Wound and Skin Infections

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.  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. 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:

• Folliculitis, furuncles, and carbuncles
• Impetigo—skin lesions and vesicles
• Pressure sores (bed sores) and ulcers—these may be found in patients who have been immobilized for long periods of time such as long-term care facility patients.  These types of wounds may contain many different types of bacteria and culturing 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
• 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.

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).

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 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 rabies viral infection with bites from unvaccinated animals and should be considered if the bite occurs outside the U.K. (although even in the wild the prevalence of rabies is low).

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.K. are immunized against tetanus, anaerobic bacteria infection is a very rare event.  Vaccination must be updated for tetanus every 10 years. Re-vaccination is often done in the emergency room where patients are treated after incurring a deep wound that may need stitches.

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.  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
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 colonized 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.

General signs and symptoms of a wound infection include redness, swelling, warmth, tenderness, and accumulation of pus.  The skin may also 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.  Skin infections often redden or discolour the skin and may cause pustules, scaling, pain, and/or itching.

Many minor and superficial skin and wound infections are diagnosed by the doctor based on a clinical evaluation and on their experience.  In addition to general symptoms, many skin infections have characteristic signs, such as the appearance of a plantar wart, and typical locations on the body, such as athlete’s foot between the toes.  A clinical evaluation cannot, however, definitively tell the doctor which microorganism is causing a wound infection or what treatment it is likely to be susceptible to.  For that, laboratory testing is required.

Laboratory Tests
Laboratory testing is primarily used to diagnose bacterial wound infections, to identify the microorganism responsible, and to determine its likely susceptibility to specific antimicrobial agents.  Sometimes testing is also performed to detect and identify fungal infections.  Sample collection may involve swabbing the surface of a wound to collect cells or pus, aspiration of fluid or pus with a needle and syringe, and/or the collection of a tissue biopsy.  If anaerobic microorganisms are suspected, then special collection and transport measures must be used to keep the sample from being exposed to oxygen.  For fungal evaluation, scrapings of the skin may be collected.

Testing may include:

• Bacterial wound culture – This is the primary test used to diagnose a bacterial infection.  The sample is streaked onto or into nutrient media and incubated at body temperature to grow and identify any bacteria present in the sample.  Results of bacterial wound cultures are usually available within 24-48 hours from the time the specimen is received in the laboratory.  Results of special cultures for slow growing organisms, such as fungi or mycobacteria, may require several weeks.
• Gram stain – Used along with the wound culture.  Special staining allows bacteria to be evaluated under the microscope along with any white blood cells that are present. They may be distinguished by their shape – cocci (spheres) or rods - and separated by colour into Gram positive or Gram-negative microorganisms.  The results of this test should be available the same day the specimen is received in the laboratory and can give the doctor preliminary information about the quality of the specimen and potential organisms that may be causing the infection.
• Antimicrobial susceptibility – A follow-up test to the wound culture.  When a pathogen is identified and isolated using the wound culture, this test is used to determine the bacteria’s likely susceptibility to certain drug treatments.  This information helps guide the doctor in selecting appropriate antibiotics for treatment.  These results are typically available about 24 hours after identification of the microorganism that is causing the infection.

Other tests that may be ordered include:

• KOH prep – A rapid test performed to microscopically detect fungal elements (cellular structures) in a sample.
• Fungal Culture – Ordered when a fungal infection is suspected.  Many fungi are slow growing and may take several weeks to identify.
• AFB culture and smear – Ordered when a mycobacterial infection is suspected.
• Blood culture – Ordered when septicaemia is suspected.
• Urine culture – Ordered when a urinary tract infection is suspected.
• DNA or RNA testing to detect genetic material of a specific organism.

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.  Most wound infections that do occur can be successfully resolved.

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 oral antimicrobial therapy.  The choice of which antimicrobials to use is based upon the results of wound culture and antimicrobial susceptibility tests.  Patients 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 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.

Types of Wound Infections and Microorganisms
Wounds may be grouped according to their cause, the environment in which they occur, their size, 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.