Nursing practice often involves undertaking procedures about which there is debate or uncertainty. In Practice Question we ask experts to determine how nurses should approach these situations
Q. If a wound is infected with anaerobic bacteria, are there specific dressings that should not be used?
A. Despite concerns that occlusive dressings may encourage anaerobic growth by providing an oxygen depleted environment, there is no evidence to support this.
Obligate anaerobic and microaerophilic bacteria live in oxygen free environments (Table 1). This has led to debate over the safety of the use of occlusive dressings, particularly hydrocolloids, for infected wounds (Finnie, 2002; Jones and Gill, 1998).
Wounds invariably have bacteria present, yet most are not infected. A wound is infected when “microbes multiply to such an extent that host tissue is invaded and wound healing is delayed” (Cooper, 2005).
To identify the causative pathogen, a microbiological sample needs to be taken, following standard operating procedures (Health Protection Agency, 2006). The primary objective for sampling is to determine sensitivity of the pathogen to antibiotics. When anaerobes are suspected or known to be a causative pathogen in wound infection, metronidazole is commonly the antibiotic of choice.
A pus/exudate sample is preferable to a wound swab because it usually provides more reliable results (HPA, 2006). The wound swab should use the Amies transport medium with charcoal (HPA, 2006). A rapid delivery to the laboratory is preferable (1-2 hours) but, if this is not possible, storage at room temperature is adequate to maintain growth of both aerobic and anaerobic bacteria (Bowler et al, 2001). In the culture of anaerobes, an incubation period of 2-7 days will be needed, depending upon the microbe. When requesting the investigation it is important that information pertaining to the sample is provided (Table 2). If anaerobes are suspected, this should be mentioned, so state “to include anaerobes”.
Anaerobes may make up 22-62% of the microbial population in a wound (Bowler et al, 2001). It is not unreasonable, therefore, to suspect anaerobic presence when signs of wound infection exist.
Occlusive dressings have provoked debate about their safe use because they create a hypoxic environment that encourages anaerobes to proliferate (Marshall et al, 1990). There has been no recent study of this, perhaps because the presence of anaerobic bacteria under these dressings neither raises the likelihood of infection developing nor delays healing (Mousa, 1997; Gilchrist and Reed, 1989). It may be that the slightly acidic environment beneath these dressings is hostile to bacteria, but this varies between products (Thomas and Loveless, 1997).
Dressing selection is based on many factors, including practicalities as well as wound type. A hydrocolloid does not have the absorptive capacity for high levels of exudate so more frequent dressing changes are required, without which complications such as peri-wound maceration could occur. This would preclude the use of this dressing type rather than the presence of anaerobes. The same could be said of hydrogel dressings, which have a high water content.
Selecting a dressing requires multiple considerations, too many for discussion here. Where a wound shows signs of clinical infection, anaerobic or not, selection of the dressing will likely be based on symptom management, wound site and patient lifestyle. Hydrocolloids and hydrogels should be used with caution, not because they may enhance infection but because they may be unsuitable for the wound conditions. l
Heidi Guy, BSc, RN, is lecturer practitioner tissue viability, East & North Hertfordshire NHS Trust/University of Hertfordshire
|Table 1. bacterial classification|
Unable to survive without oxygen
Aerobic but can be anaerobic; prefer aerobic conditions
Can tolerate low amounts of oxygen
Unable to survive in oxygen, but may survive for a few days in air (Bowler et al, 2001)
|Source: Gladwin and Trattler (2008)|
Table 2. Sample information
- Site of wound
- Medical conditions, for example type 2 diabetes
- Deep cavity
- Bowel or “dirty” surgery
- Ischaemia underlying the wound
- Gangrene or devitalised tissue
Bowler PG et al (2001) Wound microbiology and associated approaches to wound management. Clinical Microbiology Reviews; 14: 2, 244-269.
Cooper R (2005) Understanding wound infection. In: Identifying Criteria for Infection. European Wound Management Association Position Document. London: MEP.
Finnie A (2002) Hydrocolloids in wound management: pros and cons.British Journal of Community Nursing; 7: 7, 338-345.
Gilchrist B, Reed C (1989) The bacteriology of chronic venous ulcers treated with occlusive hydrocolloid dressings.British Journal of Dermatology; 121: 3, 337-344.
Gladwin M, Trattler B (2008) Clinical Microbiology Made Ridiculously Simple. Miami: Medmaster.
Health Protection Agency (2006) Investigation of Skin, Superficial and Non-surgical Wound Swabs. BSOP11 issue 4.1.
Jones V, Gill D (1998) Hydrocolloid dressings and diabetic foot lesions.The Diabetic Foot; 1: 4, 127-134.
Marshall DA et al (1990) Occlusive dressings: does dressing type influence the growth of common bacterial pathogens?Archives of Surgery;125: 9, 1136-1139.
Mousa HA (1997) Aerobic, anaerobic and fungal burn wound infections. Journal of Hospital Infection; 37: 317-323.
Thomas S, Loveless P (1997) A Comparative Study of the Properties of Twelve Hydrocolloid Dressings.