The Microbiology of Wounds

The Microbiology of Wounds
Presentation by:Neal R. Chamberlain, Ph.D.,
Department of Microbiology/Immunology
KCOM

Microbes and Chronic Wounds

* All chronic wounds are contaminated by bacteria.
* Wound healing occurs in the presence of bacteria.
* Certain bacteria appear to aid wound healing.
* It is not the presence of organisms but their interaction with the patient that determines their influence on wound healing.

Definitions

* Wound contamination: the presence of non-replicating organisms in the wound.
* All chronic wounds are contaminated.
* These contaminants come from the indigenous microflora and/or the environment.
* Most contaminating organisms are not able to multiply in a wound. (Ex. Most organisms in the soil won’t grow in a wound).
* Wound colonization: the presence of replicating microorganisms adherent to the wound in the absence of injury to the host.
* This is also very common.
* Most of these organisms are normal skin flora.
* Staphylococcus epidermidis, other coagulase negative Staph., Corynebacterium sp., Brevibacterium sp., Proprionibacterium acnes, Pityrosporum sp..
* Wound Infection: the presence of replicating microorganisms within a wound that cause host injury.
* Primarily pathogens are of concern here.
* Examples include; Staphylococcus aureus, Beta-hemolytic Streptococcus (S. pyogenes, S. agalactiae), E. coli, Proteus, Klebsiella, anaerobes, Pseudomonas, Acinetobacter, Stenotrophomonas (Xanthomonas).

Microbiology of Wounds

* The microbial flora in wounds appear to change over time.
* Early acute wound; Normal skin flora predominate.
* S. aureus, and Beta-hemolytic Streptococcus soon follow. (Group B Streptococcus and S. aureus are common organisms found in diabetic foot ulcers)
* After about 4 weeks
o Facultative anaerobic gram negative rods will colonize the wound.
o Most common ones= Proteus, E. coli, and Klebsiella.
* As the wound deteriorates deeper structures are affected. Anaerobes become more common. Oftentimes infections are polymicrobial (4-5).
* Long-term chronic wounds oftentimes contain more anaerobes than aerobes.
* Aerobic gram-negative rods also infect wounds late in the course of chronic wound degeneration. Usually acquired from exogenous sources; bath and foot water
* Ex. Pseudomonas, Acinetobacter, Stenotrophomonas (Xanthomonas).
* Organisms like Pseudomonas are not very invasive unless the patient is highly compromised (ex. Ecthyma gangrenosum in neutropenic patients).
* These organisms are associated with marked wound deterioration due to endotoxin, enzymes, and exotoxins.

* As the wounds go deeper and become more complex they can infect the underlying muscles and bone causing osteomyelitis.
* Coliforms and anaerobes are associated with osteomyelitis in these patients. You also see Staphylococcus aureus.
* Enterococcus and Candida are often isolated from wounds.
* Treating a patient for these organisms is only indicated if there are no other pathogens present and the organisms are present in high concentrations (106 CFU’s per gram of tissue)

From Colonization to Infection?
Dose of Bacteria
Bacterial Problems to Consider
Virulence
Host Resistance
Wound Depth can Result in Different Diseases
How do you know when a wound is infected?

* This can be very difficult.
* A continuum exists between when pathogens colonize the wound and then start to cause damage.
* There is no absolutely foolproof laboratory test that will aid in this diagnosis.
* One feature is common to all infected chronic wounds;
* The failure of the wound to heal and progressive deterioration of the wound.
* Unfortunately, wound infections are not the only reasons for poor wound healing.
* The typical features of wound infections:
* increased exudate
* increased swelling
* increased erythema
* increased pain
* increased local temperature
* Periwound cellulitis, ascending infection, change in appearance of granulation tissue (discoloration, prone to bleed, highly friable).

Specimen Collection and Culture Techniques.

* There is a good deal of controversy concerning specimen collection.
* The gold standard collection method is to do a tissue biopsy or needle aspirate of the leading edge of the wound after debridement.
* >105 CFU/gm of tissue= greater likelihood of sepsis developing.
* Indicate the specific anatomic site the biopsy is collected from.
* Indicate whether this is a surface or deep wound. Ask for a smear and gram stain of the tissue.
* Surface wounds are NOT cultured for anaerobes.
* Deep wounds are cultured for anaerobes.
* If a tissue biopsy is not possible;
* cleanse the wound with sterile saline
* vigorously swab the base of the lesion
* Surface wounds place the swab in a sterile container for transport.
* Deep wounds place the swab in a sterile anaerobic container for transport.

The Microbiology of Wounds.ppt