What Is Bacteria? Good vs. Bad, Benefits, and Common Types (2024)

Bacteria are single-celled, or simple, organisms that are invisible to the naked eye.

Many bacteria are found both inside and outside of organisms, including humans. Bacteria are also found on surfaces and in substances like water, soil, and food, making them key players in the Earth’s ecosystems.

While some bacteria are harmful to humans and can cause infections, most are harmless, according to the National Human Genome Research Institute (NHGRI). Your body needs certain types of bacteria to function, such as those that live in the digestive system.

Learn more about these microorganisms and what you can do to help balance helpful bacteria while keeping yourself safe from harmful ones.

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There are many types of bacteria. These tend to be classified as harmless, helpful, or pathogenic (harmful). According to the Microbiology Society, bacteria can be single-celled microorganisms, but they may also exist in clusters, chains, and pairs.

There are technically five classes of bacteria, with each group based on their shape:

  • rod (bacilli)
  • corkscrew (spirochaetes)
  • comma (vibrios)
  • spiral (spirilla)
  • spherical (cocci)

Bacteria are also described as aerobic, anaerobic, or facultative anaerobes. These terms describe how they respond to oxygen.

While aerobic bacteria need oxygen to live, anaerobic bacteria will die around oxygen. Facultative anaerobes function best with oxygen but do not need it to survive.

What do bacteria look like?

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Bacteria are classified based on their shape and can be singular or live in clusters. For example, staphylococcus aureus bacteria are found in spherical clusters. Clostridium bacteria are rod-like and singular.

Though small, bacteria are powerful and complex. They can also survive in extreme conditions. Bacteria have a tough protective coating that boosts their resistance to white blood cells in the body.

Some bacteria have a tail, known as a flagellum. The flagellum helps them move around. Other bacteria have sticky, hair-like appendages that help them stick to one another or onto hard surfaces.

While most bacteria are harmless, some may lead to infections. Below are some of the most common bacterial infections and their symptoms.

Ear infection

Ear infections may affect your middle or outer ear canal and are more common in children than adults, according to the Centers for Disease Control and Prevention (CDC). They can cause:

  • ear pain and discharge
  • fever
  • hearing difficulties

Young children may display fussiness and pull at the affected ear.

Antibiotics are typically prescribed only for a serious ear infection or one that lasts longer than 2 to 3 days.


Sinusitis develops as a secondary illness, usually after getting a common cold. It is also known as a sinus infection.

Symptoms of sinusitis include congestion and pain in your nose and forehead, as well as thick yellow or green nasal discharge. You may also feel pressure in your cheeks and experience postnasal drip, fever, and cough.

Strep throat

Unlike most cases of sore throats often caused by viruses, strep throat is an infection caused by Streptococcus group A bacteria.

Besides an extremely sore throat, this highly contagious infection can also cause a number of symptoms, according to the CDC. These include:

  • fever
  • swollen tonsils
  • trouble swallowing and talking
  • red spots on the roof of your mouth

Whooping cough

Known for causing a whooping sound after severe coughing fits, whooping cough is an extremely contagious bacterial infection caused by Bordetella pertussis. Vaccines are available to protect against this potentially deadly respiratory infection.

Bacterial meningitis

Bacterial meningitis is a serious, highly contagious infection that may quickly lead to permanent or life threatening complications, according to the CDC. The symptoms often occur suddenly and may include fever, neck stiffness, nausea, and confusion.

Urinary tract infection (UTI)

If you experience painful or frequent urination, you may have a common bladder or kidney infection called a UTI. These occur when bacteria get into the urethra and affect your urinary tract.

These bacteria can originate from the bowel, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

Other symptoms of a UTI may include fever, blood urine, or lower back pain. Antibiotics may treat most UTIs, according to the CDC.

Bacterial vaginosis (BV)

BV is a common condition that occurs when there are too many harmful bacteria in the vagin* compared to helpful bacteria, leading to a bacterial imbalance. It can occur due to antibiotics or douching, among other causes.

BV may cause:

  • gray or white discharge
  • burning sensations when urinating
  • itchiness
  • strong odors

Your doctor may recommend treating BV with antibiotics, though sometimes no treatment is necessary.


Salmonella is a type of bacteria that may lead to a salmonella infection in humans. While food is the most common source of Salmonella, the bacteria may also be passed on to humans from reptiles, according to the CDC.

Symptoms of illness from this type of bacteria may include stomach cramps, diarrhea, and fever. Unlike other types of bacterial infections, illnesses caused by Salmonella usually aren’t treated with antibiotics.


Tetanus, also known as lockjaw, is an infection caused by Clostridium tetani. While not as common in the United States due to vaccines, tetanus may still affect humans through the exposure of cuts in the skin to surfaces containing the bacteria, according to the CDC.

The nickname “lockjaw” describes some common symptoms of tetanus, such as jaw cramping and muscle stiffness.

