skin and soft tissue infections
in review
6 min read
Updated 2026-09-29
skin and soft tissue infections
Classify as purulent (abscess → S. aureus including MRSA) vs non-purulent (cellulitis/erysipelas → β-haemolytic streptococci). Non-purulent: cefazolin IV or cephalexin PO; MRSA cover adds nothing. Purulent: I&D is definitive; add TMP-SMX or doxycycline. Necrotising infection: surgical emergency — debridement + vancomycin + piperacillin-tazobactam + clindamycin. Cefazolin can be given to most penicillin-allergic patients.
cellulitis/erysipelas empiric antibiotics
- mild (outpatient): cephalexin 500 mg PO QID
- moderate (inpatient): cefazolin 2 g IV q8h → step down to cephalexin when improving
- penicillin allergy: cefazolin/cephalexin usually still appropriate — see penicillin allergy
- if β-lactams must be avoided: vancomycin IV, or clindamycin 300–450 mg PO QID (check local resistance)
- do NOT add MRSA cover for non-purulent cellulitis
- elevate limb; treat tinea pedis/interdigital maceration
- duration: 5–7 days total
purulent SSTI / abscess empiric antibiotics
- primary treatment: incision and drainage (send pus for culture)
- mild, post-I&D: TMP-SMX 1–2 DS tabs PO BID (weight-based) or doxycycline 100 mg PO BID × 5–7 days
- systemically unwell: vancomycin IV (linezolid 600 mg PO/IV BID if oral option needed)
- perineal or diabetic foot with mixed flora: piperacillin-tazobactam 4.5 g IV q6h ± vancomycin
necrotising soft tissue infection empiric antibiotics
- SURGICAL EMERGENCY → urgent surgical consult for debridement; ID consult
- vancomycin + piperacillin-tazobactam 4.5 g IV q6h + clindamycin 900 mg IV q8h (toxin suppression) — IDSA
- stop vancomycin once MRSA excluded
- confirmed group A strep: penicillin G 4 MU IV q4h + clindamycin 900 mg IV q8h
- streptococcal toxic shock: consider IVIG 1 g/kg day 1, then 0.5 g/kg days 2–3 (with ID)
local antibiogram
Community MRSA prevalence varies across Canadian regions, and clindamycin resistance is common in both S. aureus and GAS — confirm susceptibility before relying on clindamycin.
classification
| type | organism | clinical features |
|---|---|---|
| non-purulent (cellulitis, erysipelas) | β-haemolytic streptococci (usually GAS) | spreading erythema, warmth, tenderness; no drainable collection |
| purulent (abscess, furuncle, carbuncle) | S. aureus (MRSA or MSSA) | fluctuant collection, surrounding cellulitis; I&D curative |
| necrotising | polymicrobial (type I) or monomicrobial GAS (type II) | pain out of proportion, crepitus, systemic toxicity, rapid spread |
severity grading
| grade | features | management |
|---|---|---|
| mild | no systemic signs, small area | outpatient PO antibiotics |
| moderate | systemic signs (fever, tachycardia) OR failed outpatient therapy OR large area | inpatient IV antibiotics |
| severe | sepsis, rapid progression, crepitus, pain out of proportion | urgent surgical consult + broad-spectrum IV |
when to suspect necrotising fasciitis
- pain disproportionate to exam findings — most reliable early sign
- wooden-hard induration extending beyond visible erythema
- dusky/grey skin, haemorrhagic bullae, anaesthesia of overlying skin
- crepitus (gas — late sign)
- systemic toxicity; rapid progression despite antibiotics
LRINEC score
Wong (2004): CRP, WBC, haemoglobin, sodium, creatinine, glucose. ≥6 raises suspicion; ≥8 strongly predictive. Sensitivity is moderate — do not use to rule out necrotising fasciitis. Clinical suspicion alone warrants surgical exploration.
