The number of microorganisms with which an object is contaminated is referred to as the bioburden. This can be linked to an inanimate object (fomite) or an animate object (host). The bioburden of a wound is of interest to the wound care specialist because it affects the healing rate of the wound.
Wounds can be inoculated by any of the more than 200 species of bacteria that constitute the normal flora of human skin or by bacteria from the environment or within the human body.1 Patients who have impaired chemotaxis or are immunocompromised are at risk for bioburden in their wounds. In general, wounds provide a warm, moist, and nutritive environment for bacterial proliferation, and inoculation of the wound can lead to colonization and infection. Colonization is defined as the presence of proliferating bacteria without a noticeable host response, whereas infection is the invasion of bacteria into the surrounding tissue that elicits host immune response.1,2 Colonization of the wound may enhance or impede wound healing, depending on the bacterial load. In a study of diabetic foot ulcers, no detectable bacterial growth was associated with a healing rate of 0.2 cm/wk, 105-106 colony forming units/gram with a healing rate of 0.15 cm/wk, and >106 colony forming units/gram with 0.05 cm/wk.2
A further increase in the bacterial density and invasion in a given wound can elicit a host immune response, resulting in infection. Transition from colonization to infection is determined not only by the bioburden itself but also by the virulence of the organisms, the synergistic action of different bacterial species, and the ability of the host to mount an immune response.3 Advancing age, obesity, malnutrition, chronic steroid use, and diabetes are examples of conditions that increase the risk of progression to infection by altering the efficacy of immune system action. In addition, local factors such as poor perfusion, necrosis, foreign bodies, dead space, undermining, and tunneling prevent the body from launching effective immune response.4 The mechanism of injury is also important in the progression to infection. A "torn" wound, crush injury, or high-velocity missile (war wound) wound is 100 times more likely to become infected than a clean laceration closed by primary intention-a wound that does not require debridement.5 Duration of the wound is another factor associated with the risk of development of infection. It is reported that only 3.6% of surgical wounds exposed less than 30 minutes are infected, whereas 16.4% of the wounds exposed longer than 5 hours develop infection.6 In the infected wound, inflammatory cells are recruited and release proteases such as matrix metalloproteinases. These enzymes are responsible for the digestion of the extracellular matrix.7
Clinical signs of high bioburden may be obscure at best without a concomitant infection. Pink hypergranular tissue that bleeds easily may indicate a high bioburden, but a delay in wound healing often may be the only clinical finding.8 Once infection develops, the wound can present with erythema, edema, necrosis, decreased granulation tissue, and cellulitis of surrounding tissue.1 Gram's stain is a quick test that offers a rough assessment of the bacteria present in the wound and the bacterial burden.
The management of wounds should be based on a consideration of the bioburden. A colonized wound without a sign of infection that appears to follow the usual stages of wound healing may indicate a low bioburden and supportive therapy, such as removal of any mechanical stress to improve circulation, may suffice. Wounds with high bioburden may require topical antibiotics and antiseptics. Metronidazole is reported to hinder the synergistic action of bacteria, and thus, aid the healing of chronic wounds.6 However, the wound care practitioner should use metronidazole with caution in patients with peripheral neuropathy and a history of seizures. Wounds that present with obvious signs of infection warrant the use of empirical antibiotic treatment until culture results are available.
Although the assessment of bioburden during wound management is often not straightforward, the possibility of a high bioburden should always be considered by the wound care specialist or team to prevent the possible detrimental outcome that results from a large bacterial load.
1. Ovington L. Bacterial toxins and wound healing. Ostomy Wound Manage 2003;49(7A Suppl):8-12.
2. Browne AC, Vearncombe M, Sibbald RG. High bacterial load in asymptomatic diabetic patients with neurotrophic ulcers retards wound healing after application of Dermagraft. Ostomy Wound Manage 2001;47(10):44-9.
3. Wysocki AB. Evaluating and managing open skin wounds: colonization versus infection. AACN Clin Issues 2002;13:382-97.
4. Bowler PG, Duerden BI, Armstrong DG. Wound microbiology and associated approaches to wound management. Clin Microbiol Rev 2001;14:244-69.
5. Eron LJ. Targeting lurking pathogens in acute traumatic and chronic wounds. J Emerg Med 1999;17:189-95.
6. Robson MC. Wound infection. A failure of wound healing caused by an imbalance of bacteria. Surg Clin North Am 1997;77:637-50.
7. Casey G. Wound repair: advanced dressing materials. Nurs Stand 2002;17(4):49-53.
8. Edwards R, Harding KG. Bacteria and wound healing. Curr Opin Infect Dis 2004;17(2):91-6.