Shapes of Bacteria – Cocci, Bacilli and Spirals to Label

All 12 shapes on the diagram

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Start here: the three shapes 3

Coccus A bacterium shaped like a ball. The plural is cocci.
Bacillus A bacterium shaped like a rod. The plural is bacilli.
Spirillum A thick, rigid spiral. The plural is spirilla.

Cocci together 5

Diplococcus Cocci that stay in pairs after they divide.
Streptococcus Cocci in a chain. They divide in one plane and stay joined.
Staphylococcus Cocci in an irregular cluster. They divide in no regular plane.
Tetrad Cocci in a square of four. They divide in two planes.
Sarcina Cocci in a cube of eight. They divide in three planes.

Rods and spirals 4

Coccobacillus A very short rod, oval and close to a coccus in shape.
Streptobacillus Bacilli in a chain, joined end to end.
Vibrio A curved rod, shaped like a comma.
Spirochete A thin, flexible spiral with many turns.

How this list was compiled: Every name was checked on 2026-10-01 and kept only where two independent sources use it. (1) Gary Kaiser, Microbiology, section 2.1, Sizes, Shapes, and Arrangements of Bacteria, an open college textbook, https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Kaiser)/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/2%3A_The_Prokaryotic_Cell_-_Bacteria/2.1%3A_Sizes_Shapes_and_Arrangements_of_Bacteria : the three basic shapes, the five arrangements of cocci and the planes of division behind them, the arrangements of bacilli and the three spiral forms. (2) Wikipedia, text only: Bacterial cellular morphologies, https://en.wikipedia.org/wiki/Bacterial_cellular_morphologies . The American Heritage Dictionary was read for bacillus and spirillum. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

Bacteria come in three basic shapes: the round coccus, the rod-shaped bacillus and the spiral. The first drawing shows one of each. The next two show how the cells stay together after they divide.

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The name tells you the arrangement

Diplo means two, strepto means a chain, and staphylo means a bunch. Put one in front of coccus or bacillus and you have the name: a streptococcus is a chain of round cells, a streptobacillus a chain of rods.

Three kinds of curve

A vibrio is a rod with one bend, like a comma. A spirillum is a thick, stiff spiral. A spirochete is thin and flexible, with many close turns.

To draw them from memory, start with a ball, a sausage and a wave, then repeat the ball in twos, fours and a bunch.

Questions and answers

How do you name, draw and label the three shapes of bacteria?

Draw a ball and label it coccus. Draw a rod with rounded ends and label it bacillus. Draw a wavy spiral and label it spirillum. The plurals are cocci, bacilli and spirilla.

What is the difference between streptococcus and staphylococcus?

Both words describe round cells. In a streptococcus the cells divide in one plane and stay joined in a chain. In a staphylococcus they divide in no regular plane, so they pile up in a cluster shaped like a bunch of grapes.

What is the difference between a spirillum and a spirochete?

Both are spirals. A spirillum is thick and rigid. A spirochete is very thin, long and flexible, with many more turns along its length.

Why do cocci form pairs, squares and cubes?

It depends on how they divide. Division in one plane leaves pairs or chains. Division in two planes leaves a flat square of four, called a tetrad. Division in three planes leaves a cube of eight, called a sarcina.