Bacteria are the earliest forms of life on Earth discovered to be over 4 billion years old. They live in almost all land and sea habitats, they can even be found in Antarctic rocks!

Are Bacteria Animals or Plants?

Bacteria are neither animals nor plants. With bacteria sharing similar processes and characteristics to plants and animals it’s understandable why people would ask, “are bacteria animals or plants?”. Bacteria are single-celled, prokaryotic organisms in comparison to animals and plants which are multicellular, eukaryotic organisms. Because bacteria are prokaryotic, they do not have a nucleus and no membrane-bound organelles. In contrast, plants and animals are made up of eukaryotic cells, which means they have a nucleus and membrane-bound organelles like mitochondria or golgi apparatus.

Escherichia coli bacteria under microscope
Escherichia coli bacteria under microscope Image: National Institute of Allergy and Infectious Diseases
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Living things are classified into five kingdoms: animals belong to Kingdom Animalia, plants belong to Kingdom Plantae, fungi to Kingdom Fungi, protists to Kingdom Protista and bacteria is classified under their own kingdom known as Kingdom Monera.

So, are bacteria animals or plants? They are a unique category of organisms that have their own classification system.

Are bacteria animals?

No, bacteria are not animals. Although bacteria does share some characteristics with animals, for example, bacteria produces a typical nucleic acid that are found in parts of the human pancreas, spleen, and sperm.

There are 39 trillion bacterial cells in the human body, which make up about 30% of our cell composition. There are over 1000 different species of bacteria living in the human gut, these bacteria are good bacteria that keep us healthy by helping us fight diseases, make vitamins for our body, and digest complex carbohydrates.

However, not all bacteria are good bacteria, the pain you get from having a sore throat can sometimes be strep throat, a bacterial infection of the throat and tonsils caused by Streptococcus pyogenes bacteria.

Streptococcus pyogenes under microscope
Streptococcus pyogenes under microscope Image: Centre for Disease Control
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When the human body dies, the initial group of organism involved in decomposition at a microscopic level is bacteria, along with fungi and molds.This is called putrefaction. Other larger organisms such as the larvae of flies, beetles, mites and moths help speed up decay by feeding on dead matter and breaking it down to smaller pieces. This increases the surface area for the bacteria, fungi and molds. Bacteria are considered the “first colonisers” because they are already present in the body before death.

Are bacteria plants?

No, bacteria are not plants. Although early scientists wanted to classify bacteria under the plant kingdom because of their similarities with plants, modern scientists classify bacteria under their own Kingdom Monera.

Bacteria share some characteristics with plants, for example, cyanobacteria can photosynthesise to obtain energy. Just like plants, cyanobacteria use sunlight and carbon dioxide to produce food and similarly release oxygen as a by-product. Unlike plant cells, many cyanobacteria are also capable of nitrogen fixation, which is the conversion of nitrogen gas in the atmosphere to ammonia, nitrites, and nitrates, which can be absorbed by plants.

Nitrogen-fixing nodule on plant roots
Nitrogen-fixing nodule on plant roots Image: Soybean Checkoff
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Many leguminous plants like peas, beans, clovers, lentils and peanuts, develop a symbiotic relationship with nitrogen-fixing bacteria. These bacteria, called rhizobia, are located within nodules in the roots of these plants. When the plants die, the rhizobia release nitrogen into the soil creating natural fertiliser. This is why many farmers rotate crops in their fields with nitrogen-fixing plants.

While plants can only photosynthesise to obtain food, many cyanobacteria can convert chemicals such as sulfur, hydrogen sulfide, ammonia, manganese or iron, to produce food, a process known as chemosynthesis. These bacteria can be found in highly toxic environments such as deep sea hydrothermal vents and cold seeps.

Tubeworms growing near hydrothermal vents
Tubeworms growing near hydrothermal vents Image: National Geographic
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In answering the question, are bacteria animals or plants, we can deduce that bacteria are unique organisms and deserve their own separate classification system. Bacteria are neither animals nor plants.

The Australian Museum specialises in taxonomic and systematic research. To learn more about how scientists collect and classify animals and plants, visit the museum for more behind the scenes learning.