Escherichia coli

1. ESBL-producing E. coli is the most common multidrug-resistant organism in Hong Kong public hospitals — this was directly tested in the 2020 MCQ (Q83) and is a core GC 099/106 teaching point. Do NOT answer MRSA; ESBL E. coli has surpassed MRSA in HK ^[8][9].

2. Nitrofurantoin is the first-line empirical treatment for uncomplicated cystitis in a young woman — directly tested in 2025 MCQ EMQ Q17. GC 210 (UTI lecture) and GC 106 (practical antibiotic use) emphasise this. Nitrofurantoin concentrates in urine, avoids collateral resistance, and is ineffective in pyelonephritis (no tissue levels) ^[7][10].

3. SBP is diagnosed by ascitic fluid PMN > 250/mm³, treated empirically with IV 3rd-gen cephalosporin + IV albumin without waiting for culture. SBP prophylaxis = long-term fluoroquinolone. If polymicrobial growth → suspect secondary bacterial peritonitis (i.e., bowel perforation) ^[2][3].

4. EHEC O157:H7 — antibiotics are contraindicated because they increase Shiga toxin release and risk of HUS. Diagnosis is by stool antigen detection for O157:H7 or PCR for Shiga toxin, not standard stool culture alone ^[6].

5. Ciprofloxacin susceptibility of E. coli in HK is approximately 40% by first-isolate-per-patient method — this exact calculation was tested in AOS microbiology material ^[9].

6. E. coli pili determine the level of UTI: Type 1 (FimH) → cystitis; Type P (PapG) → pyelonephritis (80% of isolates). This is a favourite vignette discriminator — "Which virulence factor is associated with upper urinary tract infection?" ^[1].

• E. coli vs Klebsiella in UTI/cholangitis: Both are lactose-fermenting Enterobacteriaceae. Klebsiella is non-motile, heavily encapsulated (mucoid colonies), citrate +, and classically associated with aspiration pneumonia (alcoholics) and liver abscess (DM patients in East Asia, with endogenous endophthalmitis). E. coli is indole +, citrate −.
• E. coli vs Proteus in UTI: Proteus is urease-producing → alkaline urine → struvite (staghorn) calculi. E. coli does produce urease but not as prominently; Proteus swarms on agar.
• SBP vs secondary bacterial peritonitis: SBP = monomicrobial (usually E. coli); secondary = polymicrobial + surgical signs + lack of treatment response.
• HUS from EHEC vs TTP: Both cause MAHA + thrombocytopaenia. HUS = renal predominance, in children after bloody diarrhoea, Shiga toxin. TTP = CNS predominance, ADAMTS13 deficiency, adults. Coomb's test negative in both (except pneumococcal HUS where Coomb's positive) ^[6].
• ESBL vs CRE: ESBL E. coli is resistant to 3rd-gen cephalosporins but susceptible to carbapenems. CRE (carbapenem-resistant Enterobacteriaceae) is resistant to carbapenems → requires colistin / ceftazidime-avibactam. The HKUMed exam tests that ESBL E. coli is the #1 MDR in HK, not CRE.

2020 Fourth Summative MCQ Q83 ^[8]:

"Which of the following is the MOST COMMON multidrug-resistant organism in Hong Kong's public hospitals? A. Carbapenem-resistant Acinetobacter B. Ceftazidime-resistant Pseudomonas aeruginosa C. Extended-spectrum Beta-lactamases-producing Escherichia coli D. Methicillin-resistant Staphylococcus aureus"

Answer: C. ESBL-producing E. coli. This is a core GC teaching point — ESBL E. coli has overtaken MRSA as the most common MDR organism in HK public hospitals. Common trap: students pick MRSA out of habit.

2018 Fourth Summative MCQ Q26–27 (EMQ XII) ^[11]:

Stem list includes "E. Escherichia coli O157:H7" among outbreak pathogens. Q26: "Thirty students… vomiting with no fever approximately one hour after consuming a take-away egg and ham sandwich bought from a street vendor." Q27: "Ten children from a child care centre develop projectile vomiting, diarrhoea and fever in the past week. The first child with symptoms vomited in a classroom…"

Answer Q26: I. Staphylococcus aureus (preformed enterotoxin, rapid onset 1–6 h, vomiting predominant, no fever). Answer Q27: G. Norovirus (person-to-person/vomitus fomite spread, projectile vomiting + diarrhoea + fever, self-limiting). E. coli O157:H7 is the distractor here — it causes bloody diarrhoea after undercooked beef, not rapid-onset vomiting from sandwiches. Recognise when O157:H7 is a wrong answer.

2025 Fourth Summative MCQ EMQ IV Q17 ^[7]:

"Empirical treatment of uncomplicated cystitis in a 30-year-old woman." Options include Nitrofurantoin (H), Cefazolin (B), Cefotaxime (C), Amoxicillin-clavulanic acid (A), etc.

Answer: H. Nitrofurantoin. First-line for uncomplicated lower UTI (most commonly caused by E. coli). Concentrates in urine; minimal systemic resistance selection. Not appropriate for pyelonephritis (poor tissue penetration).

AOS Microbio Q7 ^[9]:

"What should be the cumulative ciprofloxacin susceptibility rate of Escherichia coli using the first isolate per patient method? A. 2/19 = 10.5% B. 5/19 = 26.3% C. 2/7 = 28.6% D. 2/5 = 40.0%"

Answer: D. 2/5 = 40.0%. Only the first isolate from the first admission per patient is counted. This demonstrates E. coli's poor fluoroquinolone susceptibility in HK — a key reason nitrofurantoin is preferred for uncomplicated cystitis.

2022 Fourth Summative MCQ EMQ VV Q19–21 ^[12]:

E. coli is listed as option E. Q20 stem: "A 45-year-old man with AML… severe and prolonged neutropenia… breakthrough fever and worsening oral mucositis. Blood culture was positive."

Answer Q20: G. Streptococcus mitis (not E. coli). Viridans streptococci cause breakthrough bacteraemia with mucositis in neutropenic patients on meropenem (meropenem covers G−ve but not viridans strep well enough when mucosal barrier is disrupted). E. coli is a distractor — it would be covered by meropenem.

2021 Fourth Summative SAQ Q12(a) ^[13]:

"He developed high fever on day 10 after conditioning… neutrophil count < 0.1. List common microbes which could be found in his blood culture."

Expected answer includes: G−ve bacilli (E. coli, Klebsiella, Pseudomonas), viridans streptococci, coagulase-negative Staphylococci. E. coli is a standard answer for neutropenic fever blood culture organisms.