Pseudomonas Aeruginosa

Pseudomonas Aeruginosa

Subjects: Bacteriology · Systems: Microbiology · Tags: Microbiology

Microbiology basics

It is a gram-negative rod, aerobic, and motile (polar flagella). It is oxidase-positive, catalase-positive, non-lactose fermenter (this is an important in lab ID). It is an obligate aerobe (can respire anaerobically with nitrate). Its pigments are pyocyanin (blue-green) and pyoverdine (yellow-green, fluorescent under UV). These give infected wounds/secretions a blue-green color. IT has a grape-like characteristic odor.

It also survives in water, soil, hospital sinks, ventilators, cathethers. It is very hardy and a major nosocomial pathogen.

Virulence factors

Pseudomonas is a master of survival and tissue damage.

Exotoxin A

• ADP-ribosylates EF-2, same mechanism as diphteria toxin. Inhibits host protein synthesis, which leads to cell death. Major factor in necrosis, tissue destrucion.

Elastases (LasA, LasB)

• Degrade elastin, immunoglobulins, complement system, which leads to tissue damage and hemomrrage

Phospholipase C

• Hemolysis, tissue damage.

Pyocyanin

• Generates reactive oxygen species (ROS), damages tissues, interferes with ciliary funciton.

  Biofilm formation (especially in CF patients)

  • Protects against antibiotics and immune system

Type III secretion system (T3SS)

• injects toxins (ExoS, ExoU, ExoT, ExoY) directly into host cells -> cytotoxicity, impaired phagocytosis.

Clinical syndromes

Mnemonic: PSEUDO infections.

• Pneumonia
  • Ventilator-associated pneumonia (VAP), hospital acquired pneumonia (HAP)
  • In cyscit fibrosis: chronic colonization, biofilms, multidrug resitance, progressive lung decline
• Sepsis/bacteremia
  • High mortality. Characteristic ecthyme gangrenosum = hemorragic necrotic skin lesions in septic patients.
• External otitis
  • Otitis externa
  • Maglinant otitis externa in elderly diabetics -> invasive, life threatening
• UTI
  • Especially in catheterized or hospitalized patients
• Diabetic foot/wound infections, burn wound infections (classic!)
• Ocular infections
  • Keratitis and endophtalmitis, especially in contact lens wearers

Other notes: can cause endocarditis, especially in IV drug users).

Lab diagnosis

• Culture: grows easily on standard agar, produces blue-green pigment and fruity odor. Lactose non-fermenter (distinguish from Enterobacteriaceae) Oxidase positive (differentiates from enterobacteriaceae)

Antibiotic resistance mechanisms

Pseudomonas is notorious for being multidrug resistant:

• Efflux pumps
• Porin mutations
• Beta lactamases (including AmpC and carbapenemases)
• Biofilm formation

Treatment principles

Always check local antibiogram. Resistance is commmon. Monotheerapy may work for less severe infections if the isolate is susceptible. Combination therapy sometimes used for severe infections or empiric coverage until susceptibilities are back.

Active drugs (varies by strain)

• Beta lactams
• Carbapenems (IT IS RESISTANT TO ERTAPENEM)
• Monobactam
• Aminoglycosides
• Fluoroquinolones
• Polymyxins

In cystic fibrosis: inhaled tobramycin or aztreonam is used chronically.

Disclaimer: For education only. Not medical advice; always follow your institution's guidance.