The purpose is to reduce the incidence of surgical site infection (SSI). SSIs are the most common healthcare-associated infection in surgical patients. The rationale is simple from first principles:

• When you cut through skin, you breach the body's primary barrier to infection.
• Bacteria (from skin flora, mucosal surfaces, or the environment) can be introduced into the surgical wound.
• If the bacterial inoculum exceeds the host's local immune capacity, infection establishes.
• Antibiotics present in tissues at the time of incision reduce the bacterial inoculum below the threshold needed to cause infection.

These four principles are explicitly listed as a learning outcome and are extremely high yield. ^[1]

Name the 4 classes of surgical wounds and give examples of representative procedures. ^[1]

This is the foundation for deciding whether prophylaxis is indicated.

How to decide the class: Think about these questions in order:

1. Is there existing infection or devitalized/old wound? → Dirty
2. Was there gross spillage, major break in technique, or uncontrolled entry into a contaminated tract? → Contaminated
3. Was a mucosal surface entered under controlled conditions with no significant spillage? → Clean-contaminated
4. Was no mucosal surface entered, with full aseptic technique? → Clean

Use narrow-spectrum agents; reserve broad-spectrum drugs (3rd-gen cephalosporins, fluoroquinolones, carbapenems) for treatment. ^[4]

• Give at induction of anaesthesia, within 30 minutes of first skin incision ^[4]
• The goal is to have the antibiotic at peak tissue concentration when the wound is open and vulnerable to contamination
• Re-dosing: Give an additional dose if:
  • Surgery exceeds 2× the half-life of the antibiotic (for cefazolin, this means surgery > 4 hours; for Augmentin/cefuroxime, this is ~3 hours)
  • Blood loss exceeds 1.5L (dilutes the circulating antibiotic)
• Do NOT continue prophylaxis post-operatively for clean and clean-contaminated cases ^[4][5]

Prolonged prophylaxis beyond the operative period leads to: ^[4]

• C. difficile pseudomembranous colitis
• Line sepsis
• Hospital-acquired pneumonia
• Antimicrobial resistance

This is a major public health issue. The lecture emphasises that inappropriate extension of prophylaxis is one of the most common forms of antibiotic misuse in hospitals.

Group A Streptococcus (GAS, Streptococcus pyogenes) causes pharyngitis. Through molecular mimicry (GAS M-protein cross-reacts with cardiac myosin, synovial membrane, and brain tissue), the immune response attacks the host's own tissues, causing acute rheumatic fever (ARF) 2–3 weeks after pharyngitis. Recurrent ARF episodes cause progressive valvular damage → rheumatic heart disease (RHD).

Distinguish between primary and secondary prevention of rheumatic fever. ^[1]

2025 MCQ Q20: "Treatment of group A streptococcal pharyngitis" — Answer: Penicillin V (I). This is primary prevention of ARF. ^[3]

Explain considerations in determining the duration of prophylaxis for prevention of rheumatic fever recurrences. ^[1]

The logic: the severity of cardiac involvement dictates the consequences of recurrence. A patient with existing valvular damage has the most to lose from another episode of carditis, so they get the longest prophylaxis. ^[7]

• Oral erythromycin for 10 days (primary prevention) — but note that GAS resistance to macrolides in HK is 40–50%, making this problematic ^[4]
• Azithromycin or clarithromycin as alternatives
• Clindamycin if already taking penicillin prophylaxis with good compliance and develops pharyngitis → use a non-penicillin agent ^[7]

Neisseria meningitidis is transmitted by respiratory droplets. Close contacts of a patient with meningococcal disease have a significantly elevated risk of developing invasive meningococcal disease. The purpose of post-exposure prophylaxis (PEP) is to eradicate nasopharyngeal carriage of meningococci in close contacts, thereby preventing secondary cases.

