Wound Bed Preparation: The Science Behind the Removal of Barriers to Healing

Stuart Enoch, MBBS, MRCSEd, MRCS (Eng), Keith Harding, MB ChB, MRCGP, FRCS


Wounds. 2003;15(7) 

In This Article

Bacterial Imbalance in Chronic Wounds

All chronic wounds intrinsically contain bacteria, and the process of wound healing can still occur in their presence. Therefore, it is not the presence of bacteria[86] but their interaction with the host that determines the organisms' influence on chronic wound healing.

The existence of bacteria in the wound bed can be divided into four distinct categories based on the induced host response. These categories are defined as contamination, colonization, local infection, and spreading infection. However, the relative number of microorganisms and their pathogenicity, in combination with host response and factors, such as immunodeficiency, dictate whether a chronic wound becomes infected or shows signs of delayed healing.

Contamination is defined as the presence of nonreplicating microorganisms within a wound. Most organisms entering the wound bed fall into this category and are incapable of replicating in soft human tissue. As a consequence, the host defenses rapidly clear them.

Colonization is categorized as replicating microorganisms that adhere to the wound surface but do not cause cellular damage to the host. Examples of colonizing organisms are those usually found living on the skin, such as Staphylococcus epidermidis and Corynebacterium species. Rather than causing damage to the host, these species of commensal organisms have been shown to enhance wound healing.[87,88,89,90]

Recently, the terms local infection and critical colonization have been introduced to describe a situation in which the wound has an increasing bacterial burden, which is intermediate between the category of colonization and infection. Wounds that are locally infected or critically colonized will not heal but may not display classic signs of infection.[32] During this phase, subtle clinical symptoms of local infection may, however, become apparent. The clinical signs and symptoms of critical colonization are delayed healing, pain/tenderness, increased serous exudate, change in color of the wound bed, friable granulation tissue, absent or abnormal granulation tissue, and abnormal odor.[91,92]

Wound infection is defined as the presence of replicating microorganisms within a wound with a subsequent host response that leads to a delay in healing. Because of this it is important that infection is recognized as early as possible. The signs and symptoms of local infection are redness (erythema), warmth, swelling, pain, and loss of function. Foul odor and pus may accompany this. Eventually, the local bacterial burden will increase further and becomes systemically disseminated resulting in sepsis, which if not actively treated could progress to multi-organ failure and death in some instances.

There are several factors known to affect the bacterial burden of chronic wounds and increase the risk of infection. These include the number of microorganisms present in the wound, their virulence, and host factors. For instance, experimental studies have demonstrated that regardless of the type of microorganism, impairment of wound repair may occur when there are more than 1x105 organisms per gram of tissue.[74,93,94,95] However, other studies have shown that many chronic wounds with a bacterial load greater than 1x105 will heal normally.[96] In view of this, it is clear that the type and pathogenicity of the organisms increase the risk of infection rather than the number of microorganisms alone. For example, the isolation of any highly virulent beta hemolytic streptococci from a chronic wound should be considered significant and the appropriate treatment initiated.[97]

The mix of microorganisms within a wound may also be an issue. Most chronic wounds will usually contain more than three species of microorganism.[98,99] This is significant because some combinations of bacterial species may develop synergy with each other, resulting in previously nonvirulent organisms becoming virulent and causing damage to the host.[74,100,101,102] Deep in the granulation tissue, chronic wounds may contain resident microflora that may not be reflected by routine culture. However, with the advent of new molecular techniques, like denaturing gradient gel electrophoresis (DGGE), novel organisms like Pseudomonas rhodesiae could be identified and the limitations of conventional techniques for the analysis of complex microbial populations could be overcome in the future.[103]

Although bacterial quantity and virulence are important factors in wound infection, host resistance is also of equal significance. Systemic factors also increase the risk of infection in chronic wounds ( Table 3 ). Factors, such as immunosuppression, concomitant disease, medication, and age, can all influence whether bacteria present in a wound impair healing.[74,104]

More recently, the importance of biofilms has become apparent in the context of wound infection, and these may contribute to delayed healing in chronic wounds. When bacteria proliferate they form microcolonies that become attached to the wound bed and secrete a glycocalyx or biofilm that helps to protect the microorganisms from antimicrobial agents.[105] Organisms may exist as clusters of individual bacterial types or as mixed bacterial colonies. These bacterial colonies undergo several changes, expressing different genes, which can then alter the organisms' antimicrobial sensitivity. The periodic release of motile bacteria from these colonies may result in infection. Thus, biofilms are bacterial colonies that may be resistant to the effects of antimicrobial agents, such as antibiotics and antiseptics, and could contribute to delayed healing.[105]