Normal Skin Flora
The skin is covered with microorganisms. These can be either resident organisms, those that can typically be found on the subject’s skin, or transients that are often seen on the skin surface but are quickly shed during normal body hygiene or by skin sloughing. While these organisms are usually bacteria, the yeast Pityrosporum and skin mite Demodex are also commonly found. These colonizing microbes take residence in the crypts and crevices that favor bacterial growth, and prevent pathologic species from gaining access to these areas. Human beings are protected from bacterial overgrowth and invasion at the surface by a number of defense mechanisms. A layer of dead, keratinous epithelial cells known as the stratum corneum is the outermost layer of skin. As the keratin sloughs, it removes attached organisms with it. Sebaceous glands secrete an oily, lipid-rich, acidic substance, (pH range of 4.2 to 5.6) that acts to retard bacterial growth. Bacteria become more active on the skin surface as the pH rises above 6.5, as is seen with the use of many cleansing and moisturizing agents. Should foreign organisms get past these defenses, the antigen-presenting Langerhans cells found in the epidermis and the phagocytosing macrophages and immune-stimulating mast cells present in the dermis rapidly mobilize the body’s cellular and humoral immune responses.
Contamination vs. Infection Cutaneous wounds, by definition, have lost their protective barrier and are subject to invasion by not only foreign bacteria introduced through the environment,
Bacteria Staphylococcus Micrococcus Peptococcus Corynebacterium Brevibacterium Propionibacterium Streptococcus Neisseria Acinetobacter but also the local bacterial flora that is present on intact skin.
These wounds occur in the setting of various pathologies and are usually chronic in nature before being brought to the attention of a plastic surgeon. Unlike most surgical incisions, these wounds heal by secondary intention and are always colonized by bacteria. They require extensive granulation tissue formation and keratinocyte migration for closure, involving endothelial cells and fibroblasts for the purposes of neovascularization and matrix production, respectively. For this to occur, macrophages and a varying milieu of growth factors must be present. Along with neutrophils, macrophages also act to disinfect the wound, killing foreign organisms by the generation of peroxide and superoxide radicals. The clinical spectrum of bacterial invasion exists on a continuum from least to most severe: contamination, colonization, local infection or critical contamination, invasive infection and sepsis.
- Contaminated wounds have nonreplicating organisms within their borders. These wounds will go on to heal normally.
- Colonized wounds have replicating bacteria, but these bacteria are nondestructive and contained within the wound. A hallmark of colonization is that it does not delay the wound healing process.
- Local infection or critical contamination is an intermediate level of bacterial invasion characterized by granulation tissue that has an unhealthy appearance, and wound healing that may be delayed. In this type of wound, however, tissue invasion is not present. This stage is notable for the absence of other signs of infection such as cellulitis or pus formation.
- Invasive infection occurs once bacteria have invaded through the wound bed, tissue destruction has begun, and an aggressive immune response is present. Signs and symptoms of invasive infection include pain, edema, erythema and fever. The finding of a chronic, nonhealing wound, often with pus formation and tissue necrosis is often evident.
- Sepsis occurs when the infection spread systemically, and cardiovascular instability and organ-system dysfunction develop.
The Molecular Biology of Bacterial
Infection Low levels of bacteria in wounds actually help to promote wound healing by stimulating brisk monocyte and macrophage activity. However, as their number or virulence increases, the tissue response to their presence disrupts and prolongs the inflammatory phase of wound healing, depletes the components of the complement cascade, interferes with normal clotting mechanisms, and alters leukocyte function. The level of pro-inflammatory cytokines, including interleukin-1 and tumor necrosis factor-alpha, rises and stays elevated. Elevated levels of matrix metalloproteinases and a lack of their inhibitors lead to tissue breakdown and growth factor inhibition. Bacteria also compete with local cells for oxygen, reducing its availability to these cells and stimulating an angiogenic response, leading to friable granulation tissue that is prone to bleeding.
