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Bacterial Cells
Bacterial Cells. Source: iStock

Bacterial Cells: Size, Shape & Arrangement

The Prokaryotic Cell

Prokaryotes make up a vast group of very small unicellular organisms that include bacteria and archaea. The majority are bacteria. Although bacteria and archaea look similar, their chemical composition is different. The thousands of species of bacteria are differentiated by many factors, including morphology (shape), chemical composition, nutritional requirements, biochemical activities, and sources of energy. It is estimated that 99% of the bacteria in nature exist in biofilms.

Arrangements of cocci.
Arrangements of cocci. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.
The Size, Shape, and Arrangement of Bacterial Cells

Most bacteria range from 0.2 to 2.0 µm in diameter and from 2 to 8 µm in length. They may be spherical-shaped coccus (plural: cocci, meaning berries), rod-shaped bacillus (plural: bacilli, meaning little rods or walking sticks), and spiral.

Bacilli. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

Cocci are usually round but can be oval, elongated, or flattened on one side. When cocci divide to reproduce, the cells can remain attached to one another. Cocci that remain in pairs after dividing are called diplococci; those that divide and remain attached in chainlike patterns are called streptococci. Those that divide in two planes and remain in groups of four are known as tetrads.

Those that divide in three planes and remain attached in cubelike groups of eight are called sarcinae. Those that divide in multiple planes and form grapelike clusters or broad sheets are called staphylococci. These group characteristics are frequently helpful in identifying certain cocci.

Bacilli divide only across their short axis, so there are fewer groupings of bacilli than of cocci. Most bacilli appear as single rods, called single bacilli. Diplobacilli appear in pairs after division, and streptobacilli occur in chains. Some bacilli look like straws. Others have tapered ends, like cigars. Still others are oval and look so much like cocci that they are called coccobacilli.

 Gram stained Bacillus anthracis
Gram stained Bacillus anthracis. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

“Bacillus” has two meanings in microbiology. As we have just used it, bacillus refers to a bacterial shape. When capitalized and italicized, it refers to a specific genus. For example, the bacterium Bacillus anthracis is the causative agent of anthrax. Bacillus cells often form long, twisted chains of cells.

Spiral bacteria have one or more twists; they are never straight. Bacteria that look .like curved rods are called vibrios. Others, called spirilla (singular: spirillum), have a helical shape, like a corkscrew, and fairly rigid bodies. Yet another group of spirals are helical and flexible; they are called spirochetes.

Spiral bacteria
Spiral bacteria. Source: Microbiology: An Introduction,13th Edition, Tortora et. al.

Unlike the spirilla, which use propeller-like external appendages called flagella to move, spirochetes move by means of axial filaments, which resemble flagella but are contained within a flexible external sheath. There are also star-shaped and rectangular prokaryotes.

The shape of a bacterium is determined by heredity. Genetically, most bacteria are monomorphic; that is, they maintain a single shape. However, a number of environmental conditions can alter that shape. If the shape is altered, identification becomes difficult. Moreover, some bacteria, such as Rhizobium (r¯-ZO I ¯ -be¯-um) and Corynebacterium (kor’¯-ne¯-bak-TI-re¯-um), are genetically pleomorphic, which means they can have many shapes, not just one.

We will discuss structures external to the cell wall, the cell wall itself, and structures internal to the cell wall.

About Fahmida Akter Bristi

I am currently doing my Bachelor degree. I love to write by exploring knowledge that is new to me. Hope this effort of mine benefits you all. Right now, I am the head of Project R. Franklin & Project Waksman in Society & Science Foundation. Knock me anytime. Email:

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