Progression toward Healing: Wound Infection and the Role of an Advanced Silver-containing Hydrofiber(R) Dressing

Author(s): 
Philip Bowler, MPhil

The Microbiology of Wounds
Wound microbiology and healing are unquestionably associated. Wound tissue provides a favorable environment for micro-organisms to grow and multiply, almost inevitably resulting in colonization. Understanding and managing bioburden with appropriate treatments is vital to effective wound healing.
In most cases, the micro-organisms that colonize wounds originate from multiple host sources in close proximity to the wound site, such as the mouth, the gut, and the skin. As a result, wounds usually contain unique polymicrobial ecosystems.1 A historically accepted guideline is that a bacterial load greater than 105 colony forming units (cfu)/g of viable wound tissue is an indicator of infection. However, the polymicrobial interactions that take place in this ecosystem are more significant to the development of infection than the mere presence of a specific number or specific types of organisms2 - even well-known pathogenic bacteria such as Staphylococcus aureus or Pseudomonas aeruginosa.3
It is also important to note that both aerobic and anaerobic bacteria may be present within the wound environment. Anaerobes account for 38% of the microbial population in noninfected wounds, increasing to 48% in clinically infected wounds.4
To survive in a wound, micro-organisms must adapt quickly to overcome host immune responses. To this end, they may produce a variety of cell-protecting components, adhesion molecules, enzymes, and toxins. Furthermore, in polymicrobial environments, bacteria can work together to increase their net pathogenic effect, involving processes such as bacterial synergy and quorum sensing (see Figure 1).
Synergy may involve aerobic bacteria consuming local tissue oxygen; thus, lowering the oxygen tension and favoring the growth of anaerobes. Similarly, specific nutrients produced by one bacterium may encourage potentially pathogenic cohabitors - for example, production of vitamin K by S. aureus stimulates the growth and pathogenicity of Prevotella melaninogenic.5
Quorum sensing is a process during which populations of organisms utilize specific intercellular communication molecules to help them adapt to changes in environmental conditions.6 In a wound environment, this may involve bacteria producing exopolysaccharide capsules and subsequently biofilms that provide increased protection against the host immune system together with increased production of protease enzymes that facilitates dissemination through tissue.

Progression to Infection
In a normal situation, the host is in balance with the natural microflora of the skin and mucosal surfaces. A wound represents a disruption in this balance, as the local bacteria opportunistically colonize this new environment and evade the host's immune response. In addition to these exogenous factors, endogenous factors also can affect the likelihood of wound progression. For example, the persistent and elevated inflammatory activity associated with chronic wounds can be associated with underlying pathophysiological conditions such as venous hypertension in the development of venous leg ulcers.7
When the wound bioburden exceeds a host-manageable level, a wound may become infected. This will have a negative impact on patients (eg, increased pain, inconvenience, and illness) and on healthcare providers (eg, increased treatment costs).8 The microbial progression preceding infection is shown as a continuum (see Figure 2).

References: 

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