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Gram-negativeBacilliNon-fermenters

Pseudomonas aeruginosa

Obligate aerobic, oxidase-positive, non-fermenting Gram-negative bacillus that is a leading cause of nosocomial infections, particularly ventilator-associated pneumonia, burn wound infections, and opportunistic infections in immunocompromised and cystic fibrosis patients.

Organism Card

DomainMust know
Identity
  • G−ve bacillus (rod), non-fermenter [1]
  • Obligate aerobe; motile (single polar flagellum)
  • Family Pseudomonadaceae; NOT Enterobacteriaceae
  • Produces blue-green pigment (pyocyanin + pyoverdin)
Lab discriminator
  • Oxidase positive (vs Enterobacteriaceae which are oxidase −ve) [1]
  • Lactose non-fermenter on MacConkey agar (colourless colonies)
  • Grape-like / sweet fruity odour on culture plates
  • Blue-green pigment on Mueller-Hinton agar is classic
  • Grows at 42°C (vs most other non-fermenters)
Reservoir / transmission
  • Ubiquitous in moist environments: water, soil, sinks, ventilators, humidifiers
  • Hospital-acquired: introduced by hands of medical personnel, diagnostic/therapeutic procedures [2]
  • Colonises patients during hospitalisation, especially after broad-spectrum Abx suppress normal anaerobic flora [2]
  • Community: hot tubs, contact lens solutions, contaminated ear-piercing equipment
Key virulence
  • Exotoxin A → inhibits EF-2 (same mechanism as diphtheria toxin) → tissue necrosis
  • Pyocyanin → generates ROS, impairs ciliary function, damages epithelium
  • Elastase + alkaline protease → tissue destruction, corneal ulceration
  • Alginate (mucoid exopolysaccharide) → biofilm formation on devices & in CF lungs
  • Biofilm on devices (catheters, ETT) resists eradication without device removal [2]
  • Type III secretion system → injects cytotoxins directly into host cells
  • Endotoxin (lipid A / LPS) → septic shock
Clinical syndromes
  • Nosocomial pneumonia / VAP — common cause; variable presentation, no specific features [3][4]
  • Neutropenic fever / sepsis — important G−ve cause (with E. coli) post-chemotherapy [2][5]
  • Bronchiectasis acute exacerbation — especially if long-standing disease [4][6]
  • Nosocomial / complicated UTI — catheter-associated [7]
  • CAPD peritonitis — G−ve cause; requires catheter removal if refractory [8]
  • Burns & wound infection — classic G−ve pathogen in burn units
  • Otitis externa — "swimmer's ear"; malignant otitis externa in diabetic/elderly
  • Keratitis — contact lens wearers; rapidly destructive
  • Septic arthritis in IVDU [9]
  • Ecthyma gangrenosum — necrotic skin lesion virtually pathognomonic in neutropenic patients
  • Endocarditis in IVDU — right-sided; cover in critically ill IVDU IE [10]
Diagnosis
  • Specimen: sputum / BAL, blood culture, wound swab, urine, PD effluent as appropriate
  • Gram stain: G−ve rods (non-specific morphology)
  • Culture on MacConkey: NLF colonies; oxidase +ve confirms non-fermenter
  • Colonisation is common — isolation in respiratory specimens must be correlated with clinical situation [3]
  • Antibiotic susceptibility testing essential (high resistance rates)
Treatment
  • Anti-pseudomonal agents [3][4][6]:
  • (1) Anti-pseudomonal BL/BLI: piperacillin-tazobactam (Tazocin), ticarcillin-clavulanate (Timentin)
  • (2) Selected cephalosporins: ceftazidime (Fortum), cefepime, cefoperazone [3]
  • (3) Fluoroquinolones: ciprofloxacin > levofloxacin for Pseudomonas [3]
  • (4) Carbapenems: imipenem, meropenem (NOT ertapenem) [3]
  • (5) Aminoglycosides: amikacin, gentamicin
  • Single agent generally suffices; start DUAL therapy in severe infections [3]
  • If Pseudomonas likely in sepsis: Vancomycin + TWO anti-pseudomonal agents from different classes [11]
  • MRPA (multi-resistant P. aeruginosa): may need colistin, ceftazidime-avibactam (Zavicefta), ceftolozane-tazobactam (Zerbaxa), cefiderocol [3]
  • Bronchiectasis with chronic Pseudomonas: consider inhaled anti-pseudomonal agents
Prevention
  • Infection control: hand hygiene, contact precautions for MDR strains [2]
  • Antimicrobial stewardship — avoid unnecessary broad-spectrum Abx that suppress anaerobic flora and promote colonisation [2]
  • Device removal if catheter-related / PD catheter infection refractory to Abx [2][8]
  • No vaccine in routine clinical use
  • Ceftazidime-resistant Pseudomonas is a notifiable MDR organism in HK public hospitals [12]
Classic traps
  • Oxidase +ve separates Pseudomonas from Enterobacteriaceae (exam favourite)
  • Ertapenem has NO anti-pseudomonal activity — trap MCQ answer [3]
  • Colonisation ≠ infection — do not treat sputum isolate without clinical correlation [3]
  • ESBL-producing E. coli (NOT Pseudomonas) is the most common MDR organism in HK public hospitals [12]
  • Ecthyma gangrenosum + neutropenia = Pseudomonas until proven otherwise
  • Malignant otitis externa in elderly diabetic = Pseudomonas (not S. aureus)
  • Contact lens keratitis = Pseudomonas (not Acanthamoeba, which is subacute)

Exam Intelligence

  1. GC 102 (Fever after chemotherapy): Pseudomonas aeruginosa is highlighted as a hospital-acquired coloniser that establishes after broad-spectrum antibiotics suppress anaerobic normal flora. It forms biofilm on devices (vascular catheters, ETT, Foley's) that is difficult to eradicate without device removal [2]. This is a key concept for neutropenic fever management questions.

