Bacteria that Move by Unusual Mechanisms

Some bacteria have unique mechanisms of motility that enable them to easily move to desirable locations. These organisms include the spirochetes and the magnetotactic bacteria.


The spirochetes (Greek spira for "coil" and chaete for "hair") are a group of Gram-negative bacteria with a unique motility mechanism that enables them to move through thick, viscous environments such as mud. Distinguishing characteristics include their spiral shape, flexible cell wall, and motility by means of an axial filament. The axial filament is composed of sets of flagella that originate from both poles of the cell; unlike typical flagella, these are contained within the periplasm. The opposing sets of flagella extend toward each other, overlapping in the mid-region of the cell. Rotation of the flagella within the confines of the outer membrane causes the cell to move like a corkscrew, sometimes deviating into flexing motions. Many spirochetes are very slender and can only be seen using special methods such as dark-field microscopy (figure 11.27). Many are also difficult or impossible to cultivate, and their classification is based largely on their morphology and ability to cause disease.

Spirochetes include free-living bacteria that inhabit aquatic environments, as well as species that reside on or in animals. Members of the genus Spirochaeta are anaerobes or facultative anaerobes that thrive in muds and anaerobic waters. Leptospira species are aerobic; some species are free-living in aquatic environments, whereas others thrive within animals. Leptospira

Figure 11.26 Luminescent Bacteria (a) Plate cultures of luminescent bacteria. (b) Photograph of a flashlight fish; under the eye is a light organ colonized with luminescent bacteria.

used by a variety of different bacteria to regulate the expression of certain genes. ■ quorum sensing, p. 186

Members of the genera Photobacterium and Vibrio are Gram-negative rods (the rods of Vibrio species are curved) with polar flagella. They are facultative anaerobes and typically inhabit aqueous environments. Those species that require sodium for growth are usually found in marine environments. Not all are luminescent, and some species of Vibrio cause human disease. Medically important species include V. cholerae, which causes cholera, and V. parahaemolyticus, which causes gastrointestinal disease.

The Genus Legionella

Legionella species are commonly found in aquatic environments, where they often reside within protozoa. They have even been isolated from water in air conditioners and produce misters. They are Gram-negative obligate aerobes that utilize amino acids but not carbohydrates as a source of carbon and energy. Legionella pneumophila can cause respiratory disease when it is inhaled in aerosolized droplets.

5 mm

Figure 11.27 Spirochetes Dark-field photomicrograph of spirochetes.

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Figure 11.27 Spirochetes Dark-field photomicrograph of spirochetes.

290 Chapter 11 The Diversity of Prokaryotic Organisms interrogans causes the disease leptospirosis, which can be transmitted in the urine of infected animals. Spirochetes that are adapted to reside in body fluids of humans and other animals will be discussed later. ■ leptospirosis, p. 636

Magnetotactic Bacteria

Magnetotactic bacteria such as Magnetospirillum (Aquaspirillum) magnetotacticum contain a string of magnetic crystals that align them with the earth's magnetism (see figure 3.41). This enables them to efficiently move up or down in the water or sediments. It is thought that this unique type of movement enables them to locate the microaerophilic habitats they require. Magnetospirillum species are Gram-negative spiral-shaped organisms. ■ magnetotaxis, p. 65

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