Gram-positive and Gram-negative are two distinct types of bacteria. People categorize these types based on their structures and their appearance after Gram staining. Gram staining is a process of dying bacteria and then viewing them beneath a microscope.

A researcher holding up two petri dishes.Share on Pinterest
Rafe Swan/Getty Images

There are many different types of Gram-positive and Gram-negative bacteria. These bacteria cause a wide range of different health issues.

This article discusses Gram-positive and Gram-negative bacteria. It also outlines their differences, the infections they cause, and discusses antibiotic resistance.

The following table provides an overview on Gram-positive and Gram-negative bacteria:

Gram-positive bacteriaGram-negative bacteria
Color after gram testingBlue or purple Pink or red
Cell wall thicknessThickThin
Toxins• emetic toxin
• diarrheal enterotoxins
• neurotoxins
• enterotoxin
endotoxins
Types• Staphylococcus aureus
• Staphylococcus epidermidis
• Staphylococcus saprophyticus
• Streptococcus pneumoniae
• Streptococcus pyogenes
• Streptococcus agalactiae
• Enterococci
• Corynebacterium diphtheriae
• Bacillus anthracis
• Vibrio cholerae
• Escherichia coli
• Bartonella henselae
• Campylobacter
• Legionella
• Salmonella
• Salmonella typhi

Gram-positive and Gram-negative bacteria are two different types of bacteria.

One method that scientists use to classify bacteria is Gram staining, which is the most commonly performed laboratory procedure in microbiology.

The term “Gram” refers to a specimen staining method developed by Hans Christian Gram in 1884. It involves staining an organism with crystal violet or methylene blue dye and then observing the organism under a microscope.

Gram-positive

A 2022 article notes that Gram-positive bacteria have a thick peptidoglycan cell wall. This means that they retain the dye in gram testing, causing them to have a blue color when a person views them under a microscope.

Gram-positive bacteria can be cocci- (spheres) or bacilli- (rod) shaped, or have branching filaments.

Gram-negative

Gram-negative bacteria have an outer membrane. However, they have a thinner peptidoglycan cell wall. This means they do not hold the blue dye used in Gram testing and do not appear blue. Instead, they appear red or pink in color.

Gram-negative bacteria are among the most significant public health problems in the world. This is due to their high resistance to antibiotics.

Below are the specific characteristics of Gram-positive and Gram-negative bacteria.

Gram-positive

Gram-positive bacteria have the following characteristics:

  • Membranes: Gram-positive bacteria do not have a protective outer membrane.
  • Cell wall: They have a thick peptidoglycan cell wall.
  • Shape: Gram-positive bacteria are sphere- or rod-shaped, or they have branching filaments.
  • Toxins: These bacteria also contain certain toxins that can cause a number of food-borne diseases. Toxins present in Gram-positive bacteria include:
    • emetic toxin
    • diarrheal enterotoxins
    • neurotoxins
    • enterotoxin

Gram-negative

Gram-negative bacteria have the following characteristics:

  • Membrane: Gram-negative bacteria have two membranes — one external and one internal.
  • Cell wall: These bacteria also have a thinner peptidoglycan cell wall than Gram-positive bacteria, which sits between their two membranes.
  • Shape: They can be spherical-, rod-, or spiral-shaped.
  • Toxins: If something disturbs the cell wall of Gram-negative bacteria, they can release endotoxins.

There are a number of different types of Gram-positive bacteria.

Staphylococcus aureus

Staphylococcus aureus does not normally cause infection on healthy skin. If it enters the internal tissues or bloodstream, it may cause a variety of potentially serious infections.

Infections it can cause include:

  • infective endocarditis, an infection that happens when bacteria enter the bloodstream and settles in the lining of the heart, a blood vessel, or a heart valve
  • skin and soft tissue infections, such as impetigo, folliculitis, and cellulitis
  • osteomyelitis, an infection that causes pain in the legs
  • septic arthritis, a type of join infection
  • prosthetic device infections, a complication that can happen after joint replacement surgeries
  • pulmonary infections: such as pneumonia
  • gastroenteritis, inflammation of the gastrointestinal tract that can cause vomiting and diarrhea
  • meningitis, inflammation of the lining that surrounds the brain and spinal cord
  • toxic shock syndrome, a rare and life threatening condition that happens when bacteria release harmful toxins
  • urinary tract infections (UTIs), an infection of the bladder, urethra, or kidneys

Staphylococcus epidermidis

Staphylococcus epidermidis can become infectious if it enters a human host. The bacteria are one of the most common causes of hospital-borne diseases in the United States.

It commonly infects the skin around prosthetic devices and catheters, which can then cause blood infections to develop.

Staphylococcus saprophyticus

Staphylococcus saprophyticus commonly causes uncomplicated UTIs.

This type of bacteria can also cause a number of complications such as:

Streptococcus pneumoniae

Streptococcus pneumoniae is the cause of community-acquired pneumonia (CAP). CAP is the seventh leading cause of death in the U.S.

This type of bacteria also causes:

  • ear infections
  • sinusitis
  • meningitis

Streptococcus pyogenes

Streptococcus pyogenes can cause a wide array of health issues that range from mild infections to life threatening invasive infections.

Streptococcus pyogenes can cause the following infections:

  • sore throat
  • cellulitis, erysipelas, or impetigo, which are types of skin infections
  • scarlet fever
  • necrotizing fasciitis, which causes tissue death
  • glomerulonephritis, which is damage to the small filters in the kidney
  • rheumatic fever, which refers to a condition that affects the heart, joints, skin, and brain

Streptococcus agalactiae

Streptococcus agalactiae colonizes the vagina and is often present in infants.

