atmosphere and can be found even at high altitudes in the air. The atmosphere can contain up to 2,000 distinct types of bacteria, showing a level of bacterial diversity comparable to that found in soil.

Abundance and Diversity of Airborne Bacteria

Prokaryotes typically reside in structured communities where they interact both among themselves and with larger organisms, such as humans, that they utilize as hosts.

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