Explain the considerations in choosing an antibiotic for preventing meningococcal infections. ^[1]

• Close/household contacts of the index case
• Anyone with prolonged close contact (e.g., sleeping in same room, intimate kissing)
• Healthcare workers only if they performed mouth-to-mouth resuscitation or unprotected intubation/suctioning
• Casual contacts (e.g., classmates not sitting directly next to the patient) generally do NOT require prophylaxis

Justify the use of prophylactic antibiotics in cat and dog bites. ^[1]

Bite wounds are contaminated by the oral flora of the biting animal (or human). These are polymicrobial wounds containing:

• Aerobes: Pasteurella species (especially P. multocida from cats, P. canis from dogs), Staphylococcus, Streptococcus, Capnocytophaga canimorsus (particularly dangerous in asplenic patients)
• Anaerobes: Bacteroides, Fusobacterium, Porphyromonas

Cat bites are more likely to become infected than dog bites because:

• Cat teeth are thin and sharp → create deep puncture wounds that are difficult to irrigate and seal over quickly, trapping bacteria in deep tissues
• Pasteurella multocida is present in ~75% of cat bites and causes rapidly progressive cellulitis

Step 1: Wound irrigation

• Must get to the bottom of the wound to ensure proper irrigation ^[4]
• Normal saline or povidone-iodine
• This is the single most important step in reducing infection

Step 2: Immunisation assessment

• Tetanus: check immunisation status; give booster ± tetanus immunoglobulin as indicated
• Rabies: assess risk based on animal type, location, and animal's vaccination status

Step 3: Antibiotic prophylaxis (not all bites need it)

Prophylaxis is indicated for high-risk wounds:

• Moderate/severe injury
• Bone/joint/tendon involvement
• Hand or face wounds (hand wounds have poor blood supply; facial wounds have cosmetic implications)
• Presence of prosthetic material nearby
• Cat bites (high infection rate)
• Immunocompromised patients, including post-splenectomy (risk of Capnocytophaga canimorsus causing overwhelming sepsis)

Must cover the likely pathogens in the oral flora of the biter ^[4]

Routine antibiotic prophylaxis for IE is NOT recommended. ^[8][9]

The rationale for becoming more conservative:

• Low-grade but repeated bacteraemia occurs more frequently during daily activities (tooth brushing, chewing) than during dental procedures ^[9]
• Risk of IE following dental procedures is very low in studies ^[9]
• Widespread use of prophylactic antibiotics carries risks: antimicrobial resistance, anaphylaxis ^[9]
• The single most important factor in prophylaxis is good dental hygiene! ^[10]

Only the highest-risk cardiac conditions undergoing specific procedures:

Asplenic patients are at risk of overwhelming post-splenectomy infection (OPSI) from encapsulated organisms: Streptococcus pneumoniae ( > 50% of OPSI), Neisseria meningitidis, Haemophilus influenzae type b, Capnocytophaga (dog bites). ^[11][12]

Prevention:

• Pre-operative vaccination (2 weeks before elective splenectomy): Pneumococcal, Meningococcal, H. influenzae type b, Influenza
• Post-operative antibiotic prophylaxis: "pill in the pocket" approach — early self-treatment with augmentin/amoxicillin upon any febrile episode ^[11][12]
• Some guidelines recommend lifelong penicillin prophylaxis, but compliance is poor so not routine in HK ^[11]

Patients with cirrhosis who have had spontaneous bacterial peritonitis (SBP) require long-term prophylaxis with fluoroquinolones (levofloxacin/ciprofloxacin) to prevent recurrence. ^[13]

• NOT routinely prescribed for first episode of UTI ^[14]
• Continuous cotrimoxazole prophylaxis if VUR ≥ Grade 3 ^[14]
• Cefuroxime 10 mg/kg when MCUG is performed ^[14]

1. EMQ / MCQ: Given a clinical scenario, choose the most appropriate prophylactic antibiotic from a list
2. SAQ: "State the 4 principles of surgical antibiotic prophylaxis" or "Classify this wound and state your prophylaxis plan"
3. SAQ: "Distinguish between primary and secondary prevention of rheumatic fever"
4. MCQ: "Which of the following is the most appropriate prophylaxis for a close contact of meningococcal meningitis?"

The 2025 Fourth Summative MCQ directly tested this lecture with 5 EMQ stems ^[3]: