Skin and soft tissue and bone infections ✅

Related Subjects: | Anatomy of Skin | Anatomy of the Hand | Anatomy of the Thorax | Anatomy of Muscle Groups | Anatomy of Arteries | Anatomy of Spinal Column | Skin Pathology and Lesions | Skin, Soft Tissue & Bone Infections | Waterlow Score

🦠 Skin and soft tissue infections (SSTIs) range from mild cellulitis to life-threatening necrotising infection. The key clinical skill is recognising severity early, choosing likely empirical cover, obtaining cultures where appropriate and escalating rapidly when surgery or sepsis care is needed.

📖 Overview

• SSTIs include cellulitis, erysipelas, abscesses, infected ulcers, diabetic foot infection, surgical wound infection and necrotising infection.
• Most uncomplicated cellulitis is caused by beta-haemolytic streptococci and Staphylococcus aureus.
• Infected ulcers, diabetic foot infections, bites and necrotising infections may be polymicrobial.
• Always assess for systemic illness, immunosuppression, diabetes, vascular disease, foreign body, abscess and necrotising infection.

🎯 General Principles

• 🧪 Take microbiology samples before antibiotics where possible, especially if severe, recurrent, purulent, postoperative, ulcer-related or diabetic foot infection.
• 💊 Use local antimicrobial guidelines because resistance patterns and first-line choices vary between hospitals.
• 🚨 Assess severity: fever, rigors, tachycardia, hypotension, confusion, severe pain, rapid spread or immunosuppression suggest higher risk.
• 🔁 Review at 24–48 hours and step down from IV to oral antibiotics when clinically improving.
• 🔪 Source control matters: abscesses, necrotising infection, septic arthritis and some diabetic foot infections need drainage or debridement, not antibiotics alone.
• 🤝 Involve microbiology, surgery, orthopaedics, vascular surgery, tissue viability or the diabetic foot MDT when infection is severe or complex.

🚨 Admit or Escalate Urgently If

• Sepsis, shock, confusion or rapidly deteriorating physiology.
• Severe pain out of proportion to skin findings.
• Rapidly spreading erythema, skin necrosis, bullae, crepitus or anaesthesia of the skin.
• Immunosuppression, neutropenia, poorly controlled diabetes or critical limb ischaemia.
• Facial, orbital, hand, genital or deep space infection.
• Suspected necrotising fasciitis, Fournier’s gangrene, gas gangrene, septic arthritis or osteomyelitis.

🔴 Cellulitis and Erysipelas

• 🦠 Likely pathogens: Group A streptococci, other beta-haemolytic streptococci and Staphylococcus aureus.
• 🔥 Features: unilateral erythema, warmth, swelling, tenderness, fever or lymphangitis.
• 💊 Typical oral first-line: flucloxacillin, if suitable and in line with local policy.
• 🏥 Severe disease: IV antibiotics may be required; choice depends on local guidance and severity.
• 🧦 Supportive care: elevate limb, mark edge of erythema, treat tinea pedis/eczema/wounds and optimise oedema management.

💡 Exam pearl: True lower-limb cellulitis is usually unilateral. Bilateral red swollen legs are more often venous eczema, lymphoedema, heart failure, lipodermatosclerosis or dependent oedema.

🟡 Leg Ulcers and Pressure Sores

• 🧫 Chronic wounds are often colonised; bacteria on a swab do not automatically mean infection.
• 💊 Use antibiotics only when there are clinical signs of infection: spreading erythema, warmth, increasing pain, swelling, purulent discharge, malodour with deterioration, fever or systemic illness.
• 🧼 Management includes wound care, compression if appropriate, offloading, pressure relief, nutrition and tissue viability input.
• 🚫 Do not treat colonisation alone with antibiotics.

🦶 Diabetic Foot Infection

• 🦠 Often polymicrobial: Staphylococcus aureus, streptococci, Gram-negative organisms and anaerobes depending on severity and chronicity.
• 🔍 Assess ulcer depth, cellulitis, abscess, necrosis, osteomyelitis, neuropathy and peripheral arterial disease.
• 🧪 Send deep tissue or pus samples where possible; superficial swabs are less useful.
• 💊 Mild infection may be treated orally if the patient is well; moderate or severe infection may need admission and IV antibiotics.
• 🤝 Always involve the diabetic foot MDT for significant infection, ischaemia, necrosis, spreading cellulitis or suspected bone involvement.
• 🦴 Probe-to-bone, chronic deep ulcer, exposed bone or persistent inflammation should raise suspicion of osteomyelitis.

🪡 Surgical Wound Infection

• 🦠 Common pathogen: Staphylococcus aureus, but abdominal, groin and contaminated wounds may involve Gram-negatives and anaerobes.
• 🔍 Look for increasing wound pain, erythema, warmth, swelling, discharge, fever or wound dehiscence.
• 🧪 Send pus or wound fluid for culture if present.
• 🔪 Drain collections, remove infected material where appropriate and seek surgical review for deep infection.
• 💊 Antibiotic choice depends on site, severity, procedure type and local policy.

⚫ Necrotising Fasciitis

🚨 Red flag: Severe pain out of proportion to signs is the classic warning feature. This is a surgical emergency - do not wait for imaging if the patient is unstable or clinical suspicion is high.

