PSEUDOMONAS

Pseudomonas

Pseudomonas is a genus of gram-negative bacteria, with Pseudomonas aeruginosa being the most clinically significant species. It is a versatile pathogen known for causing infections in immunocompromised patients, particularly in healthcare settings. Pseudomonas aeruginosa is notorious for its intrinsic resistance to many antibiotics and its ability to acquire further resistance, making infections difficult to treat.

Microscopic Appearance

• Gram-negative rods.

• Motile with polar flagella.

• Non-spore-forming.

• Appears as straight or slightly curved rods under the microscope.

Virulence Factors

• Exotoxins:

  • Exotoxin A: Inhibits protein synthesis by ADP-ribosylating elongation factor-2, leading to cell death.

  • Exoenzyme S: Disrupts host cell signaling and contributes to tissue damage.

• Alginate: A polysaccharide that forms a protective biofilm, particularly in the lungs of cystic fibrosis patients, protecting the bacteria from the immune system and antibiotics.

• Elastase: Degrades elastin, collagen, and immunoglobulins, contributing to tissue invasion and immune evasion.

• Pyocyanin: A blue-green pigment that generates reactive oxygen species, causing oxidative stress in host tissues.

• Efflux Pumps: Actively pump out antibiotics, contributing to the bacterium's resistance profile.

• Type III Secretion System: Injects toxins directly into host cells, disrupting cellular functions and aiding in immune evasion.

Pathogenesis

• Opportunistic Infections:

  • Pseudomonas aeruginosa primarily affects individuals with weakened immune systems, including those with cystic fibrosis, burns, cancer, or neutropenia.

  • It can infect various tissues, but it most commonly affects the lungs, urinary tract, and skin.

• Biofilm Formation:

  • The bacteria can form biofilms on medical devices such as catheters, ventilators, and indwelling tubes, making infections chronic and resistant to treatment.

• Tissue Invasion:

  • The combination of exotoxins, proteases, and elastases allows Pseudomonas to invade and damage host tissues, leading to severe infections.

Clinical Manifestations

• Pulmonary Infections:

  • Causes pneumonia, particularly in patients with cystic fibrosis or those on mechanical ventilation. Symptoms include cough, fever, difficulty breathing, and production of purulent sputum.

• Skin and Soft Tissue Infections:

  • Common in burn patients, leading to wound infections characterized by blue-green pus, pain, and delayed healing.

  • Can cause folliculitis, often associated with exposure to contaminated water, such as in hot tubs ("hot tub folliculitis").

• Urinary Tract Infections (UTIs):

  • Associated with catheter use or structural abnormalities of the urinary tract, leading to symptoms like dysuria, frequency, and fever.

• Bacteremia and Sepsis:

  • Particularly dangerous in immunocompromised individuals, presenting with fever, hypotension, and multi-organ failure.

• Otitis Externa:

  • Known as "swimmer's ear," it is an infection of the outer ear canal, characterized by pain, swelling, and discharge.

Lab Diagnosis

• Culture:

  • Pseudomonas aeruginosa grows well on standard laboratory media such as blood agar and MacConkey agar.

  • Produces characteristic blue-green (pyocyanin) and yellow-green (pyoverdine) pigments, which diffuse into the agar, giving colonies a distinct appearance.

  • Has a fruity or grape-like odor.

  • grows well at 25 C to 37 C, but can grow slowly or at least survive at higher and lower temperatures.

• Biochemical Tests:

  • Oxidase positive.

  • Lactose negative on MacConkey agar.

  • Aerobic, utilizing a variety of carbon sources, including glucose, in oxidative metabolism.

• Antibiotic Sensitivity Testing:

  • Crucial due to the high level of intrinsic and acquired resistance. Testing guides appropriate antibiotic therapy.

Treatment

• Pulmonary Infections:

  • Requires combination therapy with antibiotics such as piperacillin-tazobactam, ceftazidime, or cefepime, often combined with aminoglycosides (e.g., tobramycin) or fluoroquinolones (e.g., ciprofloxacin).

  • In cystic fibrosis patients, inhaled antibiotics like tobramycin may be used to reduce bacterial load.

• Skin and Soft Tissue Infections:

  • Debridement of necrotic tissue and topical or systemic antibiotics guided by culture sensitivity.

  • Burn wound infections may require systemic antibiotics such as piperacillin-tazobactam or meropenem.

• UTIs:

  • Treated with antibiotics like ciprofloxacin or levofloxacin, depending on the susceptibility pattern.

  • Removal or replacement of urinary catheters is often necessary.

• Bacteremia and Sepsis:

  • Treated with broad-spectrum antibiotics, often a combination of a beta-lactam (e.g., ceftazidime, piperacillin-tazobactam) and an aminoglycoside or fluoroquinolone.

  • Supportive care for sepsis, including fluid resuscitation and vasopressors, may be necessary.

• Otitis Externa:

  • Managed with topical antibiotics (e.g., ciprofloxacin ear drops) and debridement of the ear canal.

• Preventive Measures:

  • Strict infection control practices, including hand hygiene and sterilization of medical equipment, are essential in healthcare settings to prevent Pseudomonas infections.

  • In patients with cystic fibrosis, regular monitoring and prophylactic treatments can help manage chronic lung infections.