  2. GC 052 (Fever and purulent sputum): In bronchiectasis exacerbations, Pseudomonas is specifically named as a pathogen in long-standing disease. Anti-pseudomonal antibiotics listed: piperacillin-tazobactam, ceftazidime, carbapenems (meropenem), aminoglycosides (amikacin), fluoroquinolones (levofloxacin) [4].

  3. Senior notes (Ryan Ho Respiratory): Colonisation in respiratory specimens is common and must be clinically correlated before treating. Ciprofloxacin is better than levofloxacin for Pseudomonas. Ertapenem is the one carbapenem with NO anti-pseudomonal activity. Single agent generally suffices but dual therapy for severe infections. MRPA may need colistin, Zavicefta, Zerbaxa, or cefiderocol [3].

  4. GC 102: During hospitalisation, Pseudomonas is introduced by hands of medical personnel and various diagnostic/therapeutic procedures — this underpins infection control questions [2].

  5. HA guidelines (via Adrian Lui / senior notes): For hospitalised patients with chronic lung disease (bronchiectasis), use anti-pseudomonal antibiotics (e.g. piperacillin-tazobactam or cefepime) ± macrolide or doxycycline — this is distinct from standard CAP empirical therapy [6].

  6. Felix Lai (Medicine/Paeds): Pseudomonas pneumonia is characterised by severe and progressive SOB. Risk factors: bronchiectasis, repeated Abx/steroid courses, structural lung disease including COPD, immunocompromised (neutropenia, HIV, transplant) [7].

  • Oxidase test: Pseudomonas is oxidase +ve; Acinetobacter baumannii is oxidase −ve (both are non-fermenters, both cause nosocomial infections — oxidase is the key discriminator).
  • Ertapenem trap: Students commonly list "carbapenems" as anti-pseudomonal — but ertapenem specifically lacks Pseudomonas coverage. Exam favourite wrong answer.
  • ESBL E. coli vs ceftazidime-resistant Pseudomonas: The 2020 MCQ Q83 tested which is the MOST COMMON MDR organism in HK public hospitals — answer is ESBL-producing E. coli, NOT Pseudomonas [12].
  • Colonisation vs infection: Isolating Pseudomonas from sputum in a ventilated patient does NOT automatically mean pneumonia — clinical, radiological, and biochemical correlation required.
  • Ecthyma gangrenosum: Black necrotic skin lesion in a febrile neutropenic patient → think Pseudomonas bacteraemia (NOT fungal until later in course).
  • Malignant otitis externa: Elderly diabetic with severe ear pain, granulation tissue in ear canal, cranial nerve palsies → Pseudomonas (not typical otitis externa pathogens).
  • Anti-pseudomonal cephalosporins: Not all cephalosporins cover Pseudomonas. Ceftriaxone and cefotaxime do NOT. Ceftazidime and cefepime DO.
  • Septic arthritis: In IVDU, think Pseudomonas (and S. aureus) rather than typical community pathogens [9].

2020 Fourth Summative MCQ Q83 [12]:

"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"

Correct answer: C. ESBL-producing E. coli. Rationale: Despite Pseudomonas and MRSA being well-known MDR organisms, ESBL-producing E. coli is the most common MDR organism in HK public hospitals. This is a direct lecture-derived epidemiological fact. Ceftazidime-resistant Pseudomonas (option B) is a distractor — it is important but less prevalent than ESBL E. coli in HK surveillance data.

2020 Fourth Summative MCQ Q82 [12]:

"What is MORE LIKELY to be the microbial cause of neutropenic fever at day 10 post-chemotherapy? A. Aspergillus fumigatus B. Candida albicans C. Cytomegalovirus D. Streptococcus viridans"

Correct answer: D. Streptococcus viridans (for day 10 = early neutropenia; bacterial causes predominate). Relevance to Pseudomonas: Although Pseudomonas is not an answer option here, it is a key G−ve cause of neutropenic fever alongside E. coli [2][5]. Fungal causes (Aspergillus, Candida) become more important in prolonged neutropenia ( > 7–14 days without response). This question tests the timing concept from GC 102.

No other past paper questions in the indexed context directly name Pseudomonas aeruginosa in their stems.

[1] Lecture slides: GC 101. Diagnosis of infections [Handouts].pdf [2] Lecture slides: GC 102. Fever after chemotherapy infections in immunocompromised hosts [Handout].pdf [3] Senior notes: Ryan Ho Respiratory.pdf [4] Lecture slides: GC 052. Fever and purulent sputum.pdf [5] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf [6] Senior notes: Adrian Lui Pediatrics Notes.pdf [7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf [8] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf [9] Senior notes: Ryan Ho Rheumatology.pdf [10] Senior notes: Ryan Ho Cardiology.pdf [11] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (sepsis regimen section) [12] Past papers: 2020 Fourth Summative Assessment MCQ paper.pdf

Yersinia enterocolitica

Gram-negative, facultatively anaerobic coccobacillus of the family Yersiniaceae that causes enterocolitis, mesenteric lymphadenitis, and pseudoappendicitis, particularly in children.

Acinetobacter baumannii

Aerobic, non-fermenting Gram-negative coccobacillus that is a leading cause of multidrug-resistant nosocomial infections, particularly ventilator-associated pneumonia, bloodstream infections, and wound infections in critically ill and immunocompromised patients.

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