In short. How to recognise infection in a chronic wound, why biofilm makes it persistent, why a surface swab usually misleads, and how samples should be taken.
Key points
- A swab rolled across the surface of a chronic wound mostly samples colonising organisms and biofilm, not the bacteria causing deep infection.
- It commonly grows several species, prompts broad antibiotics, and directs treatment at the wrong target.
- When infection is genuinely suspected, the useful samples are deep tissue obtained during debridement, or pus aspirated from an undrained collection.
Every chronic wound carries bacteria; that is normal, and it is not the same thing as infection. The distinction matters because treating colonisation with antibiotics selects resistant organisms and achieves nothing, while missing a genuine deep infection can cost a limb or a life.
Recognising infection
Overt infection shows spreading redness, warmth, swelling, pain, and pus, sometimes with fever and malaise. Chronic wounds often present more subtly: increasing pain, a friable surface that bleeds easily, an unpleasant odour, delayed healing despite good care, and a wound bed that turns dark and gelatinous. In a person with diabetes or poor circulation the usual signs may be muted because the immune response is blunted, so a wound can be deep and dangerous while looking quiet.
Biofilm
Biofilm is a structured community of bacteria embedded in a self-made matrix that attaches to the wound surface. It is present in the majority of chronic wounds and behaves differently from free-floating bacteria: it resists antibiotics at concentrations many times higher than those that kill the same organisms in a laboratory dish, and it shields itself from immune cells. Biofilm is the reason a wound can improve briefly after antibiotics and relapse as soon as they stop. Removing it physically — by sharp debridement, irrigation, and appropriate cleansing — is an essential step, not an optional one.
Why a surface swab misleads
A swab rolled across the surface of a chronic wound mostly samples colonising organisms and biofilm, not the bacteria causing deep infection. It commonly grows several species, prompts broad antibiotics, and directs treatment at the wrong target. When infection is genuinely suspected, the useful samples are deep tissue obtained during debridement, or pus aspirated from an undrained collection. Blood cultures are taken when systemic infection is suspected.
Treatment principles
Drainage of any collection and debridement of dead tissue come first, because antibiotics cannot penetrate dead tissue or biofilm. Antibiotics are then used for a defined course when there is spreading infection, systemic illness, or bone involvement — not indefinitely. Wounds are cleansed at each dressing change, and dressings are chosen to manage exudate and bacterial burden. Where bone infection is suspected, imaging and sometimes a bone sample guide a much longer course of treatment.
Honest limits
Even with correct treatment, infection delays healing by weeks and can recur. Nothing removes the need to correct the underlying cause: pressure, poor perfusion, or uncontrolled diabetes will keep producing infection no matter how good the wound care is.
Seek urgent review if
- Redness spreads rapidly, or the wound becomes hot, swollen, and intensely painful.
- You develop fever, shivering, or confusion, or feel generally very unwell.
- The wound develops a foul odour or darkens, or you see gas bubbling in the tissue.
Frequently asked questions
What is biofilm and why does it matter?
Biofilm is a structured community of bacteria embedded in a self-made matrix on the wound surface. It resists antibiotics at concentrations many times higher than those that kill the same organisms in a laboratory, and it shields itself from immune cells, so wounds improve briefly and then relapse.
Why does a surface swab mislead?
A swab rolled across a chronic wound mostly samples colonising organisms and biofilm rather than the bacteria causing deep infection. It commonly grows several species and prompts broad antibiotics aimed at the wrong target. Deep tissue taken during debridement, or drained pus, is more useful.
Can antibiotics alone clear a wound infection?
No. Antibiotics cannot penetrate dead tissue or biofilm, so drainage of any collection and debridement of dead tissue come first. Antibiotics are then used for a defined course where there is spreading infection, systemic illness or bone involvement, not indefinitely.
Risks & Limitations
- Chronic wounds heal over months, not weeks. A wound that closes in three to six months of consistent care is a good result.
- Recurrence is common. Both diabetic foot wounds and venous leg ulcers frequently return, which is why protective footwear and compression continue after healing.
- Partial or complete failure of a skin graft or flap, which may require further surgery.
- Infection and biofilm, which delay healing by weeks and can spread deeply or into bone.
- Amputation risk in diabetes and vascular disease, which is reduced but never abolished by treatment.
- Prolonged dressings and repeated clinic visits. Offloading, compression, and follow-up are demanding and take months.
- Donor-site scarring and discomfort where a graft or flap was taken.
- Reduced or absent sensation in grafted and flap tissue, and around a healed diabetic foot wound.
When to Seek Care
- A wound that is enlarging or deepening, or that begins to expose bone, tendon, or joint.
- Black, grey, or dead tissue appearing in or around the wound.
- Spreading redness, warmth, or swelling around the wound.
- Fever, shivering, or feeling generally very unwell.
- Foul odour or pus from the wound.
- New or severe pain, including pain in the leg or foot at rest or at night.
- A toe or foot that turns black, cold, or blue.
- Loss of sensation in a foot or toe, or a new wound or blister you cannot feel.
References & Further Reading
- World Health Organization (WHO) — chronic wound and diabetes care resources
- NICE (National Institute for Health and Care Excellence) — guidance on wound care, diabetic foot problems, and pressure ulcers
- Wounds International — international consensus documents on wound infection, biofilm, and compression therapy
- American Society of Plastic Surgeons (ASPS) — plastic surgery guidance on wound closure
- Cochrane Library — systematic reviews in wound care and wound closure
This page is patient education, not medical advice, and it does not cite individual studies. Discuss your own case with a qualified clinician. To arrange an assessment at Chongqing West District Hospital, see Contact & Consultation.