Staphylococcus epidermidis

1. GC 105 — CNS are usually benign skin commensals; single blood culture isolate usually = contamination. Genuine infection considered when CNS isolated from sterile site in patient with prostheses ^[1]. This is the single most-tested conceptual point for S. epidermidis.

2. GC 105 — Expert practice tip: whenever GPC in clusters grows in blood culture, always repeat at least TWO sets before changing antibiotics ^[1]. Examiners love stems where a single culture grows "coagulase-negative Staphylococcus" and the correct action is to repeat cultures, not start vancomycin.

3. GC 102 — S. epidermidis listed as a common pathogen in neutropenic patients (neutrophil dysfunction category) and in device-related infections (implanted vascular catheters); infections are difficult to eradicate without device removal because bacteria form adherent biofilms (glycocalyx) ^[3].

4. AOS Microbio / Gen Clerk Summary — Three examinable CNS species: S. epidermidis (prosthetic/catheter biofilm), S. saprophyticus (CA-UTI in young females), S. lugdunensis (virulent, cousin of S. aureus) ^[2]. Expect MCQ distractors swapping these associations.

5. GC 102 — Antibiotic suppression of normal flora (especially anaerobes) allows colonisation by hospital-acquired resistant organisms; prolonged hospitalisation + devices → CNS bacteraemia ^[3].

6. CFB WCS27 (Surgical Infection) — Line sepsis: common organisms include Staphylococcus; management = re-site line + blood culture from infected line and distant peripheral site + antibiotics ^[10].

• Contamination vs true bacteraemia: A single positive blood culture for CNS is the classic exam trap. ≥ 2 sets positive for the same organism with matching antibiogram = likely true infection. Duke criteria explicitly place CNS in the "organisms more commonly as skin contaminants" category requiring ≥ 3 or majority of ≥ 4 positive cultures ^[5].
• Early vs late prosthetic valve IE: Early ( < 12 months) = coagulase-negative Staphylococcus (classically S. epidermidis); late ( > 12 months) = organisms similar to native valve IE (S. aureus, viridans strep, etc.) ^[4][5].
• CAPD peritonitis organism confusion: Coag-negative Staph is the commonest organism; examiners may list S. aureus, E. coli, or fungi as distractors. Bile-stained effluent → think surgical (secondary) peritonitis, not PD-related ^[6][7][11].
• S. epidermidis vs S. saprophyticus: Both are coagulase-negative. Novobiocin sensitivity separates them (epidermidis = sensitive, saprophyticus = resistant). UTI in a young woman → saprophyticus, NOT epidermidis.
• Methicillin resistance: Most hospital S. epidermidis isolates are methicillin-resistant (carry mecA); this is far more prevalent than MRSA. Therefore vancomycin is typically required for serious infections.

2020 Fourth Summative MCQ Q65 ^[11]:

"A 55-year-old man suffered from end-stage chronic renal failure and was on continuous ambulatory peritoneal dialysis (CAPD). He presented with fever for 4 days. Palpation revealed mild diffuse abdominal tenderness. His white cell count was 18 × 10⁹/L. Which of the following features is suggestive of secondary peritonitis, rather than CAPD-related peritonitis?"

Options: A. Bile-stained effluent; B. Culture of effluent yields coagulase-negative Staphylococcus species; C. Effluent white cell count > 100 cells/mm³; D. Purulent drainage at the peritoneal dialysis catheter exit site.

Answer: A — Bile-stained effluent. Bile-stained fluid indicates surgical pathology (e.g. bowel perforation). Coag-negative Staph (option B) is actually the commonest organism in PD-related peritonitis and does NOT suggest secondary peritonitis. Effluent WCC > 100 (option C) and exit site drainage (option D) are features of CAPD-related peritonitis.

2025 Fourth Summative MCQ Q39 ^[12]:

"A 36-year-old man presented to the hospital for fever which persisted for 2 weeks. He did not have any other symptoms. He has a history of eczema, but otherwise good past health. Physical examination showed small non-tender erythematous macular lesions over both palms. Chest X-ray did not show any abnormal infiltrates. Two sets of blood culture grew Gram-positive cocci in cluster."

"What is the MOST LIKELY clinical diagnosis?" A. Brucellosis; B. Extrapulmonary tuberculosis; C. Infectious mononucleosis; D. Infective endocarditis.

Answer: D — Infective endocarditis. GPC in clusters = Staphylococcus (aureus or CNS). Persistent fever + Janeway lesions (non-tender palmar macules) + 2 sets positive blood culture = IE by Duke criteria. This stem tests recognition of Staphylococcal IE. Note: although S. aureus is the most likely species here (native valve, eczema as portal), the GPC-in-clusters recognition and link to IE is the key discriminator, and the same stem framework applies to prosthetic valve scenarios where S. epidermidis would be the answer.

No other past paper questions in the indexed set directly name S. epidermidis as the answer organism, though the CAPD peritonitis Q65 (2020) and prosthetic valve IE concepts are repeatedly examinable.