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What are wound and skin infections?

Wound and skin infections are the growth and spread of microbes, usually bacteria, within the skin or a break or wound in the skin. These infections trigger the body’s immune system and cause inflammation and tissue damage within the skin or wound and slow the healing process.

Many infections remain confined to a small area, such as an infected scratch or hair follicle, and usually resolve on their own. Others may persist and, if untreated, become more severe and spread further and/or deeper into the body. Some infections spread to other organs and/or into the blood (septicemia) and cause a body-wide (systemic) infection.

Skin is the body’s largest organ and its first line of defense. Even when it is clean, the surface of the skin is not sterile but is covered with a mixture of microbes called normal flora. Most of the time, these microbes do not cause illness and do not stimulate the immune system. If there is a break in the skin or if the immune system is weakened, then the microbes may cause a wound or skin infection.

Wounds are breaks in the integrity of the skin and tissues. They may be superficial cuts, scrapes or scratches but also include deeper cuts, punctures, burns, or may be the result of surgical or dental procedures.

The skin has three layers: the outer epidermis, the dermis – where many hair follicles and sweat glands are located – and the fatty subcutaneous layer. Below these layers are membranes that protect connective tissues, muscle, and bone. Wounds can penetrate any of these layers, and skin infections can spread into them. Wound healing is a complex process that involves many related systems, chemicals, and cells working together to clean the wound, seal its edges, and to produce new tissues and blood vessels.

Skin and wound infections interfere with the healing process and can create additional tissue damage. They can affect anyone, but people with underlying conditions are at risk of slower wound healing and greater risk of wound infections. Examples of conditions that increase the risk of wound infections include:

  • Poor blood circulation
  • Diabetes
  • Weakened/suppressed immune system (e.g., HIV/AIDS, organ transplant recipient)
  • Low mobility or immobility (e.g., confined to bed, paralysis)
  • Malnutrition

When infections penetrate deep into the body into tissues such as bone, or when they occur in tissue that has inadequate circulation, they can become difficult to treat and may become chronic infections.


About wound and skin infections

Superficial skin infections

Superficial infections occur primarily in the outer layers of the skin but may extend deeper into the underlying (subcutaneous) layer.

Examples of bacteria that can cause skin infections include:

  • Bacteria that are often normally found on the skin (normal flora), such as species of Staphylococcus (staph) and Streptococcus (strep), are common causes.
  • Antibiotic-resistant bacteria, such as MRSA (Methicillin-resistant Staphylococcus aureus)
  • Vibrio or Aeromonas species, often found in brackish water
  • Pseudomonas aeruginosa is associated with hot tubs.
  • Bacteroides and Clostridium species may cause deeper wound infections.
  • Tularemia—this infection is caused by Francisella tularensis bacteria. They primarily infect rabbits and rodents, but humans can get infections through tick or deer fly bites or contact with infected animals that can result in skin ulcers.
  • Anthrax—this is an infection caused by the bacteria Bacillus anthracis. It can infect the skin as well as the respiratory or digestive tracts. The bacteria live in the soil and primarily infect animals, such as cattle, deer, sheep and goats. Humans can be infected by handling the animals or their hair, hide, or meat. Natural cases of human anthrax infection are rare in the U.S.

Typical bacterial skin infections include:

  • Infected hair follicles (folliculitis)
  • Boils (furuncles)
  • Collection of boils involving several hair follicles and deeper layers of tissue (carbuncles)
  • Impetigo—skin lesions and blisters (vesicles)
  • Pressure sores (bed sores) and ulcers—these may be found in people who are immobilized or bedridden for long periods of time
  • Cellulitis—an infection often involving the subcutaneous and connective tissue of skin, causing redness, heat, and swelling
  • Necrotizing fasciitis—a serious but uncommon infection that can spread rapidly and destroy skin, fat, muscle tissue and fascia, the layer of tissue covering muscle groups.  This type of infection often involves Group A streptococci, which are sometimes referred to as “flesh-eating bacteria.”

Examples of common fungal skin infections include:

  • Ringworm
  • Athlete’s foot
  • Yeast infections cause by Candida species may occur in the mouth (thrush) or on other moist areas of the skin.

Read the article on Fungal Infections to learn more.

Examples of viruses that cause skin infections include:

Examples of wound infections

Wound infections due to bites tend to reflect the microbes present in the saliva and mouth of the human or animal that created the bite wound. Human bites may become infected with a variety of bacteria that are part of the normal oral flora. The majority of animal bites are from dogs and cats, and the most common bacteria is Pasteurella multocida.

Trauma can be the result of any physical force, causing various wounds susceptible to infection, ranging from superficial scrapes to deep, penetrating wounds. Wounds that are initially contaminated with dirt or other material have a higher risk of becoming infected. It is not uncommon for deep and contaminated wounds to have more than one type of bacteria causing the infection.

