STEPTRACKERMICROBIOLOGY | GRAM-NEGATIVE RODS | HIGH-YIELD REVIEW

Pseudomonas & Legionella

Two water-loving, aerobic gram-negative rods: one infects burns, CF lungs and ventilators; the other comes from air conditioning and gives pneumonia with hyponatremia.

FOCUS ON
Pseudomonas aeruginosa: oxidase ⁺, non-lactose fermenter, blue-green pigment (pyocyanin), grape-like odor, biofilm, exotoxin A (EF-2) → pneumonia in CF/ventilator, burn infections, UTI (catheters), otitis externa ("swimmer's ear", malignant in diabetics), osteomyelitis after nail puncture through sneaker, hot-tub folliculitis, ecthyma gangrenosum in neutropenia. Legionella: silver stain, BCYE agar, urine antigen; high fever, GI symptoms, confusion, hyponatremia → macrolide or fluoroquinolone.
DISEASE
SETTING · VIRULENCE
CLINICAL PICTURE
CLUE · TREATMENT
Pseudomonas pneumonia
CF (mucoid alginate biofilm), ventilator, bronchiectasis
Hospital pneumonia, chronic CF colonization
Antipseudomonal β-lactam + 2nd agent
Burn wound infection
Exotoxin A, elastase
Green-blue exudate, sepsis
Topical silver sulfadiazine
Otitis externa
Moist ear canal
Swimmer's ear; malignant (necrotizing) OE in elderly diabetics → skull base osteomyelitis
Ciprofloxacin drops · IV for malignant
Osteomyelitis
Nail through rubber-soled shoe; IV drug use
Foot or vertebral osteomyelitis
—
Hot-tub folliculitis
Poorly chlorinated water
Itchy follicular pustules under swimsuit area
Self-limited
Ecthyma gangrenosum
Bacteremia in neutropenia
Painless necrotic black skin ulcers
Febrile neutropenia → cefepime
Keratitis & UTI
Contact lenses · catheters
Corneal ulcer · complicated UTI
—
Antipseudomonal drugs
Intrinsic and acquired resistance (efflux, porins)
—
Piperacillin-tazobactam, cefepime/ceftazidime, carbapenems (not ertapenem), aztreonam, fluoroquinolones (cipro, levo), aminoglycosides, colistin
Legionnaires disease
Contaminated water aerosols (AC cooling towers, showers); facultative intracellular in macrophages; no person-to-person spread
Older smokers: high fever, pneumonia, diarrhea, confusion, hyponatremia, ↑ LFTs
Urine antigen (serogroup 1), BCYE culture · azithromycin or levofloxacin
Pontiac fever
Same exposure
Mild flu-like illness, no pneumonia
Self-limited
Burkholderia cepacia
Pseudomonas relative
Severe lung infection in CF and CGD
Very resistant; may bar lung transplant

Mechanism — Pseudomonas in the CF lung

Thick, dehydrated mucusCFTR defect
→
Pseudomonas colonizes
→
Mucoid alginate biofilmResists drugs and phagocytes
→
Chronic inflammationBronchiectasis

Pseudomonas recognition

CLUE
MEANING
SETTING
Blue-green pigment
Pyocyanin, pyoverdine
Burns, wounds
Grape/corn-tortilla odor
Aminoacetophenone
Culture plates
Oxidase ⁺, non-fermenter
Lab ID
MacConkey colorless
Aerobic, water-loving
Hospital reservoirs
Ventilators, sinks

How it's tested

COMMON CONFUSIONAtypical pneumonia with prominent GI symptoms, confusion and hyponatremia is Legionella; Mycoplasma has a milder course with cold agglutinins.
THE TRAPLegionella doesn't Gram stain well and won't grow on routine media — order urine antigen and BCYE culture.
YOUR APPROACHHost (CF, burn, neutropenia, diabetic ear, ventilator) → Pseudomonas → antipseudomonal choice. Water aerosol + hyponatremia → Legionella.

If you see… think…

Blue-green pus in a burn wound → Pseudomonas
Elderly diabetic with severe ear pain and granulation tissue → malignant otitis externa
Foot osteomyelitis after a nail through a sneaker → Pseudomonas
Rash under the swimsuit after a hot tub → Pseudomonas folliculitis
Black necrotic skin lesions in a neutropenic patient → ecthyma gangrenosum
CF patient with mucoid colonies → Pseudomonas (alginate)
Pneumonia, diarrhea, confusion, sodium 128 after hotel stay → Legionella
Best test for Legionella → urine antigen
Medium for Legionella → buffered charcoal yeast extract (iron, cysteine)
CF/CGD patient with a resistant Pseudomonas relative → Burkholderia cepacia