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2.1: Sizes, Shapes, and Arrangements of Bacteria

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  • Learning Objectives

    1. List the three basic shapes of bacteria.
    2. List and describe 5 different arrangements of cocci.
    3. Define and give the abbreviation for the metric unit of length termed micrometer and state the average size of a coccus-shaped bacterium and a rod-shaped bacterium.
    4. List and describe 2 different arrangements of bacilli.
    5. List and describe 3 different spiral forms of bacteria.

    Bacteria are prokaryotic, single-celled, microscopic organisms (Exceptions have been discovered that can reach sizes just visible to the naked eye. They include Epulopiscium fishelsoni, a bacillus-shaped bacterium that is typically 80 micrometers (µm) in diameter and 200-600 µm long, and Thiomargarita namibiensis, a spherical bacterium between 100 and 750 µm in diameter.)

    1. generally much smaller than eukaryotic cells.
    2. very complex despite their small size. Even though bacteria are single-celled organisms, they are able to communicate with one another through a process called quorum sensing. In this way they can function as a multicellular population rather than as individual bacteria. This will be discussed in greater detail in Unit 2.

    To view a nice interactive illustration comparing size of cells and microbes, see the Cell Size and Scale Resource at the University of Utah.

    Bacterial cell shape is determined primarily by a protein called MreB. MreB forms a spiral band – a simple cytoskeleton – around the interior of the cell just under the cytoplasmic membrane. It is thought to define shape by recruiting additional proteins that then direct the specific pattern of bacterial cell growth. For example, bacillus-shaped bacteria that have an inactivated MreB gene become coccoid shaped, and coccus-shaped bacteria naturally lack the MreB gene. Most bacteria come in one of three basic shapes: coccus, rod or bacillus, and spiral.


    The cocci are spherical or oval bacteria having one of several distinct arrangements (Figure \(\PageIndex{2}\).1.1) based on their planes of division.


    Figure \(\PageIndex{2}\).1.1: Arrangement of cocci bacteria. image used with permission from Mariana Ruiz.

    a. Division in one plane produces either a diplococcus or streptococcus arrangement.

    diplococcus: cocci arranged in pairs (see Figure \(\PageIndex{2}\))

    - scanning electron micrograph of a Streptococcus pneumoniae, a diplococcus; courtesy of CDC
    - scanning electron micrograph of a Neisseria, a diplococcus; courtesy of Dennis Kunkel's Microscopy

    streptococcus: cocci arranged in chains (see Figure \(\PageIndex{3}\))

    - scanning electron micrograph of a Streptococcus pyogenes, a streptococcus; courtesy of Dennis Kunkel's Microscopy
    - transmission electron micrograph of Streptococcus from the Rockefeller University web page.

    - scanning Electron Micrograph of Enterococcus

    b. Division in two planes produces a tetrad arrangement.

    tetrad: cocci arranged in squares of 4 (see Figure \(\PageIndex{4}\))

    - scanning electron micrograph of Micrococcus luteus showing several tetrads

    c. Division in three planes produces a sarcina arrangement.

    sarcina: cocci in arranged cubes of 8 (see Figure \(\PageIndex{5}\))

    d. Division in random planes produces a staphylococcus arrangement.

    staphylococcus: cocci arranged in irregular, often grape-like clusters (see Figure \(\PageIndex{6}\))

    - negative image of Staphylococcus aureus
    - scanning electron micrograph of Staphylococcus aureus, a staphylococcus; courtesy of Dennis Kunkel's Microscopy
    - Scanning electron micrograph of methicillin-resistant Staphylococcus aureus (MRSA); courtesy of CDC

    An average coccus is about 0.5-1.0 micrometer (µm) in diameter. (A micrometer equals 1/1,000,000 of a meter.)

    The rod or bacillus

    Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or coccobacillus arrangement (see Figure \(\PageIndex{7}\)).

    a. bacillus: single bacilli (see Figure \(\PageIndex{8}\))
    - scanning electron micrograph of a bacillus; courtesy of CDC
    - scanning electron micrograph of Escherichia coli O157H7, a bacillus; courtesy of CDC

    b. streptobacillus: bacilli arranged in chains (see Figure \(\PageIndex{9}\))

    c. coccobacillus: oval and similar to a coccus (see Figure \(\PageIndex{9}\)A and Figure \(\PageIndex{9}\)B)

    An average bacillus is 0.5-1.0 µm wide by 1.0-4.0 µm long.

    The spiral

    Spirals come in one of three forms, a vibrio, a spirillum, or a spirochete. (see Figure \(\PageIndex{10}\))

    a. vibrio: a curved or comma-shaped rod (see Figure \(\PageIndex{11}\))

    - scanning electron micrograph of a Vibrio cholerae, a vibrio; courtesy of Dennis Kunkel's Microscopy

    b. spirillum: a thick, rigid spiral (see Figure \(\PageIndex{12}\))

    c. spirochete: a thin, flexible spiral (see Figure \(\PageIndex{13}\))

    - scanning electron micrograph of the spirochete Leptospira
    ; courtesy of CDC
    - scanning electron micrograph of the spirochete Treponema pallidum; courtesy of CDC

    Spirals range in size from 1 µm to over 100 µm in length.

    Exceptions to the above shapes

    There are exceptions to the three basic shapes of coccus, bacillus, and spiral. They include sheathed, stalked, filamentous, square, star-shaped, spindle-shaped, lobed, trichome-forming, and pleomorphic bacteria.

    Ultrasmall Bacteria

    Ultrasmall bacteria (150 could fit in a single Escherichia coli) have been discovered in groundwater that was passed through a filter with a pore size of 0.2 micrometers µm). They showed an average length of only 323 nanometers (nm) and an average width of 242 nm. They contain DNA, an average of 42 ribosomes per bacterium, and possessed pili . It is thought that they use these pili to attach to other bacteria from which they scavenge nutrients. Because the surface to volume ratio is even greater than in more traditional sized bacteria, they might be better designed to take up scarce nutrients from more nutrient-poor environments.


    1. There are three basic shapes of bacteria: coccus, bacillus, and spiral.
    2. Based on planes of division, the coccus shape can appear in several distinct arrangements: diplococcus, streptococcus, tetrad, sarcina, and staphylococcus.
    3. The bacillus shape can appear as a single bacillus, a streptobacillus, or a coccobacillus.
    4. The spiral shape can appear in several forms: vibrio, spirillum, and spirochete.
    5. The metric unit micrometer (1/1,000,000 or 10-6 of a meter) is used to measure bacterial size.

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