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4.3.3: Proteobacteria

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    Learning Objectives

    • Describe the unique features of each class within the phylum Proteobacteria: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria
    • Give an example of a bacterium in each class of Proteobacteria

    In 1987, the American microbiologist Carl Woese (1928–2012) suggested that a large and diverse group of bacteria that he called “purple bacteria and their relatives” should be defined as a separate phylum within the domain Bacteria based on the similarity of the nucleotide sequences in their genome.1 This most diverse of the Gram-negative bacterial phyla subsequently received the name Proteobacteria. It includes many bacteria that are part of the normal human microbiota as well as many pathogens. Within the phylum, the bacteria have a wide variety of metabolic abitilities, ecological roles, and cellular characteristics.  Perhaps the only characteristic they all share is that they are Gram-negative.  The Proteobacteria are further divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria.  Examples of the alphaproteobacteria and gammaproteobacteria are given below.


    Among the Alphaproteobacteria are the rickettsias, that are obligate intracellular pathogens, meaning that part of their life cycle must occur inside other cells called host cells. When not growing inside a host cell, Rickettsia are metabolically inactive outside of the host cell. They cannot synthesize their own adenosine triphosphate (ATP), and, therefore, rely on cells for their energy needs.  As mentioned earlier in the chapter, this group of bacteria is very closely related to the ancestors of mitochondria as is reflected in these characteristics.

    Rickettsia spp. include a number of serious human pathogens. For example, R. rickettsii causes Rocky Mountain spotted fever, a life-threatening form of meningoencephalitis (inflammation of the membranes that wrap the brain). R. rickettsii infects ticks and can be transmitted to humans via a bite from an infected tick (Figure \(\PageIndex{1}\)).

    A micrograph of blue cells labeled tick hemolymph cells. Inside of these cells are small red cells labeled R. rickettsia.
    Figure \(\PageIndex{1}\): Rickettsias require special staining methods to see them under a microscope. Here, R. rickettsii, which causes Rocky Mountain spotted fever, is shown infecting the cells of a tick. (credit: modification of work by Centers for Disease Control and Prevention)


    An ecologically important example of the alphaproteobacteria is Rhizobium.  As mentioned in the previous sectionRhizobium forms a symbiotic relationship with legumes (plants such as beans, peas, and clover) and through a process called nitrogen fixation converts atmospheric nitrogen to a form usable by the plant.  Prior to the development of aritifical fetilizers, crop rotation between legumes and other crops was one of the major ways nitrogen was provided to crops.


    The most diverse class of gram-negative bacteria is Gammaproteobacteria, and it includes a number of human pathogens. For example, a large and diverse family, Pseudomonaceae, includes the genus Pseudomonas. Within this genus is the species P. aeruginosa, a pathogen responsible for diverse infections in various regions of the body. P. aeruginosa is a strictly aerobic, nonfermenting, highly motile bacterium. It often infects wounds and burns, can be the cause of chronic urinary tract infections, and can be an important cause of respiratory infections in patients with cystic fibrosis or patients on mechanical ventilators. Infections by P. aeruginosa are often difficult to treat because the bacterium is resistant to many antibiotics and has a remarkable ability to form biofilms. Other representatives of Pseudomonas include the fluorescent (glowing) bacterium P. fluorescens and the soil bacteria P. putida, which is known for its ability to degrade xenobiotics (substances not naturally produced or found in living organisms).

    Enterobacteriaceae is a large family of enteric (intestinal) bacteria belonging to the Gammaproteobacteria. They are facultative anaerobes and are able to ferment carbohydrates. Within this family, microbiologists recognize two distinct categories. The first category is called the coliforms, after its prototypical bacterium species, Escherichia coli. Coliforms are able to ferment lactose completely (i.e., with the production of acid and gas). The second category, noncoliforms, either cannot ferment lactose or can only ferment it incompletely (producing either acid or gas, but not both). The noncoliforms include some notable human pathogens, such as Salmonella spp., Shigella spp., and Yersinia pestis.

    E. coli has been perhaps the most studied bacterium since it was first described in 1886 by Theodor Escherich (1857–1911). Many strains of E. coli are in mutualistic relationships with humans. However, some strains produce a potentially deadly toxin called Shiga toxin, which perforates cellular membranes in the large intestine, causing bloody diarrhea and peritonitis (inflammation of the inner linings of the abdominal cavity). Other E. coli strains may cause traveler’s diarrhea, a less severe but very widespread disease.

    Proteobacterial Phototrophs

    One large group of proteobacteria are phototrophic.  This includes the purple or green bacteria that perform photosynthesis with the help of bacteriochlorophylls, which are green, purple, or blue pigments similar to chlorophyll in plants. Some of these bacteria have a varying amount of red or orange pigments called carotenoids. Their color varies from orange to red to purple to green (Figure \(\PageIndex{4}\)), and they are able to absorb light of various wavelengths. Traditionally, these bacteria are classified into sulfur and nonsulfur bacteria; they are further differentiated by color.  In contrast to the cyanobacteria which produce oxygen when they photosynthesize, these proteobacteria all perform anoxygenic phototrophy.  Many can use either organic (heterotrophy) or carbon dioxide (autotrophy) to obtain their carbon.

    A thick glass tube filled with purple regions labeled purple bacteria and green regions labeled green bacteria.
    Figure \(\PageIndex{4}\): Purple and green sulfur bacteria use bacteriochlorophylls to perform photosynthesis.



    • Proteobacteria is a phylum of gram-negative bacteria discovered by Carl Woese in the 1980s based on nucleotide sequence homology.
    • Proteobacteria are further classified into the classes alpha-, beta-, gamma-, delta- and epsilonproteobacteria, each class having separate orders, families, genera, and species.
    • Alphaproteobacteria are oligotrophs. The rickettsias are obligate intracellular pathogens, feeding on cells of host organisms; they are metabolically inactive outside of the host cell. Some Alphaproteobacteria can convert atmospheric nitrogen to nitrites, making nitrogen usable by other forms of life.
    • Gammaproteobacteria are the largest and the most diverse group of Proteobacteria. Many are human pathogens that are aerobes or facultative anaerobes. Some Gammaproteobacteria are enteric bacteria that may be coliform or noncoliform. Escherichia coli, a member of Gammaproteobacteria, is perhaps the most studied bacterium.
    • Some proteobacteria are anoxygenic phototrophs.


    1. 1 C.R. Woese. “Bacterial Evolution.” Microbiological Review 51 no. 2 (1987):221–271.
    2. 2 H. Reichenbach. “Myxobacteria, Producers of Novel Bioactive Substances.” Journal of Industrial Microbiology & Biotechnology 27 no. 3 (2001):149–156.
    3. 3 S. Suerbaum, P. Michetti. “Helicobacter pylori infection.” New England Journal of Medicine 347 no. 15 (2002):1175–1186.


    • Nina Parker, (Shenandoah University), Mark Schneegurt (Wichita State University), Anh-Hue Thi Tu (Georgia Southwestern State University), Philip Lister (Central New Mexico Community College), and Brian M. Forster (Saint Joseph’s University) with many contributing authors. Original content via Openstax (CC BY 4.0; Access for free at

    4.3.3: Proteobacteria is shared under a CC BY license and was authored, remixed, and/or curated by OpenStax.