What Are Bacteria?

Bacteria are tiny, single-celled organisms that are found almost everywhere. The human body is estimated to contain at least as many bacterial cells as human cells. Most of these bacteria are harmless to us, and many perform beneficial purposes—such as helping to digest food. However, some bacteria are pathogenic, meaning they can make us sick.

This article will discuss what bacteria are, what they do, the types of bacteria, and what happens if there is a bacterial infection.


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What Are Bacteria?

Bacteria are microbes—living organisms that are too small to be seen without a microscope. Bacteria are so tiny that if lined up, 1,000 could span the width of a pencil eraser. Each bacterium (singular of bacteria) consists of a single cell.

Bacteria play essential roles in the Earth's ecosystems. They are also helpful to humans in many ways. For example, bacteria are used to make yogurt and cheese.

Bacteria are important inside the body too. Most of the bacteria in the human body live in the gastrointestinal tract. Bacteria are also found in any area that has contact with the environment outside the body, such as the:

Are Bacteria Bad?

Most bacteria are harmless or beneficial.

They exist in all of earth's habitats and can be found in soil, oceans, rock, snow, plants, animals, and more. This bacteria serves important purposes, including helping break down dead plant matter for nutrient cycling.

Bacteria help humans by working to digest food (making nutrients available to us), neutralizing toxins, destroying disease-causing cells, protecting against invading pathogens, and helping our bodies function normally.

Relatively few bacteria cause disease or make us sick.

Types of Bacteria

Bacteria can be classified using different sets of criteria. One common way is to separate them into gram-positive and gram-negative bacteria.

Gram-negative bacteria contain an outer membrane that gives them an extra line of defense, making them harder to kill with antibiotics. This also makes them more likely to develop resistance.

Another common way to classify bacteria is by shape. The five primary bacteria shape groups are:

  • Spherical: Cocci
  • Rod: Bacilli
  • Spiral: Spirilla
  • Comma: Vibrios
  • Corkscrew: Spirochaetes

Bacteria in these groups can exist as:

  • Single cells
  • Pairs
  • Chains
  • Clusters

Bacterial Infection

Infectious ("bad") bacteria can make people sick. These bacteria reproduce quickly and can give off toxins, causing infection, tissue damage, and illness.

Bacteria that can be infectious include:

Bacteria can infect many areas of the body, such as the skin, lungs, bowel, throat, and more. Bacteria can cause illnesses ranging from mild to severe, including:

Bacterial infections can also be "secondary infections," meaning they develop while your body is fighting another infection. For example, bacterial pneumonia could develop while your body fights a viral infection like COVID-19.

Symptoms of Bacterial Infection

Symptoms of bacterial infection depend on the location and type of infection and bacteria.

General signs you may have a bacterial infection include:

  • Fever
  • Chills
  • Pain or discomfort in the affected area
  • Swollen lymph nodes (such as in the neck, armpits, or groin)
  • Tiredness or fatigue
  • Nausea/vomiting
  • Headache

In addition to generalized symptoms, symptoms that are specific to certain infections may occur. For example:

Bacterial pneumonia may also cause symptoms like:

  • Cough, with phlegm
  • Shortness of breath
  • Chest pain with breathing
  • Sweating
  • Muscle pain

A UTI may have symptoms such as:

  • Sudden, extreme, and/or frequent urges to urinate (pee)
  • Burning, irritation, or pain with urination
  • Feeling like your bladder has not fully emptied
  • Feeling pressure in your abdomen and/or lower back
  • Thick or cloudy urine (may contain blood)

When to See a Healthcare Provider

Bacterial infections can become serious and/or require treatment. See a healthcare provider if you have symptoms of a bacterial infection, particularly if you have:

  • Difficulty breathing (including signs such as fast breathing, nostril flaring, and use of rib, stomach, or neck muscles to breathe)
  • A persistent fever
  • A persistent cough (or coughing up pus)
  • Blood in urine, vomit, or stool
  • Frequent vomiting with difficulty keeping liquids down
  • Signs of dehydration, including urinating less than three times in 24 hours or decreased tears with crying (watching for dehydration is vital for children)
  • A cut or burn that is red or has pus
  • Unexplained swelling or redness of the skin
  • Severe stomach pain
  • Severe headache
  • A lack of improvement within three to five days
  • A baby under 3 months old with a fever

Left untreated, an infection can lead to a life-threatening condition called sepsis.