This type of bacteria can cause illness in people of all ages. However, it is a common cause of severe infections in newborns during the first week of life.

Pregnant people require screening for this bacteria at 35–37 weeks of gestation.

Enterococci

Enterococci commonly reside in the gastrointestinal tracts of nearly all land animals, including humans.

They make up a core part of a person’s microbiome. The microbiome refers to the entire habitat of a person’s body.

Enterococci can also cause a variety of infections, including biliary tract infections and UTIs.

Corynebacterium diphtheriae

The toxin-producing strains of this bacteria can cause a person to develop diphtheria. Diphtheria is an acute, bacterial disease.

There are different types of diphtheria that can affect the respiratory system and the skin.

Bacillus anthracis

Bacillus anthracis is a rod-shaped, Gram-positive bacteria. This type of bacteria occurs naturally in soil around the world and commonly affects domestic and wild animals.

This bacteria can cause the seriously infectious disease anthrax.

People can develop anthrax if they come in contact with animals that have the bacterial infection or animal products containing the bacteria.

There are a number of different types of anthrax that can affect different parts of the body, such as the skin, respiratory system, or gastrointestinal system.

There are a number of different types of Gram-negative bacteria.

Vibrio cholerae

Vibrio cholerae is a type of bacteria that causes cholera.

A person usually develops cholera after the bacteria enter their body through the mouth. This type of bacteria is typically present in food or water that has been contaminated with human waste.

Cholera can cause a person to experience a severe case of watery diarrhea, which can cause extreme loss of fluid and electrolytes. This may also lead to severe dehydration which can be fatal.

Escherichia coli

Escherichia coli is a type of bacteria that medical professionals often refer to as E. coli. It can cause a number of illnesses including traveler’s diarrhea and dysentery.

Some cases of this infection are very mild, but others can be severe or life threatening.

Bartonella henselae

Bartonella henselae is a Gram-negative bacteria that causes cat scratch disease.

A person usually contracts this bacteria after they receive a bite, scratch, or lick from a cat that has an infection, which leads to the development of cat scratch disease.

Campylobacter

Campylobacter is one of the most common causes of diarrhea worldwide.

A person can get a campylobacter infection if they eat raw or undercooked poultry, or eat food that has come into contact with raw or undercooked poultry.

A person may also get this infection from:

  • eating other foods containing the bacteria, including seafood, meat, and produce
  • contact with animals that have an infection
  • drinking untreated water
  • drinking raw milk

Legionella bacteria

Legionella is a type of bacteria that is often present in freshwater environments, such as lakes and streams. This type of bacteria can become a health concern if it spreads in human-made building water systems.

This type of bacteria can cause Legionnaires’ disease. A person can develop Legionnaires’ disease if they breathe in small droplets of water that contain the bacteria.

Salmonella bacteria

Salmonella is a bacteria that causes around 1.35 million infections, 26,500 hospitalizations, and 420 deaths in the U.S. each year, according to the Centers for Disease Control and Prevention (CDC).

The most common way for a person to contract a salmonella infection is through eating food containing the bacteria.

Salmonella typhi

Salmonella typhi causes typhoid fever.

A person can develop typhoid fever if they consume food, drinks, and drinking water with fecal matter containing salmonella typhi.

Doctors can use the following treatment options.

Gram-positive

Antibiotics are the main treatment for Gram-positive bacterial infections.

Common antibiotics that healthcare professionals prescribe to treat these infections include:

  • penicillin
  • nafcillin
  • oxacillin
  • cloxacillin
  • dicloxacillin
  • trimethoprim
  • sulfamethoxazole
  • clindamycin
  • doxycycline

Gram-negative

Treatment options for Gram-negative infections can be limited. Medical professionals will often prescribe antibiotics to treat Gram-negative bacterial infections.

Common antibiotics used to treat these infections include:

  • tetracyclines
  • azithromycin
  • ciprofloxacin
  • rifampin
  • fluoroquinolones

However, the CDC notes that Gram-negative bacteria are resistant to many drugs and are becoming increasingly resistant to the currently available antibiotics.

Antibiotic resistance occurs when bacteria change and adjust in response to antibiotic medications. They can become resistant to antibiotics. This makes the antibiotics less effective or can cause them to not work at all.

The World Health Organization (WHO) states that antibiotic resistance is currently one of the biggest threats to global health, food security, and development.

The misuse and overuse of antibiotics can cause antibiotic resistance.

There are a number of things that people can do to help prevent and manage antibiotic resistance. These include:

  • only using antibiotics when a medical professional prescribes them
  • never demanding antibiotics if a medical professional advises against their use
  • always following a medical professional’s advice when using antibiotics
  • never sharing or using leftover antibiotics

Preventing infection can also help prevent and manage antibiotic resistance.

Ways a person can prevent infections include:

  • regularly washing their hands
  • preparing food hygienically
  • avoiding close contact with those who are ill
  • keeping vaccinations up to date

Gram-positive and Gram-negative are two distinct types of bacteria.

Gram-positive bacteria appear blue or purple after Gram staining, while Gram-negative bacteria appear red or pink after gram staining.

There are many types of Gram-positive bacteria. These bacteria can cause infective endocarditis, gastroenteritis, meningitis, UTIs, impetigo, and CAP.

There are also a number of types of Gram-negative bacteria. These bacteria can cause salmonella, Legionnaires’ disease, cholera, cat scratch disease, and typhoid fever.

Medical professionals can treat both types of bacteria with antibiotics. However, Gram-negative bacteria are more prone to antibiotic resistance.