Pseudomonas aeruginosa: Morphology, Biochemical reaction, Pathogenesis

• Pseudomonads are a large group of aerobic, Gram-negative, non-fermentative bacilli.
• This contains over 200 species that were originally contained within the genus Pseudomonas.
• Some of them produce water soluble pigments which get diffused through the culture medium.
• The members are generally saprophytic in nature.
• They are widely distributed in different habitat like in soil, water, sewage, or where decomposing organic matter is found.
• New genera have also been allocated based on the molecular analysis (RNA/DNA homology) which includes Burkholderia, Stenotrophomonas, Comamonas and Brevundimonas.
• Despite the many genera of pseudomonads, only a few species have been isolated which are common.
• Among them, P. aeruginosa is the most important human pathogen.
• Besides this, Burkholderia mallei, Burkholderia cepacia and Stenotrophomonas maltophilia are also frequently isolated species from humans.
• These saprophytic bacteria can behave opportunistically in debilitated or immune-compromised patients.
• Human carriage of P. aeruginosa is uncommon as part of the normal microbial flora unless the person is hospitalized or an immune-compromised.
• They frequently colonize in the gastrointestinal tract followed by other moist body sites like throat, nasal mucosa, axillae and perineum.

Morphology

• It is a slender, 1.5-3µm × 0.5µm, non-sporing, non-capsulated, Gram-negative, motile bacillus.
• It has one polar flagellum that provides motility to it.
• Occasionally, some of the strains may have two or three flagella as well.
• Most of the strains bear fimbriae.
• Though it is non-capsulated, some strains are present that may show alginate capsule.

Culture

• It is primarily aerobic but can grow anaerobically also.
• It needs standard laboratory media to grow such as nutrient agar, 5% sheep blood or chocolate agar, and MacConkey agar media.
• Selective media is used when this organism is to be isolated from the specimens with a mixed flora.
• MacConkey agar is a useful selective media for isolation of most of its species.
• Cetrimide, acetamide and nitrofurantoin is added to culture medium (e.g. cetrimide agar) as they are selective agents to isolate P. aeruginosa from clinical and environmental samples.

Image source: researchgate            

Pigment production

P. aeruginosa produces a number of pigments which get diffused into surrounding media. They are as follows:

a) Pyocyanin

i)It is a bluish- green phenazine pigment soluble in chloroform and water.

ii)It can catalyze the reduced nicotinamide-adenine dinucleotide (NADH), dependent conversion of oxygen to super-oxide and hydrogen peroxide.

iii)The presence of super-oxide and H2O2 inhibits the growth of the bacteria and causes cytotoxicity.

b) Fluorescein

i)It is greenish fluorescent material that is soluble in water but not in chloroform.

ii)It imparts a green fluorescence to bacterial colonies when they are examined under an UV light.

c) Pyoverdin

i)It is responsible for green dis-colouration of the plate of agar medium growing this organism.

ii)It is also produced by P. fluorescens and P. putida.

Note: About 10 to 15% of P. aeruginosa do not form any pigment.

Biochemical reaction

• Clinical isolates are oxidase and catalase positive.
• They do not ferment lactose but many strains oxidize glucose with acid production only.
• Acid production by P. aeruginosa is weak which gets neutralized by alkali produced from peptones.
• For this reason, an ammonium salt sugar medium (with sugar as only carbon source) should be used rather than peptone water sugars.
• All strains utilize citrate and Indole, MR, VP and H2S tests are negative.

Resistance

• It is fairly heat resistant and can be killed by heating at 55⁰C for one hour.
• Not only that, they are resistant to most of the routinely used antiseptics and disinfectants such as chloroxylenol and hexachlorophene.
• It can even sometime grow in antiseptic lotions kept for hospital use.
• It is also resistant to most commonly used antimicrobials.
• It is sensitive to acid, β-glutaraldehyde, strong phenolic disinfectants and silver slats.

Pathogenesis

• The organism is widely distributed in nature like water, sewage, soil and air and also on human skin.
• It acts as opportunistic pathogens when body resistance is lowered and tissue is damaged.
• The infection occurs either form the environment or by cross infection.
• Cross infection usually occurs from contaminated hospital equipment such as syringes, lumbar puncture needles, cystoscope, catheters, etc.
• The wound infection in the hospital usually consists of a mixed infection of P. aeruginosa and pyogenic cocci.

Determinants of pathogenicity

a)Alginate capsule

• It is formed by a polymer of manuronic acid and glucuronic acids which is present in some strains of P. aeruginosa.
• Alginate capsulated strains have a mucoid (glistening) appearance of colonies.
• The capsule protects the organism from the effect of aquatic environment.
• It protects the organisms from phagocytosis and also functions as an adhesion factor.

b)Pilin and non-pilus adhesins are essential for initial colonization of epithelial tissue.

c)Haemolysin, one of which is a phospholipase C, breaks down lipid and lecithin facilitating tissue destruction.

d)Elastase

• Two enzymes Las A (serine protease) and Las B (zinc metalloprotease) act synergistically and degrade elastin.
• This causes damage of lung parenchyma and hemorrhagic lesions (ecthyma gangrenosum) associated with disseminated P. aeruginosa infections.
• Antibodies to Las A and Las B are formed in chronic infection resulting in deposition of immune complexes in the infected issues.

e)Pyocyanin

• It catalyzes the production of super-oxide and H2O2, toxic forms of oxygen that damages tissue.
• A more toxic radical damaging to tissue is produced in presence of pyochelin (an iron binding siderophore).

f)Endotoxin

• Lipid A component of endotoxin mediates various biological effects of sepsis syndrome.

g)Exotoxins

• Exotoxin A (approx. 90% ) and Exotoxin S (approx.40%) are produced by pathogenic strains of P. aeruginosa.
• Like diphtheria toxin, exotoxin A blocks protein synthesis in eukaryotic cells by inactivating elongation of factor-2.
• Exotoxin S, ADP ribolysates host cell G proteins.
• These both toxins interfere with phagocytic killing.
• Exotoxin S is heat stable and exotoxin A is heat labile.

h)Antibiotic resistance is attributed to the relatively impermeability of the cell wall. Effective antimicrobials are transported by diffusion after binding to LPS or through porins. Thus switching in LPS side chain and mutational changes of porