Surgical Infection - Complete Overview

1. Historical Background

• Hippocrates: Used wine and vinegar to irrigate infected wounds - recognised that pus needed to be drained
• Galen: Named suppuration "pus bonum et laudabile" (good and praiseworthy pus) as it signalled localisation of infection
• Koch's Postulates (19th century): Established the scientific basis for infective disease

• The organism must be found in every case
• It must be isolatable from the host and grown in culture
• It must reproduce the disease when injected into a healthy host
• It must be recovered from the experimentally infected host
• Semmelweis: Reduced puerperal sepsis from >10% to <2% by handwashing between cases
• Pasteur: Germ theory - microorganisms responsible for infection
• Lister: Pioneered antiseptic surgery using carbolic acid spray

2. Microbiology of Surgical Infection

Gram-Positive Organisms

• Streptococcus pyogenes (Group A, β-haemolytic) - most pathogenic; causes cellulitis and tissue destruction via streptolysin, streptokinase, streptodornase
• Streptococcus faecalis (Lancefield Group D, enterococcus) - acts in synergy with other organisms
• Peptostreptococcus - anaerobic streptococcus
• All streptococci remain sensitive to penicillin and erythromycin; cephalosporins as alternative in penicillin allergy

• Staphylococcus aureus - most important pathogen; found in nasopharynx of up to 15% of the population; causes suppuration and implant infections
• MRSA (Methicillin-resistant S. aureus): Asymptomatic nasal carriage; managed by pre-admission screening in elective surgery; "search and destroy" methods used in northern Europe
• Staphylococcus epidermidis - coagulase-negative; causes prosthetic and line infections

• Clostridium perfringens - gas gangrene; produces collagenase, hyaluronidase, phospholipase (lecithinase) causing myonecrosis
• Clostridium tetani - tetanus; produces tetanospasmin (neurotoxin); prevention by immunisation and wound toilet
• Clostridium difficile (C. diff) - antibiotic-associated colitis; produces toxins A and B; risk increased after broad-spectrum antibiotics (especially clindamycin, ampicillin, cephalosporins)

Gram-Negative Organisms

• Coliforms (E. coli, Klebsiella, Proteus) - bowel flora; common in abdominal surgical infections
• Pseudomonas aeruginosa - particularly in burns and wound infections; resistant to many antibiotics
• Bacteroides fragilis - most common anaerobe; bowel operations; sensitive to metronidazole
• Haemophilus influenzae - respiratory tract infections

3. Wound Classification (CDC/NNIS)

1. Contaminated or dirty wound
2. ASA score ≥3
3. Operative time > 75th percentile for that procedure

4. Factors Determining Wound Infection (Summary box 5.4)

• Host response (immune competence)
• Virulence and inoculum of the infecting organism
• Vascularity and health of tissue (local ischaemia, systemic shock)
• Presence of dead or foreign tissue
• Presence of antibiotics during the "decisive period"

The Decisive Period

Risk Factors for Wound Infection (Summary box 5.5)

• Malnutrition (obesity or weight loss)
• Metabolic disease (diabetes, uraemia, jaundice)
• Immunosuppression (cancer, AIDS, steroids, chemo/radiotherapy)
• Gut colonisation and translocation
• Poor perfusion (systemic shock, local ischaemia)
• Foreign body material
• Poor surgical technique (devitalised tissue, dead space, haematoma)

5. Host Defence Mechanisms

• Mechanical: Intact skin and mucosal barriers
• Chemical: Gastric acid, bile salts, enzymes
• Humoral: Immunoglobulins, complement, opsonins, interferons
• Cellular: Phagocytes, macrophages, polymorphs, killer lymphocytes

6. Presentation of Surgical Infection

Major SSI

• Significant quantity of pus (spontaneous discharge or requiring secondary drainage procedure)
• Patient systemically unwell: tachycardia, pyrexia, raised WCC
• Delayed discharge from hospital

Minor SSI

• Small discharge of pus or infected serous fluid
• Not associated with systemic signs or delayed discharge
• Often resolves without antibiotics

Abscess

• Classical signs of Celsus: calor (heat), rubor (redness), dolor (pain), tumor (swelling), plus functio laesa (loss of function)
• Pus = dead/dying neutrophils succumbed to bacterial toxins
• Predominantly caused by Staphylococcus aureus
• Treatment: Incision and Drainage (I&D) - "ubi pus, ibi evacua" (where there is pus, evacuate it)
• Antibiotics alone are ineffective for an established abscess

