The Microbiology of Wounds

Neal R. Chamberlain, Ph.D.,Department of Microbiology/ImmunologyKCOM

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).

DefinitionsWound 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..

Definitions

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)

Microbiology of Wounds

After about 4 weeks Facultative anaerobic gram negative rods

will colonize the wound. 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).

Microbiology of Wounds

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).

Microbiology of Wounds

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.

Microbiology of Wounds

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.

Microbiology of Wounds

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)

Microbiology of Wounds

In summary: early chronic wounds contain mostly gram-positive organisms.

Wounds of several months duration with deep structure involvement will have on average 4-5 microbial pathogens, including anaerobes (see more gram-negative organisms).

From Colonization to Infection?

Many factors affect the progress of microorganisms in a wound from colonization to infection:

Infection= dose X virulence __________host resistance

The number of organisms.The virulence factors they produce.The resistance of the host to infection.

Dose of BacteriaDiffers depending on the organism

involved.Some organisms would need to be in high

concentrations. (ex. Candida, Enterococcus)

Various combinations of bacterial species result in more host damage (synergy)

Example; Group B Streptococcus (S. agalatiae) and Staphylococcus aureus.

Dose of BacteriaOrganisms that should be treated

regardless of the numbers present. Beta-hemolytic streptococci,

Mycobacteria sp., Bacillus anthracis, Yersinia pestis, Corynebacterium diphtheriae, Erysipelothrix rhusiopathiae, Leptospira sp., Treponema sp., Brucella sp., Clostridium sp., VZV, HSV, dimorphic fungi, Leishmaniasis.

Bacterial Problems to Consider

Streptococcus pyogenesCan result in necrotizing fasciitis or

streptococcal toxic shock syndrome. Not very common. Only about 520 cases per year of each condition.

More common to see cellulitis and erysipelas after infection of a chronic wound.

Bacterial Problems to Consider

Clostridium tetaniContamination of chronic wounds by

exogenous sources is common.Of the 41 cases of tetanus that occurred

in 1998, a total of 16 (39%) were among persons aged greater than or equal to 60 years.

Make sure your patients have gotten their tetanus vaccination.

Bacterial Problems to Consider

Erysipelothrix rhusiopathiae can infect chronic wounds. Associated with hog farmers and people who fish.

Mycobacteria marinum and M. ulcerans can infect chronic wounds. Think of people who have aquariums, pools, go fishing, etc..

Virulence

Factors an organism produces can result in host damage.

Ex. Hyaluronidase (Streptococcus pyogenes), proteases (Staphylococcus aureus, Pseudomonas aeruginosa), toxins (Streptococcus pyogenes, Staphylococcus aureus), endotoxin (gram negative organisms).

Virulence

Some organisms produce few virulence factors.

However, synergy between different bacterial factors can cause host damage.

Group B Streptococcus and Staphylococcus aureus: Synergy between two toxins results in hemolysis.

Host Resistance

Host resistance is the single most important determinant in wound infection.

Local and Systemic factors both play a role in increasing the chances a wound will become infected.

Host ResistanceLocal factors that increase chances of

wound infection: Large wound area Increased wound depth Degree of chronicity Anatomic location (distal extremity, perineal) Foreign body Necrotic tissue Mechanism of injury (bites, perforated viscus) Reduced perfusion

Wound Depth can Result in Different Diseases

Host ResistanceSystemic factors that increase chances of

wound infection: Vascular disease Edema Malnutrition Diabetes Alcoholism Prior surgery or radiation Corticosteroids Inherited neutrophil defects

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.

How do you know when a wound is infected?

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.

How do you know when a wound is infected?

The typical features of wound infections: increased exudate increased swelling increased erythema increased pain increased local temperaturePeriwound 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.

Specimen Collection and Culture Techniques.

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.

Specimen Collection and Culture Techniques.

If a tissue biopsy is not possible;cleanse the wound with sterile saline vigorously swab the base of the lesionSurface wounds place the swab in a

sterile container for transport.Deep wounds place the swab in a sterile

anaerobic container for transport.

Thank You

I would like to thank KCOM Department of Continuing Medical Education

The following article is a helpful review of this topic: Dow, G., Browne, A., and Sibbald, R.G. Infection in Chronic Wounds: Controversies in Diagnosis and Treatment. Ostomy/Wound Management. 1999;45(8):23-40.