CT with IV contrast may show fascial thickening, fat stranding, gas — should not delay surgery if suspicion is high.
toxic shock syndrome
| streptococcal (GAS) | staphylococcal | |
|---|---|---|
| criteria | hypotension + GAS isolated + ≥2 of: AKI, coagulopathy, liver involvement, ARDS, erythematous rash, soft tissue necrosis | fever ≥38.9 °C, diffuse erythroderma, desquamation at 1–2 weeks, hypotension, ≥3 organ systems |
| source | NSTI, bacteraemia; blood cultures usually positive | tampons, packing, wounds, post-op; blood cultures often negative |
| treatment | penicillin G + clindamycin ± IVIG; debridement | cloxacillin/cefazolin (vancomycin if MRSA) + clindamycin; remove foreign body |
Invasive GAS → droplet + contact precautions until 24 h of effective antibiotics; notify public health (household contact prophylaxis per local guidance).
special situations
diabetic foot infections
| syndrome | empiric therapy | duration |
|---|---|---|
| cellulitis ± non-purulent ulcer | cefazolin 2 g IV q8h or cephalexin 500 mg PO QID | 7 days |
| purulent ulcer, no osteomyelitis | I&D + cefazolin, TMP-SMX, or doxycycline | 7–14 days |
| necrosis, deep tissue, bone involvement, or sepsis | debridement ± bone biopsy + ceftriaxone 2 g IV q24h + metronidazole 500 mg PO BID, or piperacillin-tazobactam 4.5 g IV q6h | 6 weeks if osteomyelitis not resected |
- anaerobic cover only for necrotic, gangrenous, or ischaemic wounds (IWGDF/IDSA 2023)
- probe-to-bone positive in a high-risk foot → treat as osteomyelitis until proven otherwise; MRI if uncertain
- bone biopsy (percutaneous or surgical) is the diagnostic gold standard and guides therapy — superficial wound swabs mislead
- osteomyelitis after minor amputation with positive bone margin → ~3 weeks; fully resected → days only
- known ESBL or MRSA colonisation → adjust with ID
bite wounds
- prophylaxis (all human bites, deep cat bites, hand wounds, immunocompromised): amoxicillin-clavulanate 875/125 mg PO BID × 3–5 days
- severe β-lactam allergy: TMP-SMX + metronidazole, or moxifloxacin
- Pasteurella multocida (cat/dog) — penicillin-susceptible; Eikenella corrodens (human) — resistant to clindamycin and first-generation cephalosporins
water and environmental exposures
| exposure | organism | treatment |
|---|---|---|
| saltwater, raw oysters (cirrhosis → haemorrhagic bullae, sepsis) | Vibrio vulnificus | doxycycline + ceftazidime (or ceftriaxone); early debridement |
| freshwater | Aeromonas | ciprofloxacin or ceftriaxone |
| fish tank, swimming pool (indolent nodules along lymphatics) | Mycobacterium marinum | clarithromycin ± ethambutol for months |
| rose thorns, gardening (sporotrichoid spread) | Sporothrix schenckii | itraconazole |
| dog bite in asplenia or alcohol use disorder (sepsis, DIC) | Capnocytophaga canimorsus | amoxicillin-clavulanate; pip-tazo if septic |
Bites also need tetanus and rabies risk assessment — see infection control and post-exposure prophylaxis.
people who inject drugs
- always cover MRSA (TMP-SMX or vancomycin); consider mixed flora
- image for deep collections (ultrasound or CT)
- SSTI + bacteraemia → consider infective endocarditis
duration
| type | duration |
|---|---|
| uncomplicated cellulitis | 5–7 days (5 if rapid response) |
| abscess (post-I&D) | 5–7 days |
| necrotising infection | until no further debridement needed, clinically improved, and afebrile 48–72 h |
| diabetic foot (no osteomyelitis) | 1–2 weeks |
| diabetic foot + osteomyelitis | 6 weeks (shorter if resected — see above) |
recurrent cellulitis
- treat predisposing factors: tinea pedis, oedema/lymphoedema, venous insufficiency, obesity
- chronic leg oedema → compression therapy — Webb (2020): recurrence 15% vs 40%
- recurrent episodes despite this → penicillin V 250 mg PO BID — IDSA threshold 3–4 episodes/year; PATCH I (2013) enrolled ≥2 episodes and halved recurrence over 12 months, benefit waned after stopping
traps
- bilateral “cellulitis” is almost never infection — stasis dermatitis, contact dermatitis, lipodermatosclerosis, lymphoedema, DVT
- mark the erythema border; early worsening at 24–48 h is common on appropriate therapy — reassess at 48–72 h
- adding TMP-SMX to cephalexin for non-purulent cellulitis does not improve cure
- blood cultures in uncomplicated cellulitis are positive in <5% — obtain only if septic, immunocompromised, or unusual exposure
- doxycycline has unreliable streptococcal cover — not for non-purulent cellulitis
- I&D alone cures most small abscesses; TMP-SMX post-I&D modestly improves cure and reduces recurrence (Talan (2016))