The right balance of oxygen is also important — too much or too little and the wound won't heal correctly. Another type of blood cell, a white blood cell called a macrophage, takes on the role of wound protector.
This cell fights infection and oversees the repair process. You might see some clear fluid on or around the cut at this time. That is helping clean out the wound. Macrophages also produce chemical messengers, called growth factors, which help repair the wound. Growth and rebuilding. Blood cells, including oxygen-rich red blood cells, arrive to help build new tissue. Chemical signals tell the cells to create collagen. It serves as a type of scaffolding. Occasionally, you see the result of this process as a scar that starts out red and eventually dulls.
Over time, the new tissue gets stronger. You might notice stretching, itching, and even puckering of the wound as that happens. The wound gains strength quickly over the first 6 weeks of healing. Depending on the size and the severity of the wound, the entire healing process might take up to a couple of years to complete.
The process seems simple enough, but wound healing is really quite complicated and involves a long series of chemical signals. Certain factors can slow or prevent healing entirely. One of the most dramatic factors is reduced or poor blood supply to the wound. Microbiologists working in wound care have argued that tissue biopsies are not required to determine the bacterial load in acute wounds because a correlation exists between quantitative tissue biopsy and semi-quantitative superficial swab analyses.
Chronic wounds and the 10 5 guideline. For the day-to-day management of chronic wounds eg, leg ulcers and pressure ulcers where many factors influence healing, the validity of the 10 5 guideline as a primary indicator of infection is questionable, despite reports indicating that such wounds are unlikely to heal when the bacterial load exceeds the , threshold. However, an infecting dose is known to vary with the type of organism involved and the way in which it is presented.
Wound sampling and quantitative analysis.
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With awareness raised regarding the complex aerobic-anaerobic microbiology of wounds and the ability of cohabiting organisms to work in synergy, clinicians should consider what type of wound sample will generate the most meaningful data - a superficial swab or a tissue biopsy. Many argue that a deep tissue biopsy will identify a causative organism; whereas, superficial samples will only collect debris and, therefore, provide meaningless data. However, in virtually all cases, wound contamination occurs from outside of the wound; consequently, organisms that penetrate into deeper tissue will have disseminated from superficial tissue.
Thorough microbiological investigation of a superficial swab sample will isolate all organisms that may be involved in delayed healing, but it will not determine the specific deeper-penetrating organism s. With adequate clinical information regarding the wound eg, type, location, signs of infection, and current antimicrobial therapy , the microbiologist can conduct appropriate quantitative and qualitative investigations, provide an expert opinion on the causative organism s , and target them with antimicrobial therapy.
Superficial swab samples provide a general picture of the wound microbiology; tissue biopsy samples are highly localized and may not reflect the microflora in other parts of a wound see Figure 2. This is a topic of continued debate. Additionally, isolating only the deeper penetrating organisms and using the quantitative data to guide therapy may overlook superficial organisms that potentially influence the healing status of a wound.
In a study of infected diabetic foot ulcers, Sapico et al 25 reported that less invasive micro-organisms may be synergistic with more virulent ones and play a crucial role in wound infection. Although microbial synergy is not widely recognized within the field of wound care, a clear indication of its contribution in an infected leg ulcer has been reported. In this case, S. The potential significance of superficial bacteria in wound healing recently was recognized in a study evaluating an antimicrobial dressing in the management of 29 nonhealing chronic wounds.
Evidence suggests that micro-organisms residing in superficial wound tissue can contribute to delayed wound healing 28 ; this may not be a direct effect, but is likely to occur via polymicrobial interactions. Microbiology in deep and superficial tissue not withstanding, surface sampling is easier and less time-consuming to perform than tissue biopsy, and the cost is approximately one-sixth that of tissue biopsy. Microbiological analysis of samples. Another extremely important but less known aspect of wound microbiology is the microbiological analysis. If infection is to be diagnosed on the basis of a tissue count, the quality of the sample is critical.
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In the laboratory, standard practice is to aseptically weigh the tissue sample, heat with flame to remove surface contamination, and homogenize the tissue using a known volume of fluid culture medium. The resulting suspension is diluted serially fold and a known volume of each dilution is plated out onto a series of agar plates to culture a variety of micro-organisms. Following appropriate incubation, organisms are counted, the dilution factors are considered, and a total count per gram of tissue is calculated. This procedure is significantly more time-consuming and expensive than semi-quantitative analysis of a superficial sample.
Anaerobes are expensive and time-consuming to culture, but considering the prevalence of anaerobes in chronic wounds, they should not be but often are excluded in quantitative tissue biopsy counts. With this in mind, how valid is a tissue count that the practitioner receives from the laboratory? Is it appropriate to confirm that a wound is not infected based on a result that does not provide the complete microbiological picture?
