Gram-positive bacteria have thick cell walls. A Gram stain test, which involves a chemical dye, stains the bacterium’s cell wall purple. On the other hand, gram-negative bacteria stain pink instead.

Both groups of bacteria can cause disease. However, they require different treatments. If you have a bacterial infection, the Gram stain will determine what kind of medication you need.

Read on to learn about gram-positive bacteria and their associated diseases, along with typical treatments.

The hallmark trait of gram-positive bacteria is their structure. Generally, they have the following characteristics:

  • No outer membrane. Gram-positive bacteria don’t have an outer membrane, but gram-negative bacteria do.
  • Complex cell wall. The cell wall, which surrounds the cytoplasmic membrane, consists of peptidoglycan, polysaccharides, teichoic acids, and proteins. It can easily absorb foreign material.
  • Thick peptidoglycan layer. In gram-positive bacteria, the peptidoglycan is 40 to 80 layers thick.
  • Certain surface appendages. Gram-positive bacteria may have flagella, which help them move. They rarely have hair-like structures called pili.

Gram-positive and gram-negative bacteria have different structures. Typically, gram-negative organisms have the following the traits:

  • outer lipid membrane
  • thin peptidoglycan layer (2 to 3 nanometers)
  • usually doesn’t have teichoic acids
  • can have flagella or pili

The major difference is the outer lipid membrane. It’s difficult to penetrate, which gives gram-negative bacteria extra protection. Gram-positive bacteria don’t have this feature.

Because of this difference, gram-negative bacteria are harder to kill. This means gram-positive and gram-negative bacteria require different treatments.

Though gram-negative bacteria are harder to destroy, gram-positive bacteria can still cause problems. Many species result in disease and require specific antibiotics.

Gram stain testing is a method for classifying bacteria based on their cell wall. It allows scientists to determine whether an organism is gram-positive or gram-negative. The test, which uses a microscope, was created by Hans Christian Gram in 1884.

During the procedure, crystal violet dye is applied to a sample of bacteria. This chemical dye can stain thick peptidoglycan layers.

Under a microscope, gram-positive bacteria appear purple-blue because their thick peptidoglycan membrane can hold the dye. The bacteria is called gram-positive due to the positive result.

Gram-negative bacteria stain pink-red. Their peptidoglycan layer is thinner, so it doesn’t retain the blue color. The test result is negative.

In a medical setting, a doctor can send a sample of your blood, urine, or tissue to a lab for Gram stain testing. This may help them diagnose a bacterial infection.

Depending on various traits, gram-positive bacteria are further classified into the following groups:

Gram-positive cocci

Gram-positive cocci are circular or oval in shape. The term “cocci,” which means sphere, indicates the bacteria are generally round.

The following types are gram-positive cocci:


Staphylococcus grows in grape-like clusters. Normally, they exist on our skin and mucous membranes without causing problems. But if staphylococci enter the body, they can cause serious infections.


Streptococcus bacteria grow in chains. This happens because the cells don’t completely separate after they divide.

Like staphylococci, streptococci normally exist in the body. They’re typically found in the skin, mouth, intestinal tract, and genital tract.

Streptococci are divided into the following categories:

  • S. pyogenes (Group A)
  • S. agalactiae (Group B)
  • Enterococci (Group D)
  • S. viridans
  • S. pneumoniae

Gram-positive bacilli

When gram-positive bacteria are shaped like rods, they’re known as bacilli. Most of these bacteria are typically found on the skin, but some can cause serious medical conditions.

Gram-positive bacilli are further categorized based on their ability to make spores. This includes:


Bacillus and Clostridia bacteria can form spores, which help the bacteria survive in harsh conditions like high heat.

These bacilli are subdivided based on their need for oxygen. Bacillus bacteria need oxygen to survive (aerobic), while Clostridia bacteria don’t (anaerobic).


Listeria and Corynebacterium species don’t make spores. Listeria bacteria are anaerobic, while Corynebacterium are aerobic.

If a bacterium is pathogenic, it means it causes disease in humans. Many gram-positive bacteria are pathogens.

While there are more than 100 pathogenic gram-positive bacteria, the most notable species include:


Staphylococci are commonly responsible for bacterial infections.

Most cases are caused by the following species. Other pathogenic staphylococci are less common and rarely lead to disease.

Staphylococcus aureus

S. aureus is the most pathogenic staphylococci bacteria. It’s responsible for most staphylococci infections, including:

Staphylococcus epidermidis

Often, S. epidermis causes infections in people with weakened immune systems or who are in the hospital. It causes:

Staphylococcus saprophyticus

S. saprophyticus, which is normally found in the genital tract and perineum. It causes:


Streptococci bacteria are also common pathogenic bacteria. The following organisms are the most prevalent. In general, other streptococci groups may cause foodborne illnesses with a sore throat.