Cellulitis

• Diffuse spreading infection of dermis and subcutaneous tissue
• Caused by β-haemolytic Streptococcus (Group A) - spreads via lymphatics and tissue planes
• Red, hot, tender skin without discrete margin
• Treatment: Penicillin (or erythromycin/cephalosporin); mark borders to monitor spread; elevation

7. Specific Surgical Infections

Necrotising Fasciitis (Synergistic Spreading Gangrene)

• Coliforms, staphylococci, Bacteroides, anaerobic streptococci, peptostreptococci
• Aerobic bacteria destroy living tissue → anaerobic bacteria thrive

• Meleney's synergistic gangrene - abdominal wall
• Fournier's gangrene - scrotal/perineal

Fournier's gangrene - classic presentation with exposed testes following debridement

• Severe wound pain
• Spreading inflammation with crepitus (gas in tissues) and odour
• Usually in immunocompromised patients (especially diabetics)

1. Infiltrate area with local anaesthetic
2. Make 2-cm incision down to deep fascia
3. Lack of bleeding = positive sign
4. "Dishwater-coloured fluid" from wound = positive sign
5. Gentle probing at deep fascia level - if tissues dissect with minimal resistance = positive
6. Tissue biopsies for frozen section: characteristic findings - obliterative vasculitis, acute inflammation, subcutaneous necrosis

• Broad-spectrum antibiotics + aggressive circulatory support
• Wide excision and laying open of all affected areas (debridement may be very extensive)
• Survivors may need large skin grafting

8. Systemic Infection

Bacteraemia

• Uncommon after superficial SSI (drains through wound)
• Common after deep space SSI (anastomotic breakdown)
• Often transient after instrumentation through infected bile or urine
• Dangerous in patients with implanted prostheses (haematogenous seeding)
• Causative organisms: aerobic Gram-negative bacilli, S. aureus, fungi

SIRS, Sepsis, and Septic Shock

• Hyperthermia (>38°C) or hypothermia (<36°C)
• Tachycardia (>90/min) or tachypnoea (>20/min)
• WCC >12 × 10⁹/L or <4 × 10⁹/L

• Respiratory: ARDS
• Cardiovascular: Septic shock (↓ cardiac output + ↓ peripheral vascular resistance)
• Renal failure

Pathophysiology of SIRS:

Gut failure → bacterial colonisation → endotoxin release → macrophage activation → cytokine release (IL-6, TNF) → SIRS/MODS

Surviving Sepsis Campaign ("Sepsis Six" bundle)

1. Give high-flow oxygen
2. Take blood cultures
3. Give IV antibiotics
4. Start IV fluid resuscitation
5. Check serum lactate and haemoglobin
6. Measure urine output (insert catheter)

9. Antibiotic Prophylaxis

• Indicated for clean-contaminated and contaminated surgery
• Given to cover the "decisive period" (4 hours)
• Should be given 30 minutes before incision (IV at induction)
• Routine prophylaxis in clean, non-prosthetic surgery is of limited value and encourages resistant strains
• For prosthetic surgery (joint replacements, vascular grafts), prophylaxis is mandatory

• GI surgery: Cefuroxime + metronidazole
• Orthopaedic implant surgery: Cefuroxime or flucloxacillin
• Biliary surgery: Cefuroxime

10. Hospital-Acquired Infections (HAI)

1. Respiratory infections (incl. ventilator-associated pneumonia)
2. Urinary tract infections (catheter-related)
3. Bacteraemia (indwelling vascular catheter-related)
4. Surgical Site Infections (SSI)

• Strict hand hygiene (Semmelweis principle)
• Aseptic technique
• Minimise indwelling catheters and their duration
• Regular replacement of IV cannulae
• MRSA screening pre-operatively for elective surgery

11. Universal Precautions (Bloodborne Viruses: HIV, Hepatitis B/C)

• Full face mask or protective spectacles
• Fully waterproof disposable gowns and drapes
• Boots (not clogs) - prevent sharp injuries
• Double gloving (larger inner glove for comfort)
• Minimal essential personnel in theatre
• Respect for sharps - pass in a kidney dish only
• Slow, meticulous technique to minimise bleeding

• Wash under running water immediately
• Report the incident
• Occupational health advice + consider post-exposure prophylaxis (PEP)
• HIV and hepatitis testing; consider non-operative specialty if high-risk exposure

Key Summary Points for Exam