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Wound and Skin Infections

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Types of Wound Infections and Microorganisms

Wounds may be grouped according to the cause, the environment in which they occur, their extent, and whether they are clean or contaminated. Bacteriafungi, and viruses can all cause skin and wound infections.The microorganisms that typically infect wounds and the skin depend on what is present in the environment, the state of the person's immune system, and the depth of the wound.

Superficial skin infections
Bacteria may be divided according to the environment in which they grow: those that grow in air (aerobic), those that grow in reduced oxygen environments (microaerophilic), and those that grow in little to no oxygen (anaerobic). Microaerophilic and anaerobic bacteria may be found in deeper wounds and abscesses.

Superficial infections occur primarily in the outer layers of the skin but may extend deeper into the subcutaneous layer. They are primarily caused by aerobic microorganisms, but deeper wounds may also be infected with anaerobes.

Bacterial infections are typically caused by normal flora bacteria, such as species of Staphylococcus (staph) and Streptococcus (strep). They may also be caused by colonizing bacteria and antibiotic resistant bacteria, such as MRSA (Methicillin Resistant Staphylococcus aureus). Brackish water wound infections may be due to waterborne Vibrio or Aeromonas species. Hot tub-associated infections may be caused by Pseudomonas aeruginosa. When wounds are deeper, the possible pathogens include anaerobes such as Bacteroides and Clostridium species.

Typical bacterial skin infections include:

  • Folliculitis, furuncles, and carbuncles
  • Impetigo—skin lesions and vesicles
  • Pressure sores (bed sores) and ulcers—these may be found in people who have been immobilized for long periods of time such as those in long-term care facilities. These types of wounds may contain many different types of bacteria and culturing them does not always provide useful information as to how the person should be treated.
  • 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."

Other common skin infections such as ringworm and athlete's foot are not caused by bacteria but by fungi. Fungi can be found on thorns, splinters, and dead vegetation and can lead to deep wound infections that require special cultures for detection and identification. Yeast infections cause by Candida species may occur in the mouth (thrush) or on other moist areas of the skin.

A variety of warts, such as common and plantar warts, are due to human papilloma virus (HPV). Another virus, Herpes simplex types 1 & 2 (HSV-1, HSV-2), is responsible for a skin infection known as herpetic whitlow.

Bites
Wound infections due to bites tend to reflect the microorganisms present in the saliva and oral cavity of the human or animal that created the bite wound. They may involve one or more aerobic, microaerophilic, and/or anaerobic microorganisms.

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 recovered from these cultures is Pasteurella multocida

Trauma
Trauma is a wide category of injuries caused by physical force. It includes everything from burns to injuries from motor vehicle accidents, crushing injuries, cuts from knives and other sharp instruments, and gunshot wounds. The type of infections that trauma victims acquire depend primarily on the environment in which the injury took place, the extent of the injury, the microorganisms present on the skin of the affected person, the microorganisms the person is exposed to during wound healing, and the person's general health and immune status.

Wounds that are initially contaminated such as with the dirt that may be acquired during a motor vehicle accident or that involve extensive damaged tissue, such as a severe burn, are at an increased risk of becoming infected. It is not uncommon for deep and contaminated wounds to have more than one aerobic and/or anaerobic microorganism present.

A deep puncture wound could allow anaerobic bacteria such as Clostridium tetani (the cause of tetanus) to grow. Because most people in the U.S. are immunized against tetanus, this is a very rare event. Vaccination must be updated for tetanus every 10 years. Re-vaccination is often done in the emergency room where people are treated after incurring a deep wound that may need stitches.

Post surgical
Surgical sites are most commonly infected with the person's normal skin and/or gastrointestinal flora – the same organisms seen with superficial infections. They may also become infected by exposure to microorganisms in the hospital environment. Hospital-acquired bacteria, such as MRSA, often have an increased resistance to antibiotics. Deep surgical wounds may become infected both superficially with aerobic microorganisms and deep within the body by anaerobes.

Burns
Burns may be caused by scalding or flammable liquids, fires and other sources of heat, chemicals, sunlight, electricity, and very rarely by nuclear radiation. 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.

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, they are quickly colonized by the person's normal flora. The affected person is at an increased risk for wound infection, septicemia, and for multiple organ failure.  Initial infections tend to be bacterial. Fungal infections due to Candida, Aspergillus, Fusarium, and other species may arise later since they are not inhibited by antibacterial treatment. Viral infections, such as those caused by the herpes simplex virus, may also occur.

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