Is a wound culture showing 1+ E. coli with rare usual cutaneous flora after incision and drainage (I&D) of an infected epidermoid cyst a contaminant or a risk of real infection?

E. coli in Post-I&D Epidermoid Cyst Culture: Likely Contaminant

The finding of 1+ E. coli with rare usual cutaneous flora in a second wound culture from a post-I&D epidermoid cyst is most likely a contaminant rather than a true pathogen, and does not warrant antibiotic treatment in the absence of clinical signs of infection. 1, 2

Why This is Likely Contamination

Culture Interpretation in Epidermoid Cysts

• Gram stain and culture of pus from inflamed epidermoid cysts are NOT recommended by the Infectious Diseases Society of America, as inflammation in these cysts results from rupture of the cyst wall and extrusion of keratinous contents into the dermis, rather than primary bacterial infection. 1, 2

• The inflammatory process in epidermoid cysts is typically a foreign body reaction to cyst contents, not a true infectious process requiring antimicrobial therapy. 1, 2

Expected Flora vs. Pathogenic Organisms

• Normal epidermoid cyst flora includes Staphylococcus epidermidis, anaerobic gram-positive cocci, and Corynebacterium acnes—not E. coli. 3, 4

• When true bacterial infection occurs in epidermoid cysts, the predominant organisms are Staphylococcus aureus (most common), Peptostreptococcus species, and Bacteroides species—particularly in perirectal, vulvovaginal, and head locations where anaerobes predominate. 3

• E. coli is rarely a primary pathogen in epidermoid cyst infections, accounting for only 7 isolates out of 315 total bacterial isolates in one large study of infected epidermal cysts. 3

Significance of Culture Quantity and Context

• The "1+" growth with rare usual cutaneous flora" suggests minimal bacterial burden and mixed contamination from skin surface. 1

• Superficial wound swabs have a high risk of contamination with normal skin flora, even when obtained from the wound base, and can lead to inappropriate antibiotic therapy. 1

• The first wound culture was negative, making subsequent low-level growth more consistent with contamination than progressive infection. 1

Clinical Decision Algorithm

Assess for True Infection (Check ALL of the Following):

If ANY of these are present, consider true infection:

• Temperature ≥38.5°C (101.3°F) 1, 5
• Heart rate >110 beats/minute 5, 2
• WBC count >12,000 cells/µL 5
• Erythema extending >5 cm from the wound margins 1, 5
• Purulent drainage with increasing volume 1
• Wound dehiscence or spreading cellulitis 1
• Systemic signs of SIRS (tachypnea >24 breaths/minute, temperature <36°C or >38°C) 5

If NONE of these are present:

• This represents contamination, not infection
• No antibiotics are indicated 1, 5, 2
• Continue local wound care with dry dressing 5

If True Infection is Present:

• Do NOT target E. coli specifically based on this culture result alone 1, 2
• Empiric coverage should target S. aureus (including MRSA if risk factors present) with trimethoprim-sulfamethoxazole, clindamycin, or doxycycline 5, 2
• Consider anaerobic coverage if the cyst is in perirectal, vulvovaginal, or head locations 3
• Duration: 5-10 days based on clinical response 2

Critical Pitfalls to Avoid

• Do not treat culture results in the absence of clinical infection signs—this leads to unnecessary antibiotic exposure and resistance. 1

• Avoid relying on superficial wound swabs for treatment decisions, as they frequently yield contaminants that do not reflect true pathogens. 1

• Do not assume all bacteria cultured from wounds represent infection—the mere presence of organisms, even in high numbers, does not equal infection without clinical signs. 1

• Remember that adequate drainage is the cornerstone of treatment—antibiotics alone are insufficient and should never replace proper surgical management. 5, 2

Four Weeks Post-I&D Context

• At four weeks post-procedure, if the wound is healing appropriately without systemic signs, any bacterial growth is almost certainly colonization or contamination. 1

• The negative first culture followed by low-level E. coli growth strongly suggests surface contamination during the second specimen collection. 1

• If clinical concern persists, reassess the wound clinically rather than treating culture results in isolation. 1