FRCS Surgical Microbiology & Immunology 1 — Questions and Answers
Question 1: Which organism is the most common cause of surgical site infections?
- Staphylococcus aureus (Correct answer)
- Escherichia coli
- Pseudomonas aeruginosa
- Bacteroides fragilis
Correct answer: Staphylococcus aureus
Staphylococcus aureus, including MRSA, is the most frequent causative organism in surgical site infections due to its prevalence on skin and in nasal carriage.
Question 2: Which type of hypersensitivity reaction is responsible for anaphylaxis?
- Type II (cytotoxic)
- Type III (immune complex)
- Type I (IgE-mediated) (Correct answer)
- Type IV (delayed)
Correct answer: Type I (IgE-mediated)
Anaphylaxis is a Type I (immediate) hypersensitivity reaction mediated by IgE antibodies bound to mast cells and basophils, triggering rapid degranulation.
Question 3: What is the mechanism of action of beta-lactam antibiotics?
- Inhibition of bacterial protein synthesis at the 30S ribosome
- Inhibition of bacterial cell wall synthesis by binding penicillin-binding proteins (Correct answer)
- Disruption of the bacterial cell membrane
- Inhibition of bacterial DNA gyrase
Correct answer: Inhibition of bacterial cell wall synthesis by binding penicillin-binding proteins
Beta-lactam antibiotics inhibit cell wall synthesis by covalently binding to penicillin-binding proteins (PBPs), preventing cross-linking of peptidoglycan chains.
Question 4: Which Clostridium species is the primary causative organism of gas gangrene (clostridial myonecrosis)?
- Clostridium difficile
- Clostridium perfringens (Correct answer)
- Clostridium tetani
- Clostridium botulinum
Correct answer: Clostridium perfringens
Clostridium perfringens produces alpha-toxin (phospholipase C), which destroys cell membranes and is the primary virulence factor in gas gangrene.
Question 5: Which cells are the primary effector cells of innate immunity?
- B lymphocytes and plasma cells
- CD4+ and CD8+ T lymphocytes
- Neutrophils and macrophages (Correct answer)
- Natural killer T cells exclusively
Correct answer: Neutrophils and macrophages
Neutrophils and macrophages are the principal effector cells of innate immunity, providing immediate non-specific defence through phagocytosis and release of inflammatory mediators.
Question 6: What is the precise definition of bacteraemia?
- Bacteria multiplying in the bloodstream with associated systemic inflammatory signs
- The presence of viable bacteria in the bloodstream (Correct answer)
- Bacterial endotoxins circulating in the bloodstream
- A positive urine culture with more than 10^5 organisms/mL
Correct answer: The presence of viable bacteria in the bloodstream
Bacteraemia is defined as the presence of viable bacteria in the bloodstream, which may be transient (e.g. following a dental procedure) or persistent.
Question 7: Which sterilisation method is most appropriate for heat-sensitive surgical equipment such as fibreoptic endoscopes?
- Autoclaving at 134°C for 3 minutes
- Dry heat oven at 160°C for 2 hours
- Ethylene oxide (ETO) gas sterilisation (Correct answer)
- Glutaraldehyde high-level disinfection only
Correct answer: Ethylene oxide (ETO) gas sterilisation
Ethylene oxide gas achieves sterilisation at low temperatures (37–63°C) and is therefore suitable for heat-sensitive equipment that would be damaged by autoclaving.
FRCS Surgical Microbiology & Immunology 1
Which organism is the most common cause of surgical site infections?