Bacteria in Wounds
Classic teaching is that wounds with greater than 105 organisms/gram should be considered infected whereas those with a lower bacterial count should not. Although studies do show that wounds with bacterial counts higher than this heal more slowly and have a higher rate of infection, a more practical approach to diagnosing the infected wound is encouraged. As wounds mature, not only do the species of organisms present in the wound change, the wounds begin to carry a higher level of bioburden, meaning a higher baseline number of colonies without being infected. Conversely, the more virulent bacteria, such as beta-hemolytic streptococcus and some rare Clostridium species, can easily cause infection at lower quantitative levels than the more commonly occurring species. Finally, the status of the patient’s immune response has a role in the patient’s likelihood of developing an infected wound. Therefore, the surgeon is encouraged to study the appearance of the wound and the overall clinical picture when deciding whether a wound is infected. Although it is important to note the classic signs and symptoms of infection including erythema, edema, fever and an elevated white blood cell count, recent studies attempting to establish evidence-based criteria for the determination of a chronic wound infection have shown that increasing pain, friable granulation tissue, foul odor and wound breakdown are the most sensitive indicators. Bacteria in chronic wounds often establish a biofilm. This is an extracellular, polysaccharide-rich matrix in which the organisms are embedded. Within this glycocalyx is a system of channels, like a primordial circulatory system, that allows the bacteria to remain viable with less direct dependence on the host tissue. Cells in this environment become more sessile and less metabolically active. As a result, they are resistant to host immune responses and antibiotic therapy. Biofilms often coat foreign and implanted material, making infections in this setting more difficult to treat, and certain bacteria such as Pseudomonas aeruginosa have a predilection to biofilm production.
Bacteria Occurrence (%)
Staphylococcus aureus 20 Coagulase-negative staphylococci 14 Enterococci 12 Escherichia coli 8 Pseudomonas auruginosa 8 Enterobacter species 7 Proteus mirabilis 3 Klebsiella pneumonia 3 Other streptococci 3 Candida albicans 3 Group D streptococci 2
Clinical Evaluation
A thorough history includes information related to the chronicity of the wound, any changes to the wound appearance, and details that should make the clinician suspicious of a more invasive bacterial involvement (e.g., pain, fever). Mitigating factors such as comorbid conditions that could lead to immunosuppression, the use of any immunosuppressive medications, previous radiation in the wound area and the overall functional status of the patient are important to explore. In addition to a white blood cell count and blood cultures, laboratory tests can include the erythrocyte sedimentation rate and C-reactive protein. Although not specific, in a patient with no recent history of surgery or acute illness, their value is in helping to determine the level of systemic response to a wound and in helping to determine the presence of a deep wound infection. When examining a wound, its depth and width should be measured and a careful inspection and probing should be done. Attention to findings such as erythema at least 5 mm beyond the wound edges, expressed pus, necrotic debris or granulation tissue that is dark, friable or heaped above the wound edges can help to determine the extent of infection. Foreign bodies such as old strands of gauze should be removed and the presence of underlying foreign material such as sutures or mesh should be ruled out. Care must be taken to ensure that wounds overlying osseous structures do not have any exposed bone at their base that would suggest the presence of osteomyelitis. As stated earlier, bacterial cultures can help to make a diagnosis and guide appropriate therapy. In a wound that has been appropriately cleaned and prepared, a swab of the deeper tissue can give a qualitative notion of which bacteria are present. It does not, however, allow the clinician to quantitate the amount of bacteria within the wound. For this, the gold standard is a biopsy culture. A punch biopsy is taken and ground into a liquid state from which serial dilutions are cultured. A measure of colonies per milligram can then be reported.
