Getting this right requires understanding how wound cultures are collected, what the laboratory does with them, what the report means, and where the limitations of that report lie. A significant proportion of wound cultures grow organisms that are colonisers rather than pathogens, and treating colonisation with systemic antibiotics is one of the clearest examples of antibiotic stewardship failure in clinical practice.
This page works through the full wound culture pathway: from swab collection, through laboratory processing and reporting, to interpreting what the result means clinically and deciding what, if anything, to do about it.
The Problem with Superficial Swabs
Before interpreting any wound culture, it matters to know how the sample was collected. A surface swab of a wound, rolled across the wound bed without any preparation, picks up whatever microorganisms happen to be on that surface at the moment. Many of those organisms are colonisers: bacteria that live on the wound surface without invading tissue or causing clinical infection. They do not need treating. They may not even be the organisms causing any infection that is present, because the truly pathogenic organisms are deeper in the tissue, in the wound margins, or in the surrounding skin.
The Levine technique for wound swabbing is the accepted standard for obtaining a sample that better represents the wound flora in the tissue rather than merely on the surface. Clean the wound surface with sterile saline to remove surface debris and colonisers. Then apply firm, rotating pressure with the swab tip to a 1 cm area of viable-looking granulation tissue, pressing hard enough to express tissue fluid. The organism expressing from within the tissue fluid is more likely to be the one causing active infection than whatever was sitting on the wound surface.
Even with good technique, a wound swab is an imperfect sample. Wound biopsy (taking an actual piece of tissue for culture) is more sensitive and specific for identifying the causative pathogen in chronic wounds. Biopsy is appropriate for chronic non-healing wounds, suspected osteomyelitis, and wounds that are not responding to treatment.
What the Laboratory Does with Your Wound Swab
The swab arrives in the laboratory in a transport medium that maintains bacterial viability without allowing overgrowth. The microbiologist or biomedical scientist plates the swab onto multiple agar types. A standard wound culture set typically includes: blood agar (grows most organisms, allows haemolysis observation), MacConkey agar (selects gram-negative organisms, differentiates lactose fermenters from non-fermenters), and a chocolate agar or Columbia agar with additives to support fastidious organisms.
The plates are incubated overnight and colony morphologies are examined the following morning. Organisms are identified (by MALDI-TOF mass spectrometry in most modern laboratories, supplemented by biochemical panels for confirmation) and a susceptibility panel is performed by disc diffusion or automated broth microdilution.
A smear for Gram staining is usually performed on the original swab. The Gram stain gives a rapid preliminary result within hours: are gram-positive cocci (Staphylococcus, Streptococcus?) or gram-negative rods (Pseudomonas, Klebsiella?) present? Are there numerous white blood cells (polymorphonuclear cells, PMNs) visible, suggesting an active inflammatory response in the sampled tissue? A wound culture with many PMNs on Gram stain and heavy growth of a single organism is much more suggestive of true infection than a culture growing mixed flora with few PMNs.
The Common Wound Pathogens and What They Suggest
Staphylococcus aureus is the most common wound pathogen. It produces numerous virulence factors: surface adhesins that attach it to tissue and foreign material, leucocidin toxins that kill neutrophils, and coagulase that aids in formation of a protective fibrin capsule. In diabetic foot ulcers, post-surgical wounds, and traumatic wounds, S. aureus is frequently the primary pathogen. MRSA (methicillin-resistant S. aureus) requires glycopeptide or alternative non-beta-lactam therapy.
Beta-haemolytic streptococci (Streptococcus pyogenes Group A, Group B, Group C, Group G) are significant wound pathogens even when present in small numbers. Group A Streptococcus in particular can cause rapidly progressive wound infection and necrotising fasciitis, and even low counts on culture should not be dismissed. Beta-haemolytic streptococci are reliably susceptible to penicillin in most regions.
Pseudomonas aeruginosa appears frequently in chronic wounds, burns, and wounds in hospitalised patients. It has an enormous environmental resistance repertoire and multiple intrinsic antibiotic resistance mechanisms. The key clinical question with Pseudomonas in a wound is whether it is a coloniser (very common) or an invading pathogen (requires systemic anti-pseudomonal therapy). Signs that suggest Pseudomonas is pathogenic rather than colonising: surrounding cellulitis, purulent green exudate, systemic signs of infection, failure to heal. Characteristic grape-like odour.
Enterobacteriaceae (Klebsiella pneumoniae, Escherichia coli, Proteus mirabilis, Morganella morganii) often colonise chronic wounds and diabetic foot ulcers. They are frequently co-isolated with other organisms. Proteus species produce urease which raises wound pH and may contribute to an unfavourable healing environment. ESBL-producing Enterobacteriaceae in a wound infection require carbapenem or non-beta-lactam alternatives based on susceptibility.
Anaerobes are underrepresented in routine wound cultures because standard culture conditions are aerobic. In deep or necrotic wounds, bite wounds, and diabetic foot infections with significant tissue ischaemia, anaerobes play an important pathogenic role. Bacteroides fragilis, Prevotella species, Peptostreptococcus species, Clostridium species (including C. perfringens in gas gangrene). If anaerobes are suspected, specifically request anaerobic culture and transport the swab in an anaerobic transport medium.
Interpreting the Susceptibility Report
The susceptibility panel lists each antibiotic tested and the result: S (susceptible), I (susceptible at increased exposure/intermediate), or R (resistant).