The relevance of a specific number of micro-organisms in a wound also needs to be challenged because microbial synergy can reduce the infectious dose. Clearly, infection is significantly influenced by the types of bacteria and their interactions and does not rely on numbers alone. For this reason, the view that the mere presence of organisms in a wound is less important than the level of bacterial growth 11 must be put into perspective.
Kerstein 31 specifically stated that quantitative microbiology alone cannot predict the risk of wound sepsis and that no single, perfect predictor of wound infection exists. Qualitative microbiology. The presence of a specific pathogen does not necessarily mean that a wound is infected. If a wound is malodorous, the presence of anaerobes is highly probable; this also applies to samples showing no growth following aerobic incubation.
Although anaerobes are often slower to culture in the laboratory, a series of simple tests can usually determine the key groups clostridia, anaerobic streptococci, and pigmenting and non-pigmenting Gram-negative anaerobes within 48 to 72 hours. The literature indicates that quantitative microbiology ie, the 10 5 guideline is increasingly emphasized and accepted when diagnosing infection.
However, when the potential variability in quality and analysis of tissue samples is taken into account, the potential importance of superficial micro-organisms and polymicrobial synergy, the microbiological methods used, the types of micro-organisms investigated, and the validity and value of the quantitative biopsy in diagnosing chronic wound infection are questionable. Similarly, qualitative microbiology cannot be used to diagnose infection or to identify a causative organism in polymicrobial wounds because more than one organism may be involved.
Wound infection must be based primarily on clinical signs eg, increasing pain, friable granulation tissue, foul odor, and wound breakdown 34 and subsequent qualitative microbiology can confirm that empirical antibiotic treatment had been selected appropriately. In summary, wound care practitioners are encouraged to diagnose infection and base their treatment decisions on a highly specific quantitative microbiological assessment that has debatable clinical validity.
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For the day-to-day management of chronic wounds, the benefits of tissue biopsy are unlikely to outweigh the practical and cost-associated implications. A collective evaluation and appreciation of the numbers, interactions, and virulence potential of micro-organisms in complex wounds is critical to effective wound management. The host. In , Louis Pasteur stated, "The germ is nothing. It is the terrain in which it is found that is everything.
Wound tissue is potentially a favorable terrain for micro-organisms due to the presence of moisture, nutrition, and warmth. In the event of tissue contamination, a host inflammatory response designed to inactivate and remove potentially dangerous micro-organisms is provoked. The polymorphonuclear neutrophil PMN is a host inflammatory cell that possesses highly potent antimicrobial mechanisms; consequently, the presence and activity of this cell type in a wound environment is essential to healing. The presence of PMNs at the wound site and also other factors that influence wound healing, such as oxygen and nutrients is dependent on an adequate blood supply to the local tissue.
Surgical wounds in the anus exemplify the importance of tissue perfusion; despite their susceptibility to gross bacterial contamination, these wounds rarely become infected. When deprived of oxygen, cells involved in wound healing die, leading to the accumulation of devitalized tissue that provides an ideal environment for the growth of a diversity of micro-organisms.
Additionally, ischemia and a dry environment interfere with delivery and activity of PMNs in wound tissue, compromising the clearance of wound microflora. Polymorphonuclear neutrophil function can be further impaired by pathophysiological conditions eg, disease and chemotherapy and in patients with diabetes, the probability of infection in foot ulcers is five times greater than in non-diabetic patients. The inevitable contamination of wound tissue by micro-organisms provokes a series of interrelated host and microbial responses that ultimately determine whether a wound will heal.
Microbial progression leading to wound infection is believed to follow a series of stages during which the micro-organisms gain increasing control over the host. This has been referred to as an infection continuum. Initially, contaminating micro-organisms are exposed to a new environment where they must adapt quickly to survive. Early acclimatization to the wound environment may involve bacteria producing cell-protecting components eg, biofilms to resist the host immune response, adhesion molecules that enable them to quickly attach to host cells, and enzymes and toxins that facilitate dissemination through tissue virulence factors.
At this point in the continuum, a normal host inflammatory response usually maintains control over the contaminants, although the dose and virulence characteristics of the micro-organisms involved cannot be overlooked. If the local conditions are compromised eg, presence of foreign bodies, necrotic tissue, dead space, tissue hypoxia, and reduced PMN activity micro-organisms have an opportunity to colonize tissue. If, at any stage in the continuum, progression towards wound healing is interrupted, a host-microbial balance in favor of the micro-organisms should be suspected.
At this stage, the wound may be critically colonized, meaning that the micro-organisms are interfering with wound healing without inducing obvious clinical signs of infection.
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