Gram staining is one way scientists can identify bacteria. Gram-positive bacteria, such as Staphylococcus, appear blue or purple under a microscope when scientists apply a stain.

Conversely, Gram-negative bacteria do not hold the dye well. Gram-negative bacteria may appear red under the microscope because red-pink iodine is used alongside the Gram stain as a control.

Both Gram-positive and Gram-negative bacteria can cause disease. However, they require different treatments. Therefore, medical professionals need to be able to identify the type of bacteria to provide the best possible care.

Keep reading to learn more about Gram-positive bacteria, including their characteristics, common examples, and diseases they can cause.

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Gram-positive bacteria are bacteria that contain a thick cell wall.

During the Gram staining process — a test that experts use to view the bacteria under a microscope — they appear purple or blue. This is because the cell wall retains the color of the staining technique developed by Hans Christian Gram in 1884.

Experts divide them into Gram-positive cocci or bacilli, depending on their shape.

Gram-positive bacteria differ from Gram-negative in their structure. These are their key characteristics:

  • Outer membrane: Gram-positive bacteria do not have an outer membrane, while Gram-negative bacteria do.
  • Color: Gram-positive bacteria appear blue or purple under the microscope.
  • Cell wall: This structure surrounds the cell’s membrane. In Gram-positive bacteria, the cell wall is made of multiple layers of molecules and protein. It protects the bacteria from incurring damage.
  • Peptidoglycan layer: In Gram-positive bacteria, the peptidoglycan is a 20 to 80 nm (nanometer) thick structure found within its cell wall. This layer is only 2 to 3 nm thick in Gram-negative bacteria.
  • Shape: Gram-positive bacteria come in different shapes:
    • cocci (spherical or round shape)
    • bacilli (rod shape)
    • branching filaments (threadlike shape)

Gram-negative bacteria have different structures. They have a far thinner layer of peptidoglycan, which is why they do not retain the crystal violet dye.

They also have an outer lipid membrane that protects them from their environment. It means they are more resistant to antibiotics and other drugs.

Although Gram-negative bacteria are more challenging to treat, Gram-positive bacteria can also cause health problems. Many types of Gram-positive bacteria cause diseases that require specific antibiotics.

Learn more about the difference between Gram-positive and Gram-negative bacteria.

These oval or spherical bacteria are among the most common types of bacteria known. They can live in pairs, chains, or clusters.


These bacteria grow in clusters. Staphylococcus usually occupies the skin and mucous membranes without causing infections and makes up the normal flora in the body.

However, if they enter the body and start growing in number, they can cause serious bacterial infections.

Some types of bacteria and infections include:

  • Staphylococcus aureus: This is the most pathogenic staphylococci bacteria. It causes infections, such as:
    • bacterial pneumonia
    • food poisoning
    • skin infections such as cellulitis and folliculitis
    • MRSA, a superbug infection
    • infections of the lining of the heart, or endocarditis
  • Staphylococcus epidermidis: S. epidermis causes infections in hospital settings and when a person has a weakened immune system. It can cause:
    • infections of surgical sites or urinary catheters
    • eye keratitis
    • endophthalmitis, an infection of the inner eye
  • Staphylococcus saprophyticus: These bacteria inhabit the genitals and perineum. They may cause infections such as:
    • urinary tract infections
    • kidney infections,
    • infections of the urethra, prostate, and epididymis


These bacteria grow in chains. They are also part of the body’s normal flora, and they can be found on or in:

  • skin
  • mouths
  • digestive systems
  • genitals

When they invade the body and start growing, they can cause infection. Some types of bacteria and infections include:

  • Streptococcus pneumonia: Streptococcus pneumoniae is a leading cause of bacterial pneumonia. It can also cause sinus infections and meningitis.
  • Streptococcus pyogenes: These bacteria can cause a wide range of infections, including:
    • scarlet fever
    • strep throat
    • flesh-eating diseases
    • pharyngitis
    • rheumatic fever
  • Streptococcus agalactiae: This bacteria is also called Strep B strep. It can cause many serious infections in newborns, such as:
  • Enterococcus: The most common types are Enterococcus faecalis and Enterococcus faecium. They can cause:
    • urinary tract infections
    • infective endocarditis
    • bloodstream infections

Bacilli are rod-shaped bacteria that typically exist on the skin. Doctors further categorize them into spore-forming and non-spore-forming.

Forming spores and lying dormant is a strategy that helps bacteria survive in harsh conditions. When the environment becomes more favorable, they can germinate and grow into active bacteria.

There are two types:


These bacteria are aerobic, meaning they need oxygen to survive.

The spores of Bacillus anthracis produce anthrax toxin, which is potentially lethal to humans and animals. These spores can survive in the soil for years before infecting a person.

Anthrax mainly affects grazing animals, such as cattle and sheep. However, people who handle infected animal products can become infected.

Anthrax can cause symptoms ranging from skin sores to a severe — sometimes fatal — respiratory disease.

Bacillus cereus is another spore-forming bacterium. It can cause:

  • foodborne illness
  • wound infections
  • respiratory infections


Thesebacteria are anaerobic, meaning they do not require oxygen to survive. The spores also create toxins that can cause serious human diseases.

The types of bacteria and infections include:

  • Clostridium botulinum: Botulinum toxin is the most lethal toxin known. It can cause botulism, a rare but potentially fatal disease.
  • Clostridium perfringens: People can contract food poisoning from meat contaminated with this bacterium. It results in diarrhea and abdominal cramps for 24 hours or less.
  • Clostridium difficile: Infection with this bacterium canoccur following antibiotic treatment. It can also cause various gastrointestinal symptoms. This organism is associated with hospital-acquired outbreaks and those at senior residential living facilities.
  • Clostridium tetani: The spores of this bacteria produce tetanus toxin, which can cause tetanus infection. Spores exist in soil, dust, and manure and can enter the body through open wounds.

The treatment for Gram-positive bacterial infections typically includes antibiotics. The type of antibiotic depends on the specific bacteria causing the infection. Common options for treating Gram-positive infections are:


This antibiotic interferes with the bacterial cell wall in an attempt to destroy the wall and the bacteria. It can treat a wide range of bacterial infections. For example, penicillin is effective against Streptococcus infections, including strep throat and sinus infections.

It can also be used for Streptococcus infections too.


Doctors may opt for these antibiotics if a person has a serious infection involving drug-resistant bacteria, such as MRSA. They also destroy bacterial cell walls.

They can also treat Enterococcus and Clostridium difficile infections.


If a person is allergic to penicillin, erythromycin is a common alternative. It belongs to the macrolide class of antibiotics along with clarithromycin and azithromycin. These antibiotics work by preventing bacteria from making proteins. They can also treat:

  • bacterial pneumonia
  • strep throat
  • staph skin infections

If a person has anthrax, botulism, or other toxin-related illness, they can also use antibiotics. However, if spores are present and produce toxins, the person may also require antitoxin treatment. This therapy works by neutralizing the toxins produced by bacteria.

Doctors may also need to provide supportive care. This might include:

  • intravenous fluids
  • nutrition
  • mechanical ventilation

Gram-positive bacteria are a type of bacteria that have a thick cell wall. This cell wall helps protect the bacteria from antibiotics and other substances that might damage them.

Gram-positive bacteria can cause a range of infections, from food poisoning to serious respiratory diseases. Some Gram-positive bacteria produce toxins that can cause lethal illnesses. The examples given in this article are not inclusive — other types of Gram-positive bacteria exist, as do other antibiotics to treat them.

Treatment typically includes antibiotics. In some cases, doctors may recommend antitoxin therapies.