3 Common Bacteria Shapes

Bacteria are single-celled, prokaryotic organisms that come in different shapes. They are microscopic in size and lack membrane-bound organelles as do eukaryotic cells, such as animal cells and plant cells. Bacteria are able to live and thrive in various types of environments including extreme habitats such as hydrothermal vents, hot springs, and in your digestive tract. Most bacteria reproduce by binary fission. A single bacterium can replicate very quickly, producing large numbers of identical cells that form a colony.

Not all bacteria look the same. Some are round, some are rod-shaped bacteria, and some have very unusual shapes. In general, bacteria can be classified according to three basic shapes: Coccus, Bacillus, and Spiral.

Common Shapes of Bacteria

Coccus: spherical or round
Bacillus: rod shaped
Spiral: curve, spiral, or twisted

Common Bacterial Cell Arrangements

Diplo: cells remain in pairs after dividing
Strepto: cells remain in chains after dividing
Tetrad: cells remain in groups of four and divide in two planes
Sarcinae: cells remain in groups of eight and divide in three planes
Staphylo: cells remain in clusters and divide in multiple planes

Though these are the most common shapes and arrangements for bacteria, some bacteria have unusual and much less common forms. These bacteria have varying shapes and are said to be pleomorphic—they have different forms at different points in their life cycles. Other unusual bacteria forms include star-shapes, club-shapes, cube-shapes, and filamentous branches.

Cocci Bacteria

This antibiotic resistant strain of Staphylococcus aureus bacteria (yellow), commonly known as MRSA, is an example of cocci shaped bacteria.
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Cocci Cell Arrangements

Coccus is one of the three primary shapes of bacteria. Coccus (cocci plural) bacteria are round, oval, or spherical in shape. These cells can exist in several different arrangements that include:

Diplococci: cells remain in pairs after dividing.
Streptococci: cells remain in chains after dividing.
Tetrad: cells remain in groups of four and divide in two planes.
Sarcinae: cells remain in groups of eight and divide in three planes.
Staphylococci: cells remain in clusters and divide in multiple planes.

Types of Cocci

Staphylococcus aureus bacteria are cocci shaped bacteria. These bacteria are found on our skin and in our respiratory tract. While some strains are harmless, others such as methicillin-resistant Staphylococcus aureus (MRSA), can cause serious health issues. These bacteria have become resistant to certain antibiotics and can cause serious infections that may result in death. Other examples of coccus bacteria include Streptococcus pyogenes and Staphylococcus epidermidis.

Bacilli Bacteria

E. coli bacteria are a normal part of the intestinal flora in humans and other animals, where they aid digestion. They are examples of bacilli shaped bacteria.

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Bacillus Cell Arrangements

Bacillus is one of the three primary shapes of bacteria. Bacillus (bacilli plural) bacteria have rod-shaped cells. These cells can exist in several different arrangements that include:

Monobacillus: remains single rod-shaped cell after dividing.
Diplobacilli: cells remain in pairs after dividing.
Streptobacilli: cells remain in chains after dividing.
Palisades: cells in a chain are arranged side-by-side instead of end-to-end and are partially attached.
Coccobacillus: cells are short with a slight oval shape, resembling both coccus and bacillus bacteria.

Types of Bacilli

Escherichia coli (E. coli) bacteria are bacillus shaped bacteria. Most strains of E. coli that reside within us are harmless and even provide beneficial functions, such as food digestion, nutrient absorption, and the production of vitamin K. Other strains, however, are pathogenic and can cause intestinal disease, urinary tract infections, and meningitis. More examples of bacillus bacteria include Bacillus anthracis, which cause anthrax and Bacillus cereus, which commonly cause food poisoning.

Spirilla Bacteria

Spirilla Bacteria.

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Spiral shape is one of the three primary shapes of bacteria. Spiral bacteria are twisted and commonly occur in two forms: spirillum (spirilla plural) and spirochetes. These cells resemble long, twisted coils.

Spirilla

Spirilla bacteria are elongated, spiral-shaped, rigid cells. These cells may also have flagella, which are long protrusion used for movement, at each end of the cell. An example of a spirillum bacterium is Spirillum minus, which causes rat-bite fever.

Spirochetes Bacteria

This spirochete bacterium (Treponema pallidum) is spirally twisted in form, elongated and appearing thread-like (yellow). It causes syphilis in humans.

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Spirochetes

Spirochetes (also spelled spirochaete) bacteria are long, tightly coiled, spiral-shaped cells. They are more flexible than spirilla bacteria. Examples of spirochetes bacteria include Borrelia burgdorferi, which causes Lyme disease and Treponema pallidum, which causes syphilis.

Vibrio Bacteria

This is a group of vibrio cholerae bacteria which causes cholera. Science Picture Co/Getty Images

Vibrio bacteria are Gram-negative and similar in shape to spiral bacteria. These facultative anaerobes and can survive without oxygen. Vibrio bacteria have a slight twist or curve and resemble the shape of a comma. They also have a flagellum, which is used for movement. A number of species of vibrio bacteria are pathogens and are associated with food poisoning. These bacteria can infect open wounds and cause blood poisoning. An example of a Vibrio species that causes gastrointestinal distress is Vibrio cholerae which is responsible for cholera.

Bacteria Shapes