, , and can cause skin or wound infections. Bacteria may be divided according to the environment in which they grow: those that grow in air () and those that grow in little to no oxygen (). Anaerobic bacteria may be found in deeper wounds and .
Superficial skin infections
Superficial infections occur primarily in the outer layers of the skin but may extend deeper into the layer. They are primarily caused by aerobic microorganisms but deeper wounds may also be infected with anaerobes.
Bacterial infections are typically caused by bacteria, such as species of Staphylococcus (Staph) and Streptococcus (Strep). They may also be caused by 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. Swimming pool-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:
- , , and
- —skin lesions and
- 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 contain many different types of bacteria and them 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, 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.
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 verrucas, are due to human papilloma virus (HPV).
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 and/or anaerobic microorganisms.
Human bites may become infected with a variety of aerobic and anaerobic bacteria that are part of the normal oral flora. Most animal bites are from dogs and cats, and the most common bacteria recovered from these cultures is Pasteurella multocida.
Trauma is a wide category of injuries caused by physical force. It includes everything from burns to injuries from motor vehicle accidents, crushing injuries, shooting and cuts from knives and other sharp instruments. 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.K. are immunised against tetanus, anaerobic bacteria infection is a very rare event. must be updated for tetanus every 10 years. Re-vaccination is often done in the emergency room where patients are treated after incurring a penetrating injury that may need stitches.
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. Healthcare-associated 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 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 the layers of the skin and frequently damage the tissue 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, , and for multiple organ failure. Initial infections tend to be bacterial. Fungal infections due to Candida, Aspergillus, Fusarium, and other fungi may arise later since they are not inhibited by antibiotic treatment. Viral infections, such as those caused by the Herpes simplex virus, may also occur.