Streptococcus pneumoniae

The bacterium S. pneumoniae is the most common cause of community-acquired pneumonia. It’s also responsible for:

Streptococcus pyogenes

S. pyogenes is a group A streptococci. It can cause:

  • strep throat
  • cellulitis
  • pharyngitis
  • impetigo
  • scarlet fever
  • rheumatic fever
  • necrotizing fasciitis
  • glomerulonephritis

S. agalactiae

S. agalactiae usually cause infections in newborns. This includes:


Enterococci are primarily found in the colon. They cause biliary and urinary tract infections.


As spore-forming bacteria, bacilli make spores that release toxins. Most bacilli aren’t pathogenic to humans, but the following two can cause severe medical conditions.

Bacillus anthracis

B. anthracis spores produce the anthrax toxin, which causes serious illness. Humans can get anthrax through inhalation or contact with infected animals.

Depending on how anthrax is spread, it can cause a variety of symptoms. These may include:

  • itchy bump that turns into a sore with black center
  • nausea
  • vomiting
  • abdominal pain
  • coughing up blood
  • high fever

Bacillus cereus

B. cereus is a spore-forming bacterium that’s found in the soil and some foods. It’s most associated with illness due to eating undercooked or reheated rice. B. cereus causes:

  • diarrhea
  • nausea
  • wound infections
  • respiratory infections
  • endophthalmitis


About 30 Clostridia species cause disease in humans. Like bacilli, these bacteria form toxins that lead to serious conditions.

Clostridia are usually involved with foodborne illnesses, but the most concerning bacteria include:

Clostridium botulinum

The spores of C. botulinum produce the botulinum toxin, the most dangerous toxin to humans. It leads to botulism, including:

  • foodborne botulism (most common)
  • infant botulism
  • wound botulism
  • inhalation botulism

Clostridium perfringens

C. perfringens is usually associated with meat production and processing. If a human eats contaminated meat, they can get food poisoning. Symptoms include diarrhea and abdominal cramps that last for less than 24 hours.

Clostridium difficile

C. difficile, also called C. diff, usually affects older individuals in hospital. It typically occurs after taking antibiotics. C. difficile causes:

  • colitis
  • abdominal cramping
  • severe diarrhea

Clostridium tetani

C. tetani spores produce the tetanus toxin, a neurotoxic substance. The spores can be found in soil, ash, and on rusty tools.

If the toxin causes an infection, it’s called tetanus. This a serious medical emergency.

Listeria monocytogenes

The only pathogenic Listeria bacteria is L. monocytogenes. In healthy people, it typically causes mild symptoms of foodborne illness. But in people with weakened immune systems, the bacterium can cause life-threatening conditions like:

Corynebacterium diphtheriae

There are about 30 Corynebacterium bacteria associated with human disease. However, these organisms rarely cause sickness and usually affect people with compromised immune systems.

C. diphtheria is the primary pathogenic organism in this group. It’s responsible for:

  • diphtheria
  • pharyngitis
  • respiratory infections
  • septic arthritis
  • cutaneous infections
  • osteomyelitis
  • endocarditis

When treating illnesses caused by gram-positive bacteria, the best option depends on:

  • type of bacteria
  • antimicrobial resistance
  • whether the bacteria forms toxins

Common treatments include:


Penicillin is a common antibiotic that’s used for a variety of infections. It works by interfering with the bacterium’s peptidoglycan layer, which kills the organism.

The antibiotic is mainly used for Streptococcus infections, including:

  • strep throat
  • sinus infections
  • urinary tract infections
  • cellulitis


Glycopeptide antibiotics are often used to treat serious infections caused by drug-resistant bacteria. Like penicillin, they work by destroying the bacterium’s cell wall.

Glycopeptides can treat:

  • multidrug-resistant pneumonia
  • MRSA
  • colitis


Erythromycin is in a class of antibiotics known as macrolides, which also includes the better-known azithromycin and clarithromycin. It’s an antibiotic that stops bacterial growth, and works against both gram-positive and gram-negative bacteria.

Often, erythromycin is prescribed to people who are allergic to penicillin.

The antibiotic treats conditions like:

  • bacterial pneumonia
  • pink eye
  • strep throat
  • staph skin infections

Fluid therapy

In some cases, treatment might include fluid therapy. It helps by replenishing the body’s fluid levels and preventing dehydration. Generally, fluid management is required to treat conditions caused by toxins.


For toxin-related illnesses like anthrax and botulism, treatment includes an antitoxin. This medicine works by targeting and removing toxins in the body.

The appropriate antitoxin depends on the specific toxin. Additionally, it’s used in combination with other treatments.

The Gram stain test can help doctors diagnose an illness. If it’s caused by gram-positive bacteria, the doctor will prescribe the appropriate treatment. Most illnesses require antibiotics that destroy or slow down the bacteria. In severe cases, you might require additional treatment like fluid therapy.