What bacteria are

Bacteria are simple single-celled organisms, but they are highly diverse. They inhabit virtually every environment on Earth, including many places where no other organisms can survive.

Bacteria are single-celled organisms

Bacteria are very simple compared to most other organisms. They are made up of just one cell – and that cell is smaller and less complex than most of the other cell types in existence.

Bacterial cells have no nucleus. They also lack many of the organelles (such as mitochondria) that other cells have. Bacteria have a single circular chromosome that sits in the cell cytoplasm. Scientists call bacterial cells ‘prokaryotic’ – this word describes the lack of a nucleus in bacterial cells and distinguishes them from more complex cell types (which are known as ‘eukaryotic

Bacterial cells are too small to be seen with the naked eye. For this reason, they remained undiscovered until the 1600s, when Antonie van Leeuwenhoek viewed bacteria (and other single-celled organisms) using a spherical glass lens he had made himself.

Find out more about bacterial DNA – the role of plasmids.

Bacteria are highly diverse

Even though bacterial cells have a simple structure, there are a vast number of distinct bacterial species. There are also surprisingly large differences between bacterial species – one may be as different to another as we humans are to carrots!

How can you study the differences between organisms that are too small to see? In the past, differences between bacterial species were decided by studying their appearance (as seen under the microscope), their preferred food source and their habitat (the environment in which they are found). Now, you can also compare the genomes of two species of bacteria to see where the DNA sequence differs. This has led to a greater understanding of just how diverse bacteria are.

Bacteria are everywhere – even in extreme environments

There are approximately 5 nonillion (5x10³⁰ or 5,000,000,000,000,000,000,000,000,000,000!) bacteria on Earth. They inhabit virtually every possible habitat (including those where no other life can exist). Some species can thrive in extremely low or extremely high temperatures. Others live in radioactive areas. Still others inhabit very acidic or alkaline environments.

Many bacterial species make their home in, on or around other organisms – in the gut, on the skin and in ears, eyes, belly buttons and so on. The bacterium Escherichia coli, which is a mainstay of modern biotechnology, lives in the digestive system of warm-blooded animals, including humans. For this reason, it grows best at body temperature (37.4ºC).

Biofilms – bacteria working together

In many situations, individual bacteria can come together to form biofilms – large conglomerations of bacterial cells enclosed within a slimy matrix and attached to a surface.

Within biofilms, bacterial cells can act almost like cells in a multicellular organism. Individual cells can communicate with each other and can have distinct roles within the biofilm. The upshot? A community of bacteria that is relatively resistant to stress, can use food resources effectively and can respond readily to changes in the environment.

In people, biofilms have a role in many diseases where bacterial infection is important. Plaque on teeth is a biofilm, as is the mucus that coats the lungs of individuals with cystic fibrosis. Biofilms can form on the gums (in gum disease), on medical devices (such as hip implants and catheters) and elsewhere. Bacteria in biofilms are much more resistant to antibiotics than individual bacteria.

Learn more about bacteria and other microorganisms in this article.