Chapter 9: Cultures

_________swab all partial thickness or undetermined depth of burn on admission or when there is a change in wound drainage (color and consistency)

_________punch biopsy for quantitative bacteriology on all full thickness burns when there is a change in wound appearance

I.                   The Burn Wound

A.    Serves as the site of primary invasion for local and systemic infection.[i]^,[ii]  Burn wounds become infected within 3 to 5 days after admission.  Generally the infection arises from bacteria in the hair follicles and sebaceous glands.  Some exogenous organisms may become pathogenic (i.e. exposure to pond or runoff water after a burn or transmission of organisms from attendant personnel).   Perivascular bacterial growth is followed by vessel thrombosis and dermal necrosis, which will convert a partial-thickness burn to a full-thickness burn.  The likelihood of infection and invasion increases with burn depth. 

B.    Once bacterial organisms encounter viable tissue, they will either locally invade the wound to produce a septic burn wound or spread via blood or lymph to produce burn wound sepsis.  Burned and unburned skin should be inspected daily for black, brown, or grey spots; and for hemorrhagic, vesicular or necrotic areas.

C.   Signs of a septic burn wound[iii]:

1.  black, brown, or purple discoloration

2.  edema at skin margins

3.  sloughed burn tissue or eschar

4.  subcutaneous hemorrhagic discoloration

5.      conversion of partial-thickness wound to full-thickness

      wound

6.  abscess formation

                  D.  Signs of burn wound sepsis:

1.  Gram-negative sepsis

a.  burn wound biopsy >10⁵  organisms/g tissue and/or histologic evidence of tissue invasion of subadjacent unburned tissue

b.  rapid illness in 8-12 hrs

c.  temperature >38°C

d.  increased WBC

e.  hypothermia (<35°C) and decreased WBC

f.   decreased blood pressure and urine output

g.  focal wound gangrene

h.  satellite lesions away from burn wound

i.   mental status changes

2.  Gram-positive sepsis

a.  burn wound biopsy >10⁵  organisms/g tissue and/or histologic evidence of tissue invasion (except b-hemolytic strep. where only a few organisms cause wound infection, failure of primary closure, and loss of skin grafts[iv])

b.  gradual onset of symptoms

c.  temperature of 40°C or higher

d.  increased WBC

e.  decreased hematocrit

f.   decreased blood pressure and urine output

E.  Patients with signs and symptoms of burn wound sepsis should have immediate institution of antibiotics in conjunction with Infectious Disease service recommendations.

II.  Swab all partial thickness or undetermined depth of burn on admission or when there is a change in wound drainage (color and consistency)

               Swab cultures will identify the type of microorganism present.  Wounds and/or dressings over burn wounds, skin grafts, and donor sites should be inspected and swab cultures taken for any changes in the color, odor, and amount of exudates from the wound.  Fungal infection is marked by rapidly emerging new spreading dark discolorations.                             

III.  Punch biopsy on all full thickness burns when there is a change in wound appearance   

A.   Quantitative culture is obtained by a full thickness punch biopsy, which are obtained from every 18% TBSA burned.  Studies have shown that when colony counts on quantitative culture are less than 10² organisms/gram, graft survival is greater than 90%, but when counts were greater than 10⁵, only 60% of grafts survive.[v]  Quantitative cultures showing high bacterial counts correlate with histological evidence of burn wound infection in approximately 80% of cases.[vi]^{,[vii],[viii]}   The one important exception to these studies is with the Streptococcus species, in which the mere presence of a few b-hemolytic streptococci can cause wound infection or graft loss.⁴

   The full-thickness biopsy should be repeated twice weekly on each

   18% TBSA burned area until the full-thickness burn has been

   excised.  Repeat cultures will provide information as to the emergence of new bacterial infections as well as monitor the effectiveness of topical antimicrobial therapy. 

B.    If wound cultures reveal greater than10³ organisms/g, topical therapy should be changed.  If silver sulfadiazene is used on a burn wound, it should be replaced with mafenide acetate, which has better eschar penetration.  If a donor site or skin graft is infected, mafenide acetate soaks should be used after daily dressing changes.

IV.  Catheter Infections

A.     In a study of hospitalized hematology-oncology patients, culture of blood drawn through either the central catheter or peripheral vein shows excellent negative predictive value. Culture of blood drawn through an indwelling central venous catheter has low positive predictive value, apparently less than from a peripheral venipuncture. Therefore, a positive result from a catheter needs clinical interpretation and may require confirmation.[ix] 

B.     Diagnosis

1.                      Catheter infection is considered when:

a.    There is no other obvious source for sepsis and the catheter has been in place longer than for 3 days for an internal jugular or femoral venous site, or longer than 7 days for a subclavian site,

b.    There is erythema around the catheter insertion site

c.    Pus can be expressed  from the catheter insertion site.

d.   The catheter should be removed and sent for both gram stain and for routine culture and antibiotic sensitivities. 

2.                      The gram stain provides immediate information concerning organism morphology as a guide to antibiotic coverage. 

3.                      The interpretation of catheter tip cultures by the “semiquantitative” culture technique (where the catheter is rolled over the surface of a sheep blood agar plate and the number of colonies are counted[x])  in conjunction with blood cultures is listed in Table I.

Table 1

Table I:  Interpretation of Semiquantitative Catheter Cultures[xi]

4.                      As Table I indicates, negative blood cultures should not be disregarded when the catheter tip shows large numbers of organisms present.  The infected catheter in this situation may still seed the bloodstream, but the septicemia is easily missed because it may be intermittent.  Less than 50% of catheter tips with dense growth are associated with positive blood cultures.¹³ 

5.                      Once catheter sepsis has been diagnosed, the catheter should be removed and antibiotics started to prevent life-threatening endocarditis.[xii] A study comparing burn patients with major burn injuries randomly assigned to undergo site change every 48 hours versus guidewire exchange every 48 hours at the same site showed no advantage to changing the insertion site. Guidewire change did not prevent nor predict catheter bacterial contamination.[xiii]

6.                      The femoral vein site has been successfully used in burn patients. A prospective study was undertaken to determine the safety of femoral vein catheterization in patients with burns.   None of the differences in catheter colonization and catheter-related sepsis between femoral and non-femoral vein catheter sites were statistically significant, and there were no noninfectious complications from femoral vein catheterization.[xiv]  Femoral venous catheters are also considered safe in burned children and are associated with a low incidence of infectious and mechanical complications.[xv]