While bacterial infections are often associated with illnesses, they can also include certain types of skin infections. Some common types of bacterial skin infections include:

  • Cellulitis. Cellulitis is a common bacterial skin infection that causes redness, inflammation, and warmth in the affected area, according to the CDC. It may also cause your skin to have a pitted appearance, similar to an orange peel.
  • Folliculitis. With an appearance similar to acne breakouts, folliculitis is a skin infection that occurs within your hair follicles. It may be caused by anything that rubs up against your skin and irritates the follicles, such as shaving or wearing tight clothing.
  • Impetigo. This bacterial skin infection is known for creating crusty, honey-colored lesions and pustules. Impetigo may be caused by either Staphylococcus or Streptococcus bacteria and is highly contagious.

While the World Health Organization (WHO) estimates that more than 30 types of bacteria, parasites, and viruses may be passed through sexual contact, there are three common bacteria-based sexually transmitted infections (STIs) that may be cured with antibiotics. These include:

  • chlamydia, which can cause unusual discharge and permanent reproductive damage in women
  • gonorrhea, which may cause infections in the genitals, throat, and rectum
  • syphilis, which causes skin rashes and sores and may lead to serious health complications if left untreated

Other potential symptoms of a bacterial STI may include:

  • unusual vagin*l or penile discharge
  • burning sensation when urinating
  • sores around the mouth, genitals, or anus
  • bleeding between periods
  • anal discharge or bleeding
  • painful bowel movements

Just as certain bacteria cause bacterial infections, viruses cause viral infections. Some examples of viral infections include the influenza virus, certain acute upper respiratory infections, and COVID-19-related infections.

While the symptoms of both bacterial and viral infections may be similar, they require different treatments. Antibiotics may help treat certain bacterial infections, but they can’t treat viruses. On the flip side, antiviral drugs may treat some types of viruses but not bacterial infections.

It’s also possible to develop a secondary bacterial infection. This occurs when an initial viral infection but is followed by a bacterial one. You may have a secondary infection if symptoms of a viral infection last longer than 10 to 14 days.

Examples of secondary infections can include:

  • pneumonia
  • sinusitis
  • ear infections

Researchers estimate that there are 10 times more bacteria in your body than there are human cells. The majority of them are beneficial, according to the NHGRI.

Most of the beneficial bacteria in the human body are located in your digestive system or gut microbiome. These bacteria help to break down food and keep you healthy.

Some people take probiotics on a regular basis or while taking antibiotics to support gut health. These supplements contain strains of helpful bacteria, such as Bifidobacteria and Lactobacillus.

Probiotics are also used in food production to make yogurt and fermented foods, such as sauerkraut, kimchi, and kombucha.

The environmental ecosystem also relies on bacteria to function properly. For example, bacteria break down dead matter in the environment, like dead leaves, releasing carbon dioxide and nutrients in the process. Without the release of carbon dioxide, plants are unable to grow.

Although certain types of bacteria may lead to disease, most aren’t considered pathogenic. In fact, there are many more good bacteria than bad, according to the NHGRI.

If you consume or come in contact with harmful bacteria, they may reproduce in your body and release toxins that can damage your body’s tissues and make you feel ill.

Harmful bacteria are called pathogenic bacteria because they cause diseases and illnesses, such as:

  • strep throat
  • staph infection
  • cholera
  • tuberculosis
  • food poisoning

In some cases, you may need antibiotics to stop pathogenic bacteria from reproducing and harming your body. It’s important to receive an accurate diagnosis from your doctor since antibiotics only get rid of bacteria and can’t treat viral or fungal infections.

Antibiotic resistance is an increasing concern, and you may be at a higher risk if you take antibiotics when they aren’t necessary. Bacteria may evolve and learn to become resistant to antibiotic treatments.

When you have antibiotic-resistant bacteria, this could reduce the efficacy of antibiotics when you need them. Taking antibiotics when you don’t need them could increase your risk.

While antibiotics are sometimes a lifesaving tool, the CDC estimates that more than 28 percent of related prescriptions aren’t necessary.

If you do need to take antibiotics, you can help reduce your risk of resistant bacteria by:

  • taking your full prescription, even if you’re feeling better
  • never taking someone else’s prescription
  • avoiding skipping doses
  • avoiding antibiotics for viralor fungal infections

Bacteria live both inside and outside of organisms, including in the human body. They can be beneficial or potentially dangerous, depending on their type, quantity, and location in the body.

Some bacteria, such as those in your gut, help keep you healthy. Other bacteria may cause bacterial infections and require treatment.

Antibiotics can be prescribed for certain bacterial infections. However, using antibiotics when they aren’t needed may cause antibiotic-resistant bacteria, which are bacteria that cannot be treated with antibiotics. If you are prescribed antibiotics by a doctor, it’s important to follow the instructions on your prescription.

What Is Bacteria? Good vs. Bad, Benefits, and Common Types (2024)
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