Resist the impulse to simply prescribe the first antibiotic on the list marked S. The clinical decision requires answering several questions: Does this organism actually need treating (is it a pathogen or a coloniser)? If it needs treating, does the infection require systemic antibiotics (signs of systemic sepsis, cellulitis, deep tissue involvement) or is local wound management sufficient? Which antibiotic achieves good tissue penetration at the wound site? Is the patient's renal function adequate for the chosen drug? Has the patient had prior colonisation with resistant organisms that might not be the primary isolate?
Tissue penetration is a real consideration. Vancomycin, for example, has poor tissue penetration into bone and soft tissue relative to serum concentrations. For MRSA wound infections with soft tissue involvement, linezolid achieves better tissue concentrations than vancomycin and may be preferred in certain clinical scenarios. Flucloxacillin achieves excellent soft tissue concentrations for MSSA wound infections.
Polymicrobial Wounds: The Most Challenging Interpretation
Many chronic wounds, diabetic foot infections, and pressure injuries grow three, four, or five different organisms. This is normal: chronic wounds develop complex polymicrobial biofilm communities. The question is not which of these organisms is "the pathogen" (they may all be contributing) but rather what the dominant organism is, what evidence of pathogenicity exists, and whether anaerobes may be contributing but not being detected.
For diabetic foot infections in particular, IDSA and IWGDF guidelines recommend antibiotic therapy targeting the probable pathogens based on clinical presentation: mild soft tissue infection is usually monomicrobial (S. aureus, streptococci); moderate to severe infection is more likely polymicrobial including Enterobacteriaceae and anaerobes. The susceptibility result guides the definitive choice once culture data is available, but empiric therapy should not wait for culture results in moderate to severe infection.
Frequently Asked Questions
What is the Levine technique for wound swabbing?
The Levine technique involves cleaning the wound surface with saline to remove surface contamination, then applying firm rotating pressure with the swab tip to a 1 cm area of viable granulation tissue, pressing hard enough to express tissue fluid from within the tissue. This samples organisms in the tissue rather than on the surface and is more representative of the true infecting flora.
What does heavy growth of an organism on wound culture mean?
Heavy growth (semi-quantitative result of +++ or 3+, or quantitative count above 10^5 CFU per gram of tissue) suggests the organism is present in significant numbers in the sampled material. Combined with clinical signs of infection and white blood cells on the Gram stain, heavy growth supports the organism as a pathogen rather than a surface coloniser.
When should you NOT treat a wound culture result?
When the wound shows no clinical signs of infection (no surrounding erythema, warmth, swelling, pain, purulent exudate, or systemic features) and the culture grows mixed flora typical of colonisation, treatment with systemic antibiotics is not indicated. All chronic wounds are colonised. Treating colonisation selects for more resistant organisms without clinical benefit.
What is a mixed growth report?
A mixed growth report means the laboratory identified multiple different organism types that could not be reliably separated or identified individually. It usually reflects surface contamination with normal wound flora. A mixed growth report accompanied by a clinical picture of significant wound infection warrants repeat culture with careful attention to sampling technique, or tissue biopsy.
What is necrotising fasciitis and what organism causes it?
Necrotising fasciitis is a rapidly progressive, life-threatening deep soft tissue infection that destroys the fascia and subcutaneous tissue. Type 1 is polymicrobial (mixed aerobic and anaerobic bacteria). Type 2 is caused by Streptococcus pyogenes (Group A Streptococcus) alone. It is a surgical emergency: the primary treatment is immediate surgical debridement. Antibiotics are adjunctive to surgery, not a substitute for it.
What role do biofilms play in chronic wound infections?
Biofilms are structured communities of bacteria embedded in a self-produced extracellular matrix attached to the wound surface. Bacteria within biofilms are up to 1000 times more resistant to antibiotics than their planktonic (free-floating) counterparts. Biofilm explains why many chronic wounds fail to respond to antibiotic therapy: the antibiotic cannot penetrate the biofilm matrix at clinically achievable concentrations. Debridement to remove the biofilm physically is often more effective than prolonged antibiotic courses.
Should you always swab a wound?
No. Wounds showing no clinical signs of infection should not be routinely swabbed. Swabbing a clinically uninfected wound produces a result that may prompt unnecessary antibiotic prescribing. Swabbing is indicated when clinical signs of infection are present (erythema, warmth, swelling, purulence, systemic features) and when the result will change the management decision.
What is the significance of Streptococcus pyogenes in a wound culture even at low counts?
Group A Streptococcus (S. pyogenes) is a significant pathogen at any colony count in a wound. Even small numbers of this organism can cause rapidly progressive cellulitis, bacteraemia, or necrotising fasciitis. Unlike S. aureus or Pseudomonas, it should not be dismissed as a coloniser at low counts. Treatment with penicillin (or clindamycin for toxin suppression in severe cases) is almost always indicated.
What does MRSA in a wound culture mean for treatment?
MRSA is methicillin-resistant Staphylococcus aureus, resistant to all standard beta-lactam antibiotics (penicillins, cephalosporins, carbapenems). Treatment options depend on severity: topical decolonisation (mupirocin) may be sufficient for surface colonisation, but systemic infection requires intravenous vancomycin, daptomycin, or linezolid. Contact precautions are required to prevent transmission to other patients.
What is gas gangrene and what causes it?
Gas gangrene is a life-threatening infection caused by Clostridium perfringens (and occasionally other Clostridium species) producing gas within muscle tissue. It presents with severe pain disproportionate to wound appearance, rapidly spreading tissue crepitus (crackling on palpation from gas in tissue), and profound systemic toxicity. It is a surgical emergency requiring immediate wide debridement or amputation. Antibiotics (high-dose penicillin with clindamycin to suppress toxin production) are adjunctive.