Pseudomonas Aeruginosa: Characteristics and Infections

• Pseudomonads are Gram-negative, motile, aerobic rods that produce water-soluble pigments
• They are found in soil, water, plants, and animals
• P. aeruginosa is often present in small numbers in the normal intestinal flora and on the skin of humans
• P. aeruginosa is the major pathogen of the group, while other pseudomonads less frequently cause disease
• Only a few people carry P. aeruginosa in the general population, but this can increase to over 30% after hospitalization
• The invasive potential of P. aeruginosa enables it to cause various hospital infections, especially in neutropenic patients, causing fulminant septicemia and death
• Patients on artificial ventilation for extended periods in intensive therapy units are prone to P. aeruginosa colonization and secondary lower respiratory tract infections
• Extensive burns can lead to colonization with P. aeruginosa and subsequent septicemia
• Optical solutions contaminated with P. aeruginosa can cause rapid, progressive corneal infections, potentially leading to ocular perforation
• P. aeruginosa is a significant pathogen for cystic fibrosis patients, invariably leading to colonization
• Healthy individuals exposed to high doses, such as deep-sea divers and users of contaminated hydrotherapy pools and Jacuzzis, may develop skin infections

Pseudomonas aeruginosa

• Widely distributed in nature and frequently found in moist hospital environments
• It can colonize normal humans as a saprophyte
• Causes disease in those with abnormal host defenses, especially individuals with neutropenia
• There are over 100 species within the Pseudomonas genus, with P. pseudomallei and P. mallei being the two primary pathogens

Morphology and Identification

• P. aeruginosa is motile (except P. mallei) and rod-shaped, measuring about 0.6 ×2 µm
• It is Gram-negative, found as single bacteria, in pairs, and sometimes in short chains
• As an obligate aerobe, it needs oxygen for growth

Culture of P. aeruginosa

• An obligate aerobe that readily grows on various culture media
• Some strains produce a sweet, grape-like, or corn taco-like odor and hemolyze blood
• Colonies are smooth and round, producing a fluorescent greenish color due to pyoverdin
• It often produces a non-fluorescent bluish pigment called pyocyanin that diffuses into the agar
• Other Pseudomonas species do not produce pyocyanin
• Some strains produce the dark red pigment pyorubin, or the black pigment pyomelanin

Growth Characteristics

• P. aeruginosa grows well at 37-42°C, with growth at 42°C useful for differentiation
• It is oxidase positive, does not ferment carbohydrates, but can oxidize glucose

Antigenic Structure and Toxins:

• Pili facilitate adherence to epithelial cells
• Exopolysaccharide demonstrates anti-phagocytic properties/ inhibits (LPS)
• Lipopolysaccharide induces endotoxic effects on Enzymes
• Elastases digest protein (elastin, collagen, IgG)
• Proteases are present
• Hemolysins appear
• Phospholipases C (heat labile) degrades cytoplasmic membrane components
• Exotoxin A is cytotoxic, inhibiting protein synthesis
• Endotoxin, similar to other gram-negative bacteria, causes symptoms related to sepsis and septic shock

Pathogenesis

• Pseudomonas aeruginosa is primarily an opportunistic pathogen that causes infections in hospitalized patients, particularly those with extensive burns

• Patients with chronic respiratory diseases such as cystic fibrosis, experience impaired clearance mechanisms

• Immunosuppressed individuals are more susceptible

• Conditions caused include: chronic, complicated urinary tract infections associated with indwelling catheters, wound infections of burn sites, pressure sores and ulcers, and Septicaemia manifesting as “Ecthyma gangrenosum” skin lesion (haemorrhagic skin necrosis)

• Malignant external ear infections (Otitis externa) in poorly managed diabetic patients can occur

• Infection (Pneumonia) of the lung is seen in patients with cystic fibrosis

• Eye infections can result as secondary conditions from trauma or surgery

Laboratory diagnosis

• Isolation of Pseudomonas genus bacteria is easily achieved on simple media such as nutrient or blood agar
• Bacteria will grow on less inhibitory selective media like MacConkey
• Specimen types include: pus, urine, sputum, blood, eye swabs, and surface swabs
• Smears show Gram-negative rods
• Pseudomonas pseudomallei generally isolated from sputum, blood, or pus from abscesses

Culture

• Obligate aerobe, that grows on all routine media over a wide range of temperature (5-42 °C)

• Colonies are Bluish-green pigmented and large with a characteristic “fruity” odor

• Media can be made selective for Pseudomonas using antibiotics or disinfectants they are naturally resistant to, such as irgasin, cetrimide or nalidixic acid

• P. aeruginosa colonies are morphologically diverse, appearing as dwarf, rough, mucoid, rugose, coliform-like, convex, flat, and oval

• P. aeruginosa cultures have a characteristic musty odor

• The colonies of P. pseudomallei and P. mallei are slower to appear and often wrinkled with a faint pinkish colour after about five days

• P. aeruginosa exhibits lactose and fructose oxidation, arginine dihydrolase, gelatinase and lysine decarboxylase

• In Centrimide agar, Pseudomonas aeruginosa colonies (greenish-blue in color) are medium sized and characterized by an irregular growth

• In blood agar, Pseudomonas aeruginosa colonies display a wide zone of beta-hemolysis and cultivate in 48 hours in an aerobic atmosphere at 37°C

• Pseudomonas aeruginosa may or may not produce characteristic blue-green pigment

• Biochemical reactions: oxidase-positive, catalase-positive, citrate-positive, indole-negative

• It produces acid from carbohydrate by oxidation, not by fermentation.

Acinetobacter

• Acinetobacter species are aerobic, Gram-negative bacteria widely found in soil, water, and hospitals, with A. baumannii being most commonly isolated
• Acinetobacter lwoffii and other species are isolated occasionally.

Morphology and Identification

• Acinetobacters are usually coccobacillary or coccal in appearance, resembling neisseriae on smears
• Diplococcal forms predominant in body fluids and on solid media, rod-shaped forms also occur
• Bacteria sometimes appear to be Gram-positive

Culture

• Acinetobacter grows well on most types of media
• Acinetobacter found in meningitis, bacteremia, from the female genital area, sputum, skin, pleural fluid, and urine