• 🦠 Pathogens may include Group A streptococcus, anaerobes and Gram-negative organisms.
• ⚡ Features include rapidly progressive pain, systemic toxicity, skin necrosis, bullae, crepitus, shock or reduced skin sensation.
• 🔪 Management is urgent surgical exploration and debridement plus broad-spectrum IV antibiotics.
• 💬 Discuss immediately with surgery, anaesthetics/critical care and microbiology.
• 🧪 Take blood cultures and tissue cultures, but do not delay surgery.

🟣 Fournier’s Gangrene

🚨 Fournier’s gangrene is necrotising fasciitis of the perineum, genitalia or perianal region. It is rapidly fatal without urgent debridement and sepsis management.

• 🦠 Usually polymicrobial: streptococci, coliforms and anaerobes.
• ⚠️ Risk factors include diabetes, immunosuppression, obesity, alcohol excess, perianal sepsis and urological infection.
• 🔪 Requires emergency surgical debridement, broad-spectrum IV antibiotics and critical care support.

🟤 Gas Gangrene

⚠️ Gas gangrene is clostridial myonecrosis, classically caused by Clostridium perfringens. It causes severe pain, tissue necrosis, gas production, haemolysis, shock and death if not treated rapidly.

• 🦠 Consider after contaminated trauma, crush injury, penetrating wounds or postoperative infection.
• 🔪 Management is urgent surgical debridement plus IV antibiotics.
• 🫧 Hyperbaric oxygen may be considered as an adjunct in specialist centres, but it must not delay surgery.

🐶 Animal and Human Bites

• 🦷 Bites may inoculate mixed organisms deep into tissue.
• 🐱 Cat bites have high infection risk because puncture wounds seal quickly.
• ✋ Human bites, clenched-fist injuries and hand bites need careful assessment and often specialist review.
• 💊 Co-amoxiclav is commonly used first line if prophylaxis or treatment is indicated, unless contraindicated.
• 🧬 Consider tetanus status, rabies risk if bite occurred overseas, and hepatitis/HIV risk in human bites.
• 🔍 Explore for tendon, joint, bone or foreign body involvement where clinically suspected.

🦴 Osteomyelitis

• 🦠 Common cause: Staphylococcus aureus, but pathogens vary with diabetes, trauma, surgery, vascular disease and prosthetic material.
• ⚠️ Suspect with persistent bone pain, fever, raised inflammatory markers, chronic ulcer over bone or non-healing wound.
• 🧪 Blood cultures, imaging and bone/deep tissue sampling may be needed.
• 💊 Treatment usually requires prolonged antibiotics, guided by microbiology.
• 🔪 Chronic osteomyelitis often requires surgical debridement as well as antibiotics.

🦵 Septic Arthritis

🚨 Septic arthritis is an orthopaedic emergency. It can rapidly destroy cartilage and cause sepsis, so urgent aspiration, antibiotics and joint drainage are required.

• 🦠 Common pathogens include Staphylococcus aureus, streptococci, Gram-negative organisms and Neisseria gonorrhoeae in relevant patients.
• 🔥 Features: hot swollen painful joint, reduced range of movement, fever or inability to weight bear.
• 🧪 Send blood cultures and aspirate joint fluid for Gram stain, culture, cell count and crystals.
• 🔪 Treatment is antibiotics plus joint drainage or washout.
• ⚠️ Crystals do not exclude infection - gout and septic arthritis can coexist.

💊 Antibiotic Principles

🚨 Red Flags in SSTI

• Severe pain out of proportion to clinical signs.
• Rapidly spreading erythema or swelling.
• Skin necrosis, bullae, purple discolouration or crepitus.
• Hypotension, tachycardia, confusion, rigors or lactate elevation.
• Immunosuppression, neutropenia or uncontrolled diabetes.
• Facial/orbital infection, hand infection, perineal infection or infected prosthetic material.
• Hot swollen joint or inability to weight bear.

🧠 Exam Pearls

• 🦵 Bilateral red legs are usually not cellulitis.
• 🔪 Abscess = drainage; antibiotics alone may fail.
• 🚨 Pain out of proportion suggests necrotising fasciitis until proven otherwise.
• 🦶 Diabetic foot infection needs assessment of depth, ischaemia and bone involvement.
• 🦴 Septic arthritis needs urgent aspiration and washout.
• 🧫 Culture results should be used to narrow antibiotics where possible.
• 💊 Always check local antimicrobial guidance, renal function, allergies, interactions and pregnancy status.

✅ Key message: SSTI management depends on severity, site, likely organisms and the need for source control. Mild cellulitis may need oral antibiotics, but necrotising infection, septic arthritis, deep abscess and severe diabetic foot infection require urgent escalation.

References

• NICE / BNF: Skin infections antibacterial treatment summaries.
• NICE CKS: Cellulitis - acute.
• NICE guidance on diabetic foot problems and antimicrobial prescribing.
• Local NHS antimicrobial guidelines should always be checked before prescribing.

Disclaimer

This material is for education and exam revision. It does not replace clinical judgement. Always check local antimicrobial guidelines, allergies, renal function, pregnancy status and microbiology advice in severe or complex infection.