Treatment
Antibiotics are ineffective in penetrating chronic, nonhealing wounds. Debridement is the best option for clearing bacterial loads and removing nonviable tissue. If not performed, necrotic material can release endotoxins that inhibit keratinocyte migration and matrix production, and can prolong the inflammatory response, promoting matrix-destroying proteases. Methods of debridement include sharp, mechanical, chemical and biodebridement. Sharp, or surgical debridement affords the luxury of speed, since it can be performed at the bedside with nothing more than scissors and a pair of forceps. More extensive debridement may require anesthesia, and should be performed in the operating room. Mechanical debridement is the eradication of dead tissue by the sequential changes of dressings that are inserted moist into the wound and removed after they are allowed to dry. Exudative and necrotic tissues adhere to the drying gauze and are pulled out of the wound with the gauze. This technique has only a limited ability to remove structurally intact or strongly adherent devitalized tissue. The classic “wet-to-dry” dressing is a mechanically debriding dressing. Chemical debriding agents are enzymatic compounds that break down tissue. They are most effective in moderately sized areas of necrosis or in those patients that will not tolerate an operation. In order to gain maximum benefit, larger eschars should be cross-hatched or excised to allow for better penetration of the agent. The papain-containing cream, Accuzyme®, is commonly used for this purpose. Biodebridement involves the application of sterile maggots into a wound for periods of 48-72 hours. The maggots feed on, and thus remove dead tissue before being irrigated out. This process can be repeated as necessary. Needless to say, it is not a commonly used technique. Antibiotics do have a role in the treatment of chronic, infected wounds once debridement has achieved healthy wound borders. Empiric antibiotics should be selected based on the bacteria that are likely to be involved. For example, empiric antibiotics for wounds near the oropharynx and diabetic foot wounds should include coverage for anaerobic species. The Gram stain can give a general idea of whether Gram-positive, Gram-negative or a combination of bacteria is present. Once culture results return over the subsequent 2-3 days, antibiotic coverage should be tailored to the involved organisms. Topical antibiotic preparations can help to reduce bacterial load and can be used with some success in an adjuvant setting in the select wound population. Prudence should be taken with their use, however, because many of these preparations also impair the function of the superficial cells necessary for wound healing. They should never be used in wounds related to venous disease, as these wounds are more prone to sensitivity reactions. Examples include: iodine or iodophor paint, sodium hypochlorite solution, hydrogen peroxide, acetic acid, antibiotic creams or the newer cadhexomer iodine and nanocrystalline silver. For wounds that arise in the setting of underlying pathology, treating the disease process can increase the speed and likelihood of wound healing. For venous stasis ulcers, reducing edema fluid with Unna boot compression, elevation and diuresis can improve oxygen delivery and thus cellular function. Patients with diabetic foot ulcers should have their blood sugar strictly controlled given the deleterious effects of hyperglycemia on neutrophil and monocyte function. If ischemia is believed to be contributing to the etiology or chronicity of a wound, smoking cessation and elimination of dehydration and anemia should all be considered in the treatment plan. Ultimately arterial revascularization with or without surgical reconstruction using local or microvascular flaps may be necessary.
Hidradenitis Suppurativa
This condition is due to infection of the apocrine sweat glands, most commonly in the axillary, perineal and groin regions. It results in recurrent, draining abscesses and sinus tracts that can lead to severe pain and debilitation. Lesions in the axilla that heal may scar and secondarily cause contracture limiting arm motion. Active infection should be treated with a 1-2 week course of oral antibiotics and is usually due to Gam positive cocci. Cultures should always be taken since other bacterial infections can occur, and the antibiotic should be appropriately selected. Surgical treatment consists of full-thickness excision of the infected dermis and any involved subcutaneous fat. Primary closure can be obtained in small- to moderate-sized wounds without active infection. Larger defects, or those that are grossly infected, should not be closed primarily. They should be allowed to granulate with dressing changes, followed by split-thickness skin grafting or healing by secondary intention. Incomplete excision of the involved tissue is common due to retained sinus tracts or deep infected glands. When this occurs, there is a high likelihood of recurrence and skin graft failure.
