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Survey of Bacteria

Physical Traits of Bacteria

Bacteria are classified based on physical traits such as morphology, arrangement of cells, and cell wall content.

Because classification of bacteria according to their evolutionary relationships is rarely very useful in microbiology, many other systems of classification have been devised. Bacteria can be classified based on morphology (shape), nutrition, and biochemistry, among other characteristics.

One of the most common classifications of bacteria is morphology, or shape. The shape of a bacterium can be seen using a microscope. Simple light microscopes are sufficient for obtaining this classification criterion. Bacteria can be divided into three basic shapes. Coccus (plural, cocci) are round or spherical shaped. Bacillus (plural, bacilli) are rod-shaped. Spirillum (plural, spirilla) are spiral-shaped. Spirilla are further divided. Vibrio are comma-shaped. Spirochete are curved or corkscrew-shaped. Some bacteria show modifications of these shapes and are sometimes classified as "other," although most can be considered to fall into one of these five categories. There are also genera of bacteria called Bacillus, Spirillum, and Vibrio. These genera should not be confused with the bacterial shape designations, though each genus does conform to the shape suggested by its name.

Shapes of Bacteria

Bacterial Shape Example Micrograph Other Example Organisms

Bacilli (rod-shaped)

Credit: NIAID
  • Salmonella enterica
  • Clostridium botulinum

Cocci (spherical or ball shaped)

Credit: CDC
  • Neisseria gonorrhoeae
  • Staphylococcus haemolyticus

Spirilla (spiral or helical shaped)

Credit: Robert Weaver, Ph.D.
  • Helicobacter pylori
  • Spirillum volutans

Vibrio (comma shaped)

Credit: CDC/Dr. William A. Clark
  • Vibrio parahaemolyticus
  • Vibrio vulnificus

Spirochete (corkscrew or helical shaped)

Credit: CDC/Susan Lindsley
  • Borrelia burgdorferi
  • Leptospira interrogans

The five basic shapes of bacteria are cocci (round), bacilli (rod-shaped), spirilla (spiral), vibrio (comma), and spirochete (corkscrew). Some bacteria show variations or combinations of these shapes. (E. coli: scanning electron microscope; S. aureus: light microscope; C. jejuni: light microscope; V. cholerae: light microscope, 320x; T. pallidum: light microscope)

In addition to shape, bacteria may be classified according to their arrangement of cells. Bacterial cell arrangements occur based upon the plane of cell division. A prefix is used to identify the arrangement. Diplo- denotes two bacteria cells stuck together. Strepto- denotes cells forming a line. Staphylo- denotes cells that clump together. Cocci are found in the diplo-, strepto-, and staphylo- arrangements. Bacilli are found in the diplo- and strepto- arrangements.

Bacterial Cell Division Patterns

Bacterial Cell Arrangement Example Micrograph

Coccus (single cell)

Very few discovered; no known micrographs

Diplococcus (two cells)

Credit: NIAIDLicense: CC BY 2.0

Streptococcus (linear cells)

Credit: NIAID

Staphylococcus (clumped cells)

Credit: CDC/Matthew J. Arduino, DRPH

Bacillus (single cell)

Credit: CDC/Dr. William A. Clark

Diplobacillus (two cells)

Credit: NIAID

Streptobacillus (linear cells)

Credit: CDC/Larry Stauffer, Oregon State Public Health Laboratory

Vibrio (single cell)

Credit: CDC/James Gathany

Spirillum (single cell)

Credit: Wolframm Adlassnig

Spirochete (single cell)

Credit: CDC/Dr. Edwin P. Ewing, Jr.

The arrangement of bacterial cells relative to one another in a pure culture depends on the shape of the organism and its reproductive growth pattern. (N. gonorrhoeae: scanning electron microscope; S.pyrogenes: scanning electron microscope; S. aureus: scanning electron microscope, 50,000x; B. cereus: light microscope; K. pneumonia: scanning electron microscope; B. anthracis: light microscope, 1,000x; V. cholerae: scanning electron microscope, 13,184x; S. volutans: light microscope; T. pallidum: light microscope)

Furthermore, many bacteria have flagella, which are long, whiplike appendages, or cilia, which are short hairlike protrusions. Other bacteria are atrichous, meaning they have neither cilia nor flagella. These characteristics also aid in classification.
Cilia are tiny hairlike structures that help bacteria move and obtain food. The flagellum, a whiplike appendage, helps a bacterium move.
Credit: CDC/Dr. William A. Clark (right)
The flagellum consists of three main parts: a hook, a motor, and a filament. The filament, a long, thin structure, is the part that is most visible outside the bacterial cell. The hook is an L-shaped structure that connects the filament to the body of the bacterium. The motor, embedded in the cell wall, turns the hook, which in turn causes the filament to whip back and forth, generating motion for the cell.


The filament of the flagellum extends out beyond the bacterium. It connects to the rest of the cell via the hook. The motor turns the hook, causing the filament to whip back and forth, which moves the bacterium.
Bacteria are also identified by a Gram stain, a method of staining used to differentiate types of bacteria based on cell wall structure. The Gram stain was developed in 1884 by Danish scientist Hans Christian Gram (1853–1938). The stain differentiates bacteria based on the makeup of their cell walls. The bacteria are washed first in a purple stain called crystal violet, then in a pink stain called safranin. Gram-positive bacteria contain a thick mesh of polymer chains of sugar and protein molecules called peptidoglycan in their cell walls, which retains both stains, but the crystal violet masks the safranin and thus stains purple. The gram-negative bacteria contain considerably less peptidoglycan, so they retain only the safranin and stain pink. Gram staining is frequently used as a first step in identifying a bacterium. Some bacteria are difficult to definitively classify using the Gram stain. However, most bacteria are frequently referenced according to their Gram stain classification (gram-negative or gram-positive).
The Gram stain differentiates between bacteria with cell walls rich in the structural polymer peptidoglycan, or gram-positive bacteria, and those with little peptidoglycan, or gram-negative bacteria.
Credit: CDC/Dr. William A. Clark (left), CDC (right)