What Is Bacteria? A Clear Answer
What is bacteria?
Bacteria are tiny living things made of a single cell with no nucleus. They live almost everywhere on Earth, including in soil, water and inside your body. Most are harmless and many are helpful, such as the ones in your gut and in yogurt. A small number cause disease.
One note on the word itself: “bacteria” is plural. One cell is a bacterium, as Cleveland Clinic points out. People search “what is bacteria” all the time, and the answer is the same either way.
What a bacterium is made of
Bacteria are prokaryotes, meaning their DNA is not wrapped inside a nucleus the way it is in human, animal and plant cells. The Medical Microbiology textbook chapter on bacterial structure describes a nucleoid (a loose tangle of DNA) instead of an enclosed nucleus, a cytoplasm densely packed with ribosomes, and a tough cell wall made of peptidoglycan that protects the cell.
They are small. The same chapter says bacteria are best measured in micrometers, or thousandths of a millimeter. A large one such as Bacillus anthracis is about 1 micrometer wide and 3 to 10 long; some are a fifth of a micrometer across. Most multiply by binary fission: the cell copies its DNA and splits into two.
According to the Merck Manual, some bacteria also have flagella (whip-like tails for moving), a protective capsule, or the ability to form spores, a dormant form that survives dry or harsh conditions and springs back to life later. Many stick together in a slimy layer called a biofilm. Dental plaque is one example.
Where bacteria fit in the tree of life
In 1990 microbiologist Carl Woese and colleagues proposed dividing all life into three domains: Bacteria, Archaea and Eucarya. Archaea are also single cells without a nucleus, but their genes and chemistry set them apart. Eucarya, or eukaryotes, include every organism whose cells have a nucleus, from yeast to people.
Bacteria are also among the earliest known life forms on Earth, the Merck Manual notes, and they live in soil, seawater, deep in the Earth’s crust and even in radioactive waste. The National Human Genome Research Institute adds that some species survive extreme temperatures and pressures.
How scientists sort bacteria
Bacteria get a two-part scientific name: genus, then species, as in Streptococcus thermophilus. Within a species there are strains, which can differ in their genes and in what they do. That is why probiotic research, and many labels, name the exact strain. Merck and Cleveland Clinic describe these other common ways of grouping them:
| Way to group | Main types | What it tells you |
|---|---|---|
| Shape | Spheres (cocci), rods (bacilli), spirals (spirochetes) | A first clue under the microscope |
| Gram stain | Gram-positive (blue to purple) or gram-negative (red to pink) | Cell wall type, which helps doctors choose antibiotics |
| Oxygen needs | Aerobes need it, anaerobes do not, facultative bacteria can go either way | Where the bacteria can live; many gut bacteria are anaerobes |
| Genetics | Genus, species and strain | The precise identity, down to one line of a species |
“Positive” and “negative” in the Gram stain have nothing to do with good or bad. They only describe how the cell wall takes up the dye.
Bacteria in your body
You carry a lot of them. A widely cited 2016 analysis in PLoS Biology by Sender, Fuchs and Milo estimated that a 70 kg (154 lb) adult holds about 38 trillion bacteria and about 30 trillion human cells. That updated the old claim of ten bacterial cells for every human cell, which you will still see repeated online. The ratio is closer to one to one, and all those bacteria weigh only about 0.2 kg (half a pound).
The same paper found that the vast majority live in the colon, the main part of the large intestine. The skin and mouth hold far fewer. Together with fungi and viruses, these communities make up your microbiome. The National Institute of Environmental Health Sciences (NIEHS) says the microbiome helps protect against pathogens, helps the immune system develop and helps you digest food.
One clear example is fiber. Gut bacteria ferment fiber your own enzymes cannot break down and produce short-chain fatty acids such as butyrate, which a 2016 review in Cell by Koh and colleagues describes as key signals between gut microbes and the body. When the balance of these communities shifts in an unhealthy direction, researchers call it dysbiosis.
Helpful and harmful bacteria
Most bacteria never cause trouble. Merck notes that only a few kinds are usually linked to disease, and these are called pathogens. They can make you sick by releasing toxins, invading tissue or both. Even normally harmless gut bacteria can cause infection if they get somewhere they do not belong, such as the bloodstream.
Helpful bacteria include the ones used to make food. U.S. yogurt must be cultured with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus under the FDA’s yogurt standard. Probiotics are a related idea: the National Center for Complementary and Integrative Health (NCCIH) defines them as live microorganisms intended to have health benefits, and notes that different strains may have different effects. See probiotic foods for where to find them.
Antibiotics work by attacking parts of the bacterial cell, such as its wall or DNA, Cleveland Clinic explains, so they do nothing against viruses such as colds and flu. Overuse helps resistant strains spread. The World Health Organization (WHO) reports that about 1 in 6 lab-confirmed bacterial infections worldwide were resistant to antibiotics in 2023. If you do need antibiotics, take them as prescribed; our guide to gut health after antibiotics covers the days that follow.
Common questions
Is it “bacteria is” or “bacteria are”?
“Bacteria” is plural, so “bacteria are” is correct, and a single cell is a bacterium (Cleveland Clinic).
Are bacteria alive?
Yes. Each bacterium is a complete living cell that takes in nutrients and reproduces, usually by splitting in two.
What is the difference between bacteria and viruses?
They are different kinds of germs that need different treatment: antibiotics can treat many bacterial infections but do not work on viruses (Cleveland Clinic).
How many bacteria live in the human body?
About 38 trillion in a typical adult, roughly the same as the number of human cells, by one widely cited estimate (Sender et al., 2016).
Are all bacteria bad for you?
No. Most are harmless and many help with digestion and immune development, while a relatively small number of species cause disease.
Related topics
- Gut Health: The Complete Guide: how gut bacteria fit into the bigger picture.
- Fermented Foods for Gut Health: foods that carry live bacteria.
- Probiotic vs Yogurt: capsules and cultured foods side by side.
- Streptococcus thermophilus: a close look at one of the two classic yogurt bacteria.
- How to Make Yogurt at Home: put helpful bacteria to work in your kitchen.
Sources
- Bacteria: Definition, Types, Benefits, Risks and Examples (Cleveland Clinic)
- Overview of Bacteria (Merck Manual Consumer Version)
- Bacteria (National Human Genome Research Institute, Talking Glossary of Genomic and Genetic Terms)
- Salton MRJ, Kim KS. Structure (in Baron S, ed. Medical Microbiology, 4th ed., University of Texas Medical Branch at Galveston, 1996)
- Woese CR, et al. Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya (Proceedings of the National Academy of Sciences, 1990)
- Sender R, et al. Revised Estimates for the Number of Human and Bacteria Cells in the Body (PLoS Biology, 2016)
- Koh A, et al. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites (Cell, 2016)
- Microbiome (National Institute of Environmental Health Sciences)
- Probiotics: What You Need To Know (National Center for Complementary and Integrative Health)
- 21 CFR 131.200 Yogurt (Electronic Code of Federal Regulations, U.S. Food and Drug Administration)
- Antimicrobial resistance fact sheet (World Health Organization)
