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Gram-Positive Bacterial Infection

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Gram-positive bacteria exhibit a blue stain in the Gram staining technique, which is used to classify them.

Medically reviewed by

Dr. Shubadeep Debabrata Sinha

Published At May 23, 2023
Reviewed AtJuly 27, 2023


Gram-positive bacteria are distinguished by their hue after the Gram stain test is performed on them. The result is that they appear blue, while gram-negative bacteria appear red. This difference in coloration is due to variations in their cell wall structures. These bacteria also cause varying types of infections and respond differently to antibiotics.

Bacteria can be categorized into three shapes: spherical (cocci), rod-shaped (bacilli), and spiral or helix (spirochetes). Gram-positive bacteria can be either cocci or bacilli. While some gram-positive bacteria can cause illnesses, others are a part of the normal flora in the body, such as on the skin, and do not typically cause harm.

Gram-positive cocci include Staphylococcus (catalase-positive), which grows in clusters, and Streptococcus (catalase-negative), which grows in chains. The staphylococci further subdivide into coagulase-positive (S. aureus) and coagulase-negative (S. epidermidis and S. saprophyticus) species. Streptococcus bacteria subdivide into Strep. pyogenes (Group A), Strep. agalactiae (Group B), enterococci (Group D), Strep viridans, and Strep pneumonia.

Gram-positive bacilli (rods) subdivide according to their ability to produce spores. Bacillus and Clostridia are spore-forming rods while Listeria and Corynebacterium are not. Spore-forming rods that produce spores can survive in environments for many years. Also, the branching filament rods encompass Nocardia and actinomyces. Gram-positive organisms have a thicker peptidoglycan cell wall compared with gram-negative bacteria. It is a 20 to 80 nm thick polymer while the peptidoglycan layer of the gram-negative cell wall is 2 to 3 nm thick and covered with an outer lipid bilayer membrane.

What Are the Key Features of Gram-Positive Bacteria?

Gram-positive bacteria are distinct from Gram-negative bacteria due to their structural differences. The key characteristics of Gram-positive bacteria are:

  • Outer Membrane: Gram-positive bacteria are missing an outer membrane, whereas Gram-negative bacteria possess one.

  • Color: Gram-positive bacteria have a blue or purple appearance when viewed under a microscope.

  • Cell Wall: This structure surrounds the cell membrane. The cell wall of Gram-positive bacteria is composed of several layers of molecules and proteins and serves as a protective barrier for the bacteria.

  • Peptidoglycan Layer: The peptidoglycan, which is a layer within the cell wall, is 20 to 80 nanometers thick in Gram-positive bacteria and only 2 to 3 nanometers thick in Gram-negative bacteria.

  • Shape: Gram-positive bacteria come in various shapes, including spherical (cocci), rod-shaped (bacilli), and threadlike (branching filaments).

What Is a Gram-Positive Cocci?

Oval or spherical bacteria are a commonly occurring type of bacteria that can exist in different arrangements such as pairs, chains, or clusters.

  • Staphylococcus: Staphylococcus bacteria are typically found in clusters and are often present on the skin and mucous membranes without causing harm as part of the normal body flora. However, if these bacteria proliferate within the body, they can lead to severe infections. Some examples of these types of bacteria and their associated infections include Staphylococcus aureus (causes bacterial pneumonia, food poisoning, skin infections, MRSA, endocarditis), Staphylococcus epidermidis (causes infections in hospital settings, surgical site infections, eye infections), Staphylococcus saprophyticus (causes urinary tract infections, kidney infections, infections in the urethra, prostate, and epididymis).

  • Streptococcus: These bacteria grow in a chain formation and are part of the normal flora found on or in various body parts, such as the skin, mouth, digestive system, and genitals. However, if they penetrate the body and increase in number, they can cause infections. Some examples of these types of bacteria and the infections they can cause include: Streptococcus pneumoniae is a significant contributor to bacterial pneumonia. It can also lead to sinus infections and meningitis. Streptococcus pyogenes can result in various infections, such as:

  • Scarlet fever.

  • Strep throat.

  • Flesh-eating diseases.

  • Pharyngitis.

  • Rheumatic fever.

