By Jennifer Stearns, Michael Surette

Prokaryotic cells come in many different shapes and sizes that you can see under a microscope. A description of the shape of a cell is called the cell morphology. The most common cell morphologies are cocci (spherical) and bacilli (rods).

Coccibacillus are a mix of both, while vibrio are shaped like a comma, spirilla are shaped like a helix (a spiral, sort of like a stretched-out Slinky), and spirochetes are twisted like a screw. The illustration shows these common cell morphologies.

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Although prokaryotes are unicellular organisms, their cells can be arranged in a few different ways, like chains or clusters, depending on how the cells divide:

  • Cocci bacteria that divide along a single plane form small chains of two cells called diplococci or long chains of multiple cells called streptococci.

  • Cocci bacteria can also divide along multiple planes to form tetrads (two planes), cubelike sarcinae (three planes), or grapelike clusters called staphylococci (multiple planes).

  • Similarly to the cocci, rod-shaped bacteria can divide to form double-celled diplobacilli or longer chains called streptobacilli.

The shape of a cell is encoded in its genes. Although it’s known how cell shape is controlled, the reason behind the many different shapes remains a mystery.

You may notice that some of the morphologies are also the names of bacteria — for example, Streptococcus pneumoniae, Staphylococcus aureus, Bacillus anthracis, and Vibrio cholerae. That’s because morphologies are sometimes characteristic of bacterial genera.

Morphology is a descriptive characteristic — it doesn’t give you enough information to know exactly what type of bacteria you’re looking at or its function.