A deep puncture wound could allow bacteria such as Clostridium tetani (the cause of tetanus) to grow. Because most people in the U.S. are immunized against tetanus, this is very rare. Vaccination must be updated for tetanus every 10 years. Booster shots are often given in the emergency room, where people receive treatment for deep or puncture wounds.

Other types (species) of Clostridium bacteria, such as Clostridium perfringes, can cause serious wound and surgical infections, such as gas gangrene.

Surgical sites are most commonly infected with the person’s normal skin and/or digestive tract flora, the same microbes seen with superficial infections. They may also become infected by exposure to microbes in the hospital environment. Hospital-acquired bacteria often have an increased resistance to antibiotics, such as MRSA.

Burns can range from mild to severe, affecting different layers of the skin. First-degree burns involve the epidermis. Second-degree burns penetrate to the dermis. Third-degree burns penetrate through all of the layers of the skin and frequently damage the tissues below it. With increasing tissue damage, risk of infection increases.

Burn wounds are initially sterile but because of the dead tissue at their center – the eschar (scab) – and the loss of the skin’s protection, bacteria normally found on the outer layer of skin will begin to grow within the wound. The affected person is at an increased risk for infection and more serious complications. Initial infections tend to be bacterial. Fungal infections due to CandidaAspergillusFusarium, and other species may arise later since they are resistant to antibiotics. Viral infections, such as those caused by the herpes simplex virus, may also occur.

Signs and Symptoms

General signs and symptoms of a wound infection include:

  • Redness or discoloration
  • Swelling
  • Warmth
  • Pain, tenderness
  • Scaling, itching
  • Pustules, pus drainage

The skin may harden or tighten in the area and red streaks may radiate from the wound. Wound infections may also cause fevers, especially when they spread to the blood.


Many minor and superficial skin and wound infections are diagnosed by a healthcare practitioner based on a physical examination, signs and symptoms, and experience. A clinical evaluation cannot, however, definitively tell the healthcare practitioner which microbe is causing a wound infection or what treatment is likely to be effective. For that, laboratory testing is required.

Laboratory tests
Examples of common tests include:

  • Bacterial culture – This is the primary test used to diagnose a bacterial infection. Results are usually available within 24-48 hours.
  • Gram stain – This is usually performed in conjunction with the wound culture. It is a special staining procedure that allows bacteria to be evaluated under the microscope. The results are usually available the same day and provide preliminary information about the microbe that may be causing the infection.
  • Antimicrobial susceptibility – A follow-up test to a positive wound culture, this is used to determine the bacteria’s likely susceptibility to certain drugs and helps the healthcare practitioner select appropriate antibiotics for treatment. Results are typically available in about 24 hours. This testing can identify resistant bacteria such as MRSA.

Other tests may include:

  • KOH prep – This is a rapid test performed to detect fungi in a sample. The sample is treated with a special solution, placed on a slide, and examined under a microscope.
  • Fungal culture – This is ordered when a fungal infection is suspected. Many fungi are slow-growing and may take several weeks to identify.
  • AFB testing – This is ordered when a mycobacterial infection is suspected. Most AFB are slow-growing and may take several weeks to identify.
  • Blood culture – This is ordered when infection from a wound may have spread to the blood.
  • Molecular testing to detect genetic material of a specific microbe
  • Basic metabolic panel (BMP) or Comprehensive metabolic panel (CMP) – This may be ordered to detect underlying conditions that can affect wound healing, such as a glucose test to detect diabetes.
  • Complete blood count (CBC) – An elevated white blood cell (WBC) count may be a sign of infection.

Non-laboratory tests
In some cases, imaging scans such as ultrasounds or x-rays may be ordered to evaluate the extent of tissue damage and to look for areas of fluid/pus.


The risk of wound infection can be minimized with prompt and proper wound cleansing and treatment. Many superficial bacterial infections and viral infections will resolve on their own without treatment. Other bacterial infections may require only a topical antimicrobial, and some cases require incision and drainage.

Deeper infections, and those that are persistent, typically require antibiotics. The choice is based upon the results of wound culture and antimicrobial susceptibility tests. People with antibiotic-resistant bacteria or with an infection in a location that is difficult for drug therapy to penetrate (such as bone) may require extended treatment and/or treatment with intravenous (IV) medications.

Wounds may also require removal of dead tissue (debridement) and/or drainage, sometimes more than once. Topical antimicrobials and debridement are also used for burn treatment. With extensive injuries, grafting and other surgeries may be required.

Treatment plans can be affected by the presence of underlying conditions that can slow wound healing, such as diabetesmalnutritionHIV/AIDS, and other disorders that compromise the immune system. A healthcare practitioner may need to perform tests to detect these underlying conditions.

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