Rod, Sphere and Spiral: The 3 Main Shapes of Bacteria

Bacteria (singular = bacterium) are often seen as organisms that cause disease, however there are useful bacteria too. Some are used to produce antibiotics, others live in your intestines and help you digest your food and there are even helpful bacteria that live in the roots of plants.

They occur in a wide variety of forms but the three main basic shapes of bacteria are rods (also known as bacilli), spheres (also known as cocci) and spirals.

Escherichia coli

Escherichia coli is found in the human gut [photo: US National Institutes of Health]

Rod-Shaped Bacteria

These are what most people would draw if they were asked to draw bacteria. They are longer than they are wide (in the case of coccobacilli, only just) and usually exist as single cells. There are both helpful and harmful rod-shaped bacteria; the bacterium that causes the deadly disease anthrax is rod-shaped. A common helpful rod-shaped bacteria is Lactobacillus. You come across this, or at least the results of its work, every time that you eat a yogurt.

The general word used for this type of bacteria is bacilli (singular bacillus) which comes from the ancient Greek word meaning ‘rod’.

Spherical Bacteria

Examples of these are strains of streptococcus and staphylococcus, some of which can be quite dangerous to humans. Staphylococcus bacteria can often be found on your skin, where they do no harm but if they get into the body or the blood, they can cause problems such as minor as sinusitis or as serious as pneumonia, septicemia (blood poisoning) and peritonitis.

Bacterial meningitis is also caused by a spherical bacteria. You have probably heard of MRSA which is a resistant strain of bacteria — this is Staphylococcus aureus, which has become resistant to many types of antibiotics. But the cocci aren’t all bad, some types of streptococcus are used safely in the production of cheese and yogurt.

Spiral-Shaped Bacteria


Spirochete bacteria [photo: MarcoTolo]

These are the longest of the bacteria and can be further sub-divided into spirillium bacteria and spirochete variations. The spirillium bacteria are rigid, but the spirochetes are flexible.

They are generally free-living single bacteria and can move around using a flagellum to propel themselves. Their spiral shape makes it easy for them to penetrate other organisms. The spirochete Borrelia burgdorferi is the bacteria strain responsible for the unpleasant Lyme disease which is caught from deer ticks. This bacterium can transform itself into different forms, making Lyme disease so difficult to treat.

Spiral bacteria have been found in some of the most inhospitable places on Earth — extremely saline (salty) conditions, high temperature environments and a highly alkaline lake in Africa.

Although bacteria are single-celled organisms, the cell that forms their body is usually much smaller than the cells of your body and can only be seen individually under the microscope. Their sizes are measured in micrometres (millionths of a metre) and you can fit about a thousand bacteria in one millimetre! The spiral bacteria are usually longer than the others, but still microscopically small.

Taught science for 16 years at a secondary school in the East Midlands.