Wound Swab

Wound swab culture identifies bacteria (and sometimes fungi) in infected wounds, including surgical site infections, burns, traumatic wounds, bite wounds, pressure ulcers, diabetic foot ulcers and abscesses.

A swab is rolled over the wound after cleaning, placed in transport medium (Amies with charcoal) and sent to the laboratory, where it is Gram stained, inoculated onto blood agar, chocolate agar, MacConkey agar and anaerobic media, and incubated for 24 to 48 hours (up to 5 days for anaerobes). Isolates are identified and tested for antimicrobial susceptibility.

Better samples are tissue biopsy, curettage, needle aspirate of pus or deep tissue specimens at debridement, which reflect the infecting organisms more accurately than surface swabs.

Surgical site infection complicates 2 to 5% of surgical procedures in high income countries and up to 10 to 20% or more in low income settings.

Wound infection is a clinical diagnosis (purulence, erythema, warmth, swelling, pain, delayed healing, fever), and swab culture is indicated when infection is suspected, never for routine surveillance of clinically uninfected wounds, because all open wounds are colonised.

Common pathogens are Staphylococcus aureus (including methicillin resistant strains), beta haemolytic streptococci, Enterobacterales, Pseudomonas aeruginosa and anaerobes, with polymicrobial infection typical in diabetic foot and pressure ulcers.

Indications. Clinical signs of wound infection (purulent discharge, spreading cellulitis, abscess, fever). Surgical site infection. Bite wounds with signs of infection. Burns with suspected infection (fever, change in appearance, rapid deterioration). Chronic wounds failing to heal despite optimal care. Diabetic foot infection (deep tissue sample preferred).

Suspected necrotising soft tissue infection (urgent surgery and tissue sampling). Suspicion of unusual organisms (mycobacteria, fungi, Nocardia, Actinomyces) after soil, water or animal exposure. Not for clinically uninfected wounds.

Specimen. Pus aspirated with needle and syringe (preferred), tissue biopsy or curettage from the base of the wound after cleaning with sterile saline, or a swab (Levine technique: rotate a swab over a 1 cm square area of viable wound bed with enough pressure to express fluid) placed in transport medium. Label the exact site. Surface exudate and eschar are avoided.

Specimens are transported within 2 hours at room temperature (anaerobic specimens in anaerobic transport). Request mycobacterial and fungal culture separately when needed.

Reference values. Gram stain: no or few organisms and few cells in a clean wound. Culture: no growth, or mixed skin flora (not reported as significant). Heavy growth of a single pathogen, or moderate to heavy growth of one or two pathogens with many pus cells on Gram stain, is significant.

Interpretation.

• Pure or predominant growth of Staphylococcus aureus, beta haemolytic Streptococcus (group A, B, C or G), Pseudomonas aeruginosa or Enterobacterales with pus cells and clinical infection: probable pathogen, and treatment follows susceptibility results.
• Mixed growth of three or more organisms (polymicrobial) in a chronic wound: colonisation versus infection depends on clinical signs, and anaerobes (Bacteroides, Peptostreptococcus, Clostridium) are likely in necrotic, malodorous wounds.
• Gram stain showing many neutrophils and organisms supports infection, and a swab with squamous epithelial cells indicates contamination.
• Skin commensals (coagulase negative staphylococci, Corynebacterium, Cutibacterium) are usually contaminants unless isolated from a deep specimen in the setting of prosthetic material.
• Methicillin resistant Staphylococcus aureus (MRSA), extended spectrum beta-lactamase-producing Enterobacterales and carbapenem resistant organisms are reported, with infection control alerts.
• Quantitative tissue culture (over 100 000 colony forming units in each gram) correlates with impaired healing, but is limited to research and specialist centres.
• Negative culture with clinical infection: consider anaerobes, fungi, mycobacteria, antibiotic pre-treatment or deep infection (osteomyelitis, abscess), and biopsy.
• Bite wounds: Pasteurella, Capnocytophaga (dog), Eikenella (human), Streptobacillus, with polymicrobial anaerobic flora. Freshwater and marine exposure: Aeromonas, Vibrio, Mycobacterium marinum.

Critical values and findings needing immediate action.

• Wound with rapidly spreading erythema, severe pain out of proportion, bullae, crepitus, skin necrosis, systemic toxicity: necrotising fasciitis, urgent surgical exploration and broad spectrum antibiotics, without waiting for swab results.
• Gram positive bacilli (suspect Clostridium) in a necrotic wound or muscle: gas gangrene, emergency debridement.
• Group A Streptococcus with toxic shock (hypotension, rash, multi-organ failure): intensive care, clindamycin with beta lactam and notification.
• MRSA or multidrug resistant organism: contact precautions and targeted treatment.
• Tetanus prone wound with unknown immunisation: tetanus toxoid and immunoglobulin as indicated.

Interferences and pitfalls.

• Surface swabs reflect colonising flora rather than deep infection, and poorly correlate with tissue cultures. Swabbing of pus, necrotic slough or wound edges and dressings gives misleading results.
• Prior antibiotics and antiseptics (silver, iodine, chlorhexidine) reduce yield.
• Delay in transport reduces anaerobes and fastidious organisms.
• Anaerobic cultures need anaerobic transport and appropriate media, and are requested when deep or necrotic infection is suspected.
• Colonisation is expected in chronic wounds, and treatment of positive swabs in the absence of clinical infection promotes resistance and does not improve healing.
• Reporting of susceptibility results for contaminants and mixed flora is discouraged.
• Specific tests are added for mycobacteria (Mycobacterium marinum, M. ulcerans, M. fortuitum), fungi (Sporothrix, mycetoma), Nocardia and Actinomyces, with prolonged incubation.
• Bone involvement (probe-to-bone positive, raised inflammatory markers, radiographic change) calls for bone biopsy and culture.
• Quantitative swab methods and fluorescence imaging for bacterial load are not routine.

Special groups.

• Diabetic patients: polymicrobial infection with limb threat, deep tissue sampling at debridement is preferred, and osteomyelitis is assessed by imaging.
• Burns patients: Pseudomonas, Staphylococcus aureus, and Candida, with rapid deterioration and sepsis.
• Children: Staphylococcus aureus and group A streptococcus, and impetigo.
• Immunocompromised and post-transplant patients: opportunistic organisms (Nocardia, fungi, atypical mycobacteria).
• Regions: surgical site infection rates are high in low income settings, with community acquired MRSA and unusual causes such as Buruli ulcer, mycetoma, tropical ulcers and tetanus, and limited access to culture and susceptibility testing.

Follow up tests. Blood cultures if febrile, tissue biopsy or deep culture, anaerobic culture, mycobacterial and fungal culture, MRSA screening swabs, full blood count, C-reactive protein, procalcitonin, plain radiograph, ultrasound, CT or MRI for deep collections and bone infection, HbA1c, vascular assessment (ankle brachial pressure index) in leg wounds, and tetanus immunisation status.