Strep B strep, also known as Streptococcus agalactiae, can cause serious infections in newborns, including sepsis, pneumonia, and meningitis. The most prevalent types of Enterococcus are Enterococcus faecalis and Enterococcus faecium, which can cause urinary tract infections, infective endocarditis, and bloodstream infections.

What Is a Gram-Positive Bacillus?

Bacilli are a bacterium with a rod shape and are commonly found on the skin. They are further divided into spore-forming and non-spore-forming categories. Spore formation and dormancy are survival mechanisms that allow bacteria to endure tough conditions, and when the environment improves, they can sprout and grow into active bacteria. There are two main types of bacillus.

  • Bacillus: Bacilli are rod-shaped bacteria that usually occur on the skin and are classified into spore-forming and non-spore-forming categories. These bacteria require oxygen to live, can form spores, and lie dormant to survive tough conditions before growing into active bacteria when the environment improves.

One type, Bacillus anthracis, generates anthrax toxin, which is potentially deadly to humans and animals. These fragments can endure in the soil for many years before infecting a person and mainly impact grazing animals, such as cattle and sheep. People handling contaminated animal products can also become infected. Anthrax can result in symptoms ranging from skin sores to fatal respiratory diseases. Another spore-forming bacterium, Bacillus cereus, can cause food poisoning, wound infections, and respiratory infections.

  • Clostridia: These bacteria are anaerobic and do not require oxygen to exist. They also generate toxins that can result in dangerous human illnesses. Included among the different types of bacteria and their associated infections are:

  1. Clostridium Botulinum: The botulinum toxin produced by this bacterium is the most potent known, and it can lead to the potentially deadly disease of botulism.

  2. Clostridium Perfringens: This bacterium can cause food poisoning when present in contaminated meat, leading to brief bouts of diarrhea and abdominal cramps.

  3. Clostridium Difficile: This bacterium can cause infection following antibiotic treatment and result in various gastrointestinal symptoms. It is commonly associated with outbreaks in hospitals and senior living facilities.

  4. Clostridium Tetani: This bacterium generates tetanus toxin, leading to tetanus infections. The bacterium's spores can be in soil, dust, and manure and penetrate the body through open wounds.

What Is the Treatment for Bacterial Infection?

Treatment of Gram-positive bacterial infections typically involves antibiotics. The choice of antibiotic depends on the specific type of bacteria causing the infection. Common antibiotics used to treat Gram-positive infections include:

  • Penicillin: Penicillin is an antibiotic that targets the bacterial cell wall to destroy the wall and bacteria. It has a broad range of effectiveness against bacterial infections, including Streptococcus infections like strep throat and sinus infections.

  • Glycopeptides: These are antibiotics that doctors commonly prescribe in cases of severe infections caused by drug-resistant bacteria such as MRSA. These antibiotics work by breaking down the bacterial cell wall. They are effective in treating infections caused by Enterococcus and Clostridium difficile.

  • Erythromycin: A common alternative for those allergic to Penicillin, this macrolide antibiotic prevents bacteria from producing proteins. It is often used to treat various bacterial infections, such as bacterial pneumonia, strep throat, and staph skin infections.

  • Antitoxin Treatment: For individuals with illnesses such as anthrax, botulism, or others caused by toxins, antibiotics may be used in conjunction with antitoxin treatment. Antitoxin therapy neutralizes bacteria's harmful toxins, while antibiotics aim to eliminate the fragments. In addition to these treatments, supportive care such as intravenous fluids, proper nutrition, and mechanical ventilation may also be necessary.


To sum up, Gram-positive bacteria can be identified by their color after a Gram stain test, which shows them as blue or purple under a microscope. These bacteria come in various shapes, such as spherical (cocci), rod-shaped (bacilli), and threadlike (branching filaments). Some gram-positive bacteria are part of the natural flora in the body, while others can cause serious infections. Examples of gram-positive cocci include Staphylococcus and Streptococcus, which can result in infections such as bacterial pneumonia, food poisoning, skin infections, MRSA, endocarditis, and urinary tract infections. It is crucial to determine the type of gram-positive bacteria causing the infection for appropriate treatment and to prevent its spread.

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Dr. Shubadeep Debabrata Sinha
Dr. Shubadeep Debabrata Sinha

Infectious Diseases


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