Bacteria are tiny, single-celled living organisms. There are millions of different types of bacteria. Many can be found in and on your body and are beneficial to you. These bacteria make up your microbiome, which keeps your body healthy. Other bacteria can make you sick. Healthcare providers can treat many bacterial infections with antibiotics.

What are bacteria?

Bacteria are microscopic living organisms that have only one cell. The word for just one is “bacterium.” Millions (if not billions) of different types of bacteria can be found all over the world, including in your body. They’re on your skin and in your airways and mouth. They’re also in your digestive system, reproductive system and urinary tract. Scientists estimate you have 10 times more bacterial cells than human cells in your body.


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What are the benefits of bacteria?

Most types of bacteria aren’t harmful. Some are even good for you. These helpful bacteria are mainly located on your skin or in your gut or digestive system. They’re called resident flora, or your microbiome, which are groups of microbes living in and on your body. Gut bacteria keep you healthy by absorbing nutrients, breaking down food and preventing the growth of harmful bacteria.

What are the risks of bacteria?

Most bacteria are harmless, but certain types can make you sick. These bacteria are a type of pathogen. Pathogens are microorganisms that can cause disease. They can reproduce quickly in your body and give off poisons (toxins) that can cause infection. Harmful bacteria examples include:

These bacteria are the most common germs responsible for septicemia, or blood poisoning. This infection occurs when bacteria enter your bloodstream. Bacteria in blood can spread and lead to sepsis. Sepsis is a systemic overreaction to widespread infection in your body.

Other examples of pathogenic bacteria include:

Antibiotics can treat most types of bacterial infections. However, the more you take an antibiotic, the greater the chances your body will become resistant to it. Bacterial resistance is also more likely if you don’t finish or take your antibiotics as prescribed.


What are the different types of bacteria?

Scientists classify and define bacteria in several ways.

Scientific name

One way scientists classify bacteria is by their scientific name. The scientific name includes their genus — based on the characteristics of the bacteria — and within the genus, their species. For example, “Clostridium botulinum” is the scientific name for the bacterium that causes botulism. Within a species, scientists may discover different types, or strains, of a bacterium.

One way scientists classify bacteria is by their shape. The three basic bacterial shapes include spheres, rods and spirals.

One way scientists classify bacteria is by their shape.

Bacteria shapes

Another way scientists classify bacteria is by their shape. There are three basic bacterial shapes:

  • Spheres or ball-shaped (cocci bacteria).
  • Rod-shaped bacteria (bacilli).
  • Spirals or helixes (spirochetes).

Need for oxygen

Scientists also classify bacteria based on their need for oxygen to live and grow. Bacteria that need oxygen are called aerobes. Bacteria that can’t live or grow when oxygen is around are called anaerobes. Certain bacteria can live and grow with or without oxygen. These are called facultative bacteria.

Genetic makeup

Another way scientists classify bacteria is by their genetic makeup. Each bacterium contains a different genetic makeup. This is called their genotype. Specialized tests can determine the differences in each bacterium’s genotype.


Scientists classify bacteria by the color they turn after they apply special chemicals (stains) to them. One common staining process is called Gram staining. Bacteria may be classified as gram-positive or gram-negative. Gram staining also helps guide treatment because gram-positive and gram-negative bacteria respond differently to certain types of antibiotics.

What are gram-positive bacteria?

Scientists classify bacteria as gram-positive or gram-negative based on which color they turn under a Gram stain. They stain differently because their cell walls are different. “Positive” and “negative” don’t mean “good” or “bad.” Gram-positive bacteria look blue to purple under a Gram stain. Examples of gram-positive bacteria include:

  • Corynebacterium.
  • Clostridium.
  • Listeria.

What are gram-negative bacteria?

Gram-negative bacteria look red to pink under a Gram stain. They cause different types of infections than gram-positive bacteria. They also need different types of antibiotics to treat them. Examples of gram-negative bacteria include:

  • Pseudomonas.
  • Proteus.
  • Klebsiella.

Bacteria vs. viruses — what’s the difference?

Bacteria and viruses are different kinds of germs, or microorganisms. Both can cause infections that may result in similar symptoms. However, bacterial infections and viral infections require different methods of treatment. Healthcare providers may use antibiotics to treat some bacterial infections. But antibiotics won’t work on viruses. Providers may treat some viruses with antivirals, but antivirals won’t treat infections caused by bacteria.

What are antibiotic-resistant bacteria?

Your immune system can fight off some bacteria, but in other cases, you may need an antibiotic to treat a bacterial infection. Antibiotics work by destroying a bacterium’s cell wall or DNA.

Overuse of antibiotics can sometimes lead to problems over time. This is because some bacteria can become resistant to antibiotics, making it hard to treat infections caused by new strains. Each time you take an antibiotic, you increase your chances that the bacteria will learn to resist it. An example of antibiotic-resistant bacteria is MRSA (methicillin-resistant Staphylococcus aureus).

How do bacteria reproduce?

Most bacteria reproduce through binary fission. This means that each bacterium cell duplicates its DNA and then divides into two parts, with each new cell receiving one copy of DNA.

Are bacteria prokaryotic or eukaryotic?

Bacteria don’t have a nucleus, so they’re classified as prokaryotes. They’re microbes with a very simple cell structure. Bacteria have cell walls. Within the cell walls, a bacteria diagram would show the structure of each cell. Each bacterium contains cytoplasm, ribosomes and DNA. Outside the cell wall, one or more bacteria flagella help the bacterium move.

What are plastic-eating bacteria?

In 2016, scientists in Osaka, Japan, discovered a species of bacteria called Ideonella sakaiensis. It was eating its way through plastic bottles outside a recycling facility.

Bacteria usually consume dead organic matter, but this bacterium was eating a type of plastic called polyethylene terephthalate (PET). Plastic bottles are commonly made with PET. The scientists studied the bacterium and found it produced two digestive enzymes that decompose the plastic.

The enzymes only decompose PET plastic, but scientists hope that one day this type of plastic-eating bacteria could help tackle the world’s plastic pollution problem.

A note from Cleveland Clinic

There are thousands of different types of bacteria. Most types of bacteria aren’t harmful; many are even helpful. They make up your microbiome, which keeps your gut healthy. Other bacteria, called pathogens, can cause infections that require treatment. Healthcare providers can prescribe antibiotics to treat many of these infections. Always take antibiotics as directed.

Medically Reviewed

Last reviewed by a Cleveland Clinic medical professional on 12/02/2022.

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