Pseudomonas aeruginosa is a significant pathogen responsible for many infections, particularly in immunocompromised individuals and those with chronic lung diseases. Understanding its characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Pseudomonas. Continuous surveillance and research into antibiotic resistance are necessary to address the challenges posed by this versatile and resilient bacterium effectively.
General Character
- Genus: Pseudomonas
- Species: Pseudomonas aeruginosa
- Family: Pseudomonadaceae
- Gram Staining: Pseudomonas species are Gram-negative bacteria, appearing pink due to their thin peptidoglycan layer and outer membrane.
- Shape and Arrangement:
- Shape: Rod-shaped (bacilli).
- Arrangement: Typically found as single cells but can form short chains.
- Oxygen Requirements: Pseudomonas species are obligate aerobes but can grow anaerobically using nitrate as a terminal electron acceptor.
Morphology
- Cell Wall Structure:
- It comprises a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharides (LPS), contributing to virulence.
- Flagella: Pseudomonas aeruginosa is motile and has polar flagella, allowing for swimming motility.
Cultural Characteristics
- Growth Media:
- MacConkey Agar: Selective for Gram-negative bacteria; P. aeruginosa typically appears as colourless colonies due to non-lactose fermentation.
- Cetrimide Agar: Selective for P. aeruginosa; produces greenish colonies due to pyocyanin production.
- Nutrient Agar: Grows well and produces a characteristic green pigment.
- Colony Appearance:
- On nutrient media, colonies may appear green or blue-green due to the production of pigments like pyocyanin and pyoverdine.
- Temperature and pH Range:
- The optimal growth temperature is around 37°C, with a favourable pH range of 6.0 to 8.0.
Biochemical Reactions
- Catalase Test: Pseudomonas is catalase-positive.
- Oxidase Test: Pseudomonas is oxidase-positive, which helps differentiate it from Enterobacteriaceae.
- Lactose Fermentation: Pseudomonas does not ferment lactose.
- Indole Production: P. aeruginosa is typically indole-negative.
- Methyl Red Test: Negative.
- Voges-Proskauer Test: Negative.
Pathogenicity
- Virulence Factors:
- Biofilm Formation: P. aeruginosa can form biofilms on surfaces, enhancing its ability to cause chronic infections.
- Exotoxins: Produces various exotoxins, including exotoxin A, which inhibits protein synthesis in host cells.
- Enzymes: Produce a range of enzymes (e.g., elastase, phospholipase C) that damage tissues and promote invasion.
- Clinical Infections:
- Respiratory Infections: Common in cystic fibrosis patients, leading to chronic lung infections.
- Wound Infections: These can cause infections in burns and surgical wounds.
- Urinary Tract Infections (UTIs): Often seen in individuals with catheters or urinary tract abnormalities.
- Bacteremia: This can lead to sepsis, particularly in immunocompromised patients.
- Ear Infections: Can cause otitis externa, especially in swimmers (often called “swimmer’s ear”).
Laboratory Diagnosis
- Specimen Collection: Clinical specimens may include sputum, blood, urine, or wound samples.
- Microscopic Examination:
- Gram staining reveals Gram-negative bacilli.
- Culture Techniques:
- Inoculation on selective media like Cetrimide or MacConkey agar, followed by incubation.
- Biochemical Testing:
- Confirmatory tests for oxidase positivity, lactose fermentation, and other biochemical reactions to identify Pseudomonas species.
- Molecular Methods: PCR and other nucleic acid amplification techniques can rapidly identify and type Pseudomonas strains.
Antibiotic Resistance
- Emergence of Resistance: Pseudomonas aeruginosa is known for its intrinsic and acquired resistance to multiple antibiotics.
- Multidrug-Resistant Strains: Resistance to beta-lactams, aminoglycosides, and fluoroquinolones is common, making treatment challenging.
- Treatment Options: Antibiotic susceptibility testing is crucial; treatment may involve combinations of antibiotics, including piperacillin-tazobactam, ceftazidime, or carbapenems, depending on resistance profiles.
Prevention
- Infection Control Practices: Good hygiene and infection control measures in healthcare settings are vital to prevent Pseudomonas infections.
- Environmental Control: Regular cleaning and disinfection of surfaces in hospitals, particularly in areas with high risk of infection (e.g., ICUs).
- Monitoring: Surveillance for antibiotic resistance patterns and outbreak investigations are essential for effective public health management.