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Practice questions

If a wound is infected with anaerobic bacteria, are there specific dressings that should not be used?

  • 2 Comments

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.

Anaerobes

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).

Classifying infection

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.

Dressing selection

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.

Conclusion

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

CLASSIFICATION

 

DESCRIPTION

 

Obligate aerobes

 

Unable to survive without oxygen

 

Facultative anaerobes

 

Aerobic but can be anaerobic; prefer aerobic conditions

 

Microaerophilic bacteria

 

Can tolerate low amounts of oxygen

 

Obligate anaerobes

 

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
  • Undermining
  • Malodour
  • Bowel or “dirty” surgery
  • Ischaemia underlying the wound
  • Gangrene or devitalised tissue
  • 2 Comments

Readers' comments (2)

  • Hassan Sharifi

    this was a good debate about applicability of new brand dressing in clinical area. in exudative wounds, it prefer to use a dry and close dress that change every day. this cause less maceration in wound area and like a suction, suck the pus around wound.

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  • Ian Mansell

    Quantifying bacteria ie >10 power 6 tends to be the bench mark to determine infection. This needs to be considered with the host reaction ie do they display signs & symptoms of infection ie Redness (erythema) swelling,heat, induration, pain, purulent discharge, wound dehiscence or wound deterioration, abnormal granualtion tissue, systemic response ie tachycardia, pyrexia, poor appetite lethargy. The latter sypmtoms are probably more diagnostic as its impossible to quantify bacteria by observation. But the host reaction will differ depending upon the host other co-morbidities & general health & medication (immunosuppresives & steroids). This would apply to aerobes as well as anaerobes. With the latter malodour could be a major issue and there are raft of treatments for that symptom eg Charcoal dressings we use Carboflex (particulary good for highly exudative wounds) or Clinisorb. Remember the efficacy of charcoal is greatly diminished once it becomes wet, consequently Carboflex with its combination of hydrofibre, alginate layer will increase the longevity of the dressing in use. As mentioned there is a vast array of anti-microbial dressings on the market, perhaps too confusing at times. Generically these include silvers, iodines, mannuka honey,Polyhexanide Biguanide (PHMB) variations. DACC technology eg Cutimed Sorbact.The clinician needs to appreciate the difference also between colonisation, & the concept of critical colonisation. this suggests the level of bacteria is high enough to elicit a local response ie wound deterioration but is not manifested systemically, this maybe a situation when local wound management needs to consist of some anti-microbial treatment, but again this wound be host specific. Colonisation & contamination is a fact of life with all chronic wounds, these can progress to healing despite this. When useing anti-microbial dressings set a specific end point for evaluation & discontinue if no improvement as with all chronic wounds intransigent healing is usually due to an underlying systemic cause ie Treat the Whole of the patient not the hole in the patient

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