Sepsis Emergency

Sepsis is a medical emergency that requires immediate medical attention. Seek emergency medical care if you or your child have any symptoms of sepsis, including:

  • High heart rate or weak pulse
  • Fever, shivering, or feeling very cold
  • Shortness of breath
  • Confusion or disorientation
  • Clammy or sweaty skin
  • Extreme pain or discomfort

If you aren't sure if you should seek an assessment for sepsis, get the assessment and ask the emergency healthcare provider if you could have sepsis.

Causes of Bacterial Infection

For infection to develop, bacteria need to enter the body. This can happen through:

  • An opening in the skin (such as a cut, bug bite, burn, or surgical wound)
  • Through your airway
  • Ingestion (such as food poisoning)

When infectious bacteria are in the body, they rapidly increase in number, kill cells, and cause a reaction, such as a damaging immune reaction.

Some bacterial infections are highly contagious. To avoid spreading infectious bacteria, take measures such as:

  • Frequent hand-washing
  • Covering sneezes and coughs
  • Not sharing items such as cups, utensils, towels, etc.

Treatment of Bacterial Infection

Bacterial infections are typically treated with antibiotic medication. Antibiotics kill the bacteria or make it difficult for them to grow and multiply.

Antibiotics do not work against viruses and should not be taken for viral illnesses like colds and flu.

Some bacterial infections can go away on their own without antibiotics, such as some ear infections and some sinus infections.

The type, method, dosage, and length of antibiotic treatment depend on the infection and the type of bacteria. Sometimes broad-spectrum antibiotics are prescribed. These cover a wide range of bacteria.

Antibiotics can be taken:

  • Orally (pill, liquid, or capsule)
  • By injection
  • As drops
  • Topically (cream or ointment)
  • Intravenously

How to Properly Use Antibiotics

Overuse and misuse of antibiotics can lead to antibiotic resistance, meaning bacteria adapt to antibiotics, making the antibiotics less effective. Proper use of antibiotics is critical to minimizing the development of antibiotic resistance. This includes:

  • Taking antibiotics precisely as prescribed
  • Not skipping doses
  • Finishing all of the prescribed antibiotics (do not save any for future infections)
  • Not taking "leftover" antibiotics or those prescribed for someone else (this can be dangerous and may not be effective or appropriate for your current illness)
  • Talking to a healthcare provider or pharmacist about how to properly take your antibiotics with each prescription and discussing any side effects you may experience


Bacteria are single-celled microbes found nearly everywhere and in every habitat. They are also found in the human body, primarily in the gastrointestinal tract.

Most bacteria are harmless or beneficial. Some bacteria can be harmful and cause infections such as pneumonia, UTIs, or food poisoning,

Some bacterial infections can go away on their own, but bacterial infections are usually treated with antibiotics. It is important to take antibiotics exactly as prescribed.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol. 2016;14(8):e1002533. doi:10.1371/journal.pbio.1002533

  2. Microbiology Society. Bacteria.

  3. National Human Genome Research Institute. Bacteria.

  4. Institute for Molecular Bioscience. What’s the difference between bacteria and viruses?

  5. MedlinePlus. Bacterial infections.

  6. Sepsis Alliance. Bacterial infections.

  7. Duke Health. Is it a bacterial infection or virus?

  8. Centers For Disease Control and Prevention. What is sepsis?

  9. Cedars-Sinai. Viruses, bacteria and fungi: what's the difference?

  10. Centers for Disease Control and Prevention. Antibiotic use questions and answers.

  11. U.S. Food and Drug Administration. Combating antibiotic resistance.

By Heather Jones
Heather M. Jones is a freelance writer with a strong focus on health, parenting, disability, and feminism.