Bacterial Cell Morphology and Classification: Definition, Shapes & Arrangements

Bacterial Cell Morphology and Classification: Definition, Shapes & Arrangements
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  • 0:05 Bacterial Morphology
  • 0:33 Shapes of Cells
  • 2:01 Arrangement of Cells
  • 3:46 Spiral Cell Variations
  • 5:02 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Do all bacteria look the same? Definitely not! These tiny, singled-celled organisms come in a variety of morphologies, from cocci to spirals to tetrads. Many bacterial names even have clues to their morphology!

Bacterial Morphology

I remember getting sick as a kid and having to sit in the doctor's office. After what seemed like an hour, the doctor would look at me for two minutes, swab the back of my throat and declare that I had strep throat. As a kid, all that meant to me was a few days off from school and a week of antibiotics. As a microbiologist, that short diagnosis actually tells me a lot about the bacteria that are making my throat scratchy and sore.

Shapes of Cells


The first shape is called coccus, plural cocci. Cells that have a cocci shape are spherical, resembling tiny balls. The 'strep' in strep throat actually refers to the bacterium Streptococcus, which exists in tiny, spherical cells.

The second shape is bacillus, plural bacilli. These bacteria are shaped like small rods, longer than they are wide. A bacillus cell looks a lot like a pill. Have you ever heard of anthrax poisoning? Anthrax is caused by a rod-shaped bacteria called Bacillus anthracis. Some bacilli bacteria have round ends, while others are square. It is important to note that the term 'bacillus' can describe the cell shape as well as bacteria in the genus Bacillus.

The third bacterial shape is spiral. These bacterial cells are twisted in helices and resemble little corkscrews. Not to gross you out, but if you scrape some gunk off your teeth and look at it under the microscope, you will find many spiral bacteria!

Arrangement of Cells

Different species of cocci and bacilli also arrange themselves into groups characteristic to the specific bacteria. A bacteria that lives as one cell all alone is called a single cell. Some bacteria remain grouped together after cell division. A bacteria that lives in pairs has a diplo arrangement, with 'di-' meaning 'two.' So, Diplococcus is a cocci bacteria that is found in pairs.

A group of four cells forming a flat square is called a tetrad. Micrococcus roseus can form tetrads and is a normal bacteria found on the skin of mammals.

A sarcina is a cube-like group of eight cocci. Members of the genus Sarcina are harmless inhabitants of your large intestine that take this shape.

If the bacteria form long chains, the prefix strepto- is added. Going back to that pesky bacteria that causes strep throat, let's break down the genus Streptococcus. 'Strepto-' refers to long chains and '-coccus' refers to a spherical cell. So, the strep throat bacteria Streptococcus forms long chains of cocci.

If the dividing bacteria form an irregular, grape-like cluster, the prefix staphylo- is added. So, members of the Staphylococcus form clusters of cocci. An infamous member of this genus causes the dangerous infection known as MRSA, or methicillin-resistant Staphylococcus aureus.

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