Alphaproteobacteria is a class of Proteobacteria that includes several obligate and facultative intracellular pathogens, such as the rickettsias. Additionally, some members of this class are oligotrophs capable of surviving in nutrient-poor environments, while others can convert atmospheric nitrogen into nitrites to make it usable for other forms of life.

Alphaproteobacteria

Betaproteobacteria are a diverse class of Proteobacteria whose members utilize a wide range of metabolic strategies to survive in various environments. This group includes human pathogens, such as species from the genus *Neisseria* and the species *Bordetella pertussis*.

Betaproteobacteria

Gammaproteobacteria is the largest and most diverse class of Proteobacteria. This group contains aerobes or facultative anaerobes, many of which are human pathogens. Some Gammaproteobacteria are enteric bacteria, which can be further classified as coliform or noncoliform, with *Escherichia coli* being the most extensively studied species.

Gammaproteobacteria

Deltaproteobacteria is a small class of gram-negative bacteria that includes species capable of reducing sulfate or elemental sulfur. Some members of this class act as scavengers and can form myxospores with multicellular fruiting bodies.

Deltaproteobacteria

Epsilonproteobacteria is the smallest class of Proteobacteria. They are gram-negative, microaerophilic bacteria, meaning they only require small amounts of oxygen in their environment. Clinically relevant genera within this class include *Campylobacter* and *Helicobacter*, both of which include human pathogens.

Epsilonproteobacteria

- Alphaproteobacteria
- Betaproteobacteria
- Gammaproteobacteria
- Deltaproteobacteria
- Epsilonproteobacteria

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Proteobacteria is a major phylum of gram-negative bacteria whose establishment as a separate phylum within the domain Bacteria was proposed in 1987 by American microbiologist Carl Woese, based on the similarity of their genomic nucleotide sequences. Originally termed 'purple bacteria and their relatives' by Woese, this diverse phylum contains both normal human microbiota and pathogens, and is divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria.

Proteobacteria

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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.

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.

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