The Microbiology of Wounds

1
The Microbiology of Wounds

• Neal R. Chamberlain, Ph.D.,
• Department of Microbiology/Immunology
• KCOM

2
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.

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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 wont grow in a wound).

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Definitions

• 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..

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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).

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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)

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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).

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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).

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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.

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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.

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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 CFUs per gram of tissue)

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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).

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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.

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Dose of Bacteria

• Differs 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.

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Dose of Bacteria

• Organisms 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.

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Bacterial Problems to Consider

• Streptococcus pyogenes
• Can 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.

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Bacterial Problems to Consider

• Clostridium tetani
• Contamination 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.

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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..

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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).

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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.

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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.

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Host Resistance

• Local 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

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Wound Depth can Result in Different Diseases
24
Host Resistance

• Systemic factors that increase chances of wound
  infection
• Vascular disease
• Edema
• Malnutrition
• Diabetes
• Alcoholism
• Prior surgery or radiation
• Corticosteroids
• Inherited neutrophil defects

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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.

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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.

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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 temperature
• Periwound cellulitis, ascending infection, change
  in appearance of granulation tissue
  (discoloration, prone to bleed, highly friable).

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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.
• gt105 CFU/gm of tissue greater likelihood of
  sepsis developing.

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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.

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Specimen Collection and Culture Techniques.

• 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.

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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.
  199945(8)23-40.