Microbiological Analysis of Wounds: Controversies and Clinical Relevance

Factors Influencing Microbial Populations in Wounds

From a microbiological perspective, the primary function of normal, intact skin is to control microbial populations that live on the skin surface and to prevent underlying tissue from becoming colonized and invaded by potential pathogens. Exposure of subcutaneous tissue following a loss of skin integrity (i.e., a wound) provides a moist, warm, and nutritious environment that is conducive to microbial colonization and proliferation. However, the abundance and diversity of microorganisms in any wound will be influenced by factors such as wound type, depth, location, and quality, the level of tissue perfusion, and the antimicrobial efficacy of the host immune response.

Whereas the microflora associated with clean, surgical wounds would be expected to be minimal, the presence of foreign material and devitalized tissue in a traumatic wound is likely to facilitate microbial proliferation unless early prophylactic antibiotic treatment and surgical debridement is implemented.

Since wound colonization is most frequently polymicrobial (25, 27, 44, 166, 226), involving numerous microorganisms that are potentially pathogenic, any wound is at some risk of becoming infected. In the event of infection, a wound fails to heal, the patient suffers increased trauma, treatment costs rise, and general wound management practices become more resource demanding. An analysis of postsurgical wound infections following head and neck surgery demonstrated an increase in the average hospitalization period from 14 days when wounds healed without complication to 24 days when the wounds became infected (118). In a similar analysis of 108 postsurgical wounds, Zoutman et al. (249) concluded that 10.2 days per case was directly attributable to wound infection and that the associated hospital cost was $3,937 per infected patient.

Thus, concern among health care practitioners regarding the risk of wound infection is justifiable not only in terms of increased trauma to the patient but also in view of its burden on financial resources and the increasing requirement for costeffective management within the health care system. From a clinical perspective, fears associated with wound infection have paralleled the increasing use of occlusive dressings since the 1960s. The primary function of dressings such as polyurethane films, polyurethane foams, and hydrocolloids is to maintain a moist and optimal environment for wound healing. Although they have been reported to encourage microbial proliferation in wounds (95, 128), the infection rate is lower under occlusive dressings than under conventional dry dressings (24, 113) and wound healing is not impaired (95).

Although microorganisms are responsible for wound infection, widespread controversy still exists regarding the exact mechanisms by which they cause infection and also their significance in nonhealing wounds that do not exhibit clinical signs of infection. One school of thought is that the density of microorganisms is the critical factor in determining whether a wound is likely to heal (100, 102, 151, 196, 202). However, a
second school of thought argues that the presence of specific pathogens is of primary importance in delayed healing (59, 130, 149, 181, 216, 217), while yet others have reported microorganisms to be of minimal importance in delayed healing (4, 70, 80, 95, 98, 214, 237).

There is also debate about whether a wound should be sampled for culture, the value of wound sampling in determining the cause of infection and subsequent treatment, and the sampling technique required to provide the most meaningful data. Regarding the role of the microbiology laboratory, consideration must be given to the relevance of culturing polymicrobial specimens, the value of identifying one or more microorganisms, and which microorganisms (if any) should be assayed for antibiotic susceptibility. By questioning and justifying the need to sample and perform microbiological analyses on any problematic wound, long-term savings in cost, labor, and time to both the wound management team and the microbiology laboratory could be considerable. In this respect, the value of the Gram stain as a quick and inexpensive additional or alternative test is also worthy of consideration.

In clinical practice, the presentation of a devitalized acute or chronic wound or a clinically infected wound is likely to prompt a practitioner to sample the wound for microbiological analysis. However, from a wound management perspective, there is little consensus regarding whether sampling is relevant, when and how a wound should be sampled, how a specimen should be transported to the laboratory, and what analyses should be requested. Confusion also exists in view of the fact that health care practitioners often consider a microbiological report to provide definitive information on whether a wound is infected (76, 173), and the provision of an antibiogram for a particular pathogen can often be misleading and prompt unnecessary treatment.

The aim of the following sections is to clarify current controversies in wound sampling and discuss the role of the health care practitioner and the microbiology laboratory in achieving clinically relevant outcomes.

Conclusions

Dermal wounds involve exposed tissue, which, under normal circumstances would be sterile, i.e., free from microbial contamination. However, like normal intact skin, a newly formed wound will naturally become colonized by microorganisms and compromised tissue will encourage their proliferation. In addition to the warm, moist, and nutritious conditions, ischemic wounds (usually chronic) are often characterized by tissue hypoxia, necrosis, and an impaired immune response. Consequently, commensal aerobic and anaerobic microfloras of the human body (with primary sources being the skin, the oral cavity, the gut, and the genitourinary tract) are presented with an opportunity to become established in an abnormal but favorable environment, where their survival strategies may render them pathogenic rather than commensal. Since microorganisms from a variety of sources are presented with an opportunity to colonize a common but unnatural habitat, microbial interactions unique to this particular environment may significantly influence wound pathogenesis and healing.