Planctomycetes is a phylum of aquatic bacteria inhabiting freshwater, saltwater, and brackish environments. They reproduce by budding rather than binary fission, producing a motile swarmer cell from a maternal cell. This swarmer cell eventually attaches to a surface using an appendage called a holdfast and differentiates into an immobile (sessile) cell, which is the only stage capable of reproduction.

Planctomycetes

Gram-negative cells possess a cell envelope with a significantly thinner layer of peptidoglycan—no more than about $${}4$$ nm thick—compared to gram-positive cells. Their overall cell envelope structure is highly complex, featuring a periplasmic space and an outer membrane located external to the peptidoglycan layer.

Gram Negative Cell Walls

The *Chlamydia* genus consists of many pathogens. They spread very easily from host to host because they form elementary bodies during their life cycle. These are spore-like which allows *Chlamydia* to be metabolically inactive until they are in a host environment.

*Chlamydia*

The *Cytophaga-Flavobacterium-Bacteroides* (CFB) group is a phylum of phylogenetically diverse, rod-shaped, gram-negative nonproteobacteria. These bacteria are anaerobic organoheterotrophs and avid fermenters that share similarities in their DNA nucleotide sequences and are adapted to anaerobic environments, such as the human mouth or gut, where they can be mutualistic or pathogenic.

*Cytophaga-Flavobacterium-Bacteroides* (CFB)

Spirochetes are gram-negative bacteria characterized by long, spiral-shaped bodies that can reach lengths up to $${}250$$ μm. Due to their extreme thinness, they are difficult to view under conventional brightfield microscopes, typically requiring darkfield fluorescent microscopy instead, and are highly difficult or impossible to culture in a laboratory.

Spirochete

Non-proteobacteria gram-negative bacteria are a group of gram-negative bacteria that do not belong to the Proteobacteria phylum. This group includes spirochetes, the Cytophaga-Flavobacterium-Bacteroides (CFB) group, Planctomycetes, and Chlamydia species.

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Modern bacterial taxonomy classifies bacteria using physiological, biochemical, and genetic traits. This classification includes a unique category of deeply branching bacteria, which represent the oldest evolutionary lineages. Additionally, gram-negative bacteria are further divided into Proteobacteria, Cytophaga-Flavobacterium-Bacteroides (CFB), and spirochetes, while gram-positive and other bacterial groups continue to be categorized based on their distinct physiological and genomic properties.

Bacterial Diversity

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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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Deeply branching bacteria are the phylogenetically oldest forms of life and the closest relatives to the last universal common ancestor. They often live in extreme environments that mimic the conditions of ancient Earth billions of years ago, making them crucial for studying evolutionary history and useful for industrial processes.

Deeply Branching Bacteria

Non-proteobacteria Gram Negative Bacteria

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

Phototrophic bacteria represent a diverse category of bacteria, rather than a single taxonomic group, characterized by their ability to use sunlight as an energy source. They include both Proteobacteria and nonproteobacteria, as well as purple or green sulfur and nonsulfur bacteria, with some species being capable of nitrogen fixation.

Phototrophic Bacteria

Gram-positive bacteria represent a large and diverse group of microorganisms whose taxonomy and unique traits are critical for diagnosing and treating infectious diseases. These bacteria are categorized into high G+C and low G+C gram-positive groups, depending on the abundance of guanine and cytosine nucleotides within their genomes.

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