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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. 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.

Bacteria, fungi and viruses can cause skin and wound 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 skin infections
Superficial infections occur primarily in the outer layers of the skin but may extend deeper into the subcutaneous layer.

Bacterial infections are typically caused by normal flora bacteria, such as species of Staphylococcus (Staph) and Streptococcus (Strep). They may also be caused by colonising bacteria which may be resistant to some antibiotics, 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 patients who have been immobilised for long periods of time such as long-term care facility patients. These types of wounds may be colonised by many different types of bacteria and culturing them often does not provide useful information as to how the patient should be treated.
  • Cellulitis - an infection often involving the subcutaneous and connective tissue of skin causing redness, heat, pain and swelling
  • Necrotising 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 superficial skin infections such as ringworm and athlete's foot are not caused by bacteria but by fungi such as Trychophyton sp. However, fungi can be found on thorns, splinters and dead vegetation and can lead to deep invasive wound infections that require special cultures for detection and identification. Yeast infections caused by Candida species may occur on moist areas of the skin such as in nappy rash.

A variety of warts, such as common and plantar warts, are due to human papilloma virus (HPV).

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.

Human bites may become infected with a variety of aerobic and anaerobic 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. Although rare, there is a risk of a rabies viral infection with bites from unvaccinated animals and should be considered if the bite occurs outside Australia (although even in the wild the prevalence of rabies is low). In Australia, if bitten or scratched by a bat, then prophylaxis against Australian Bat Lyssa Virus should be considered by your health care provider.

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 Australia are immunised against tetanus, this is a rare event but need to be considered. Routine vaccination of adults every 10 years is no longer recommended but you should discuss this with your health care provider. Re-vaccination is often done in the emergency room where patients are treated after incurring a deep wound that may need stitches particularly if more than 5 years has elapsed since the last dose.

Post surgical
Surgical sites are most commonly infected with the patient'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 centre - the eschar (scab) - and the loss of the skin's protection, they are quickly colonised by the patient's normal flora. The affected person is at an increased risk for wound infection, septicaemia, 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.


Last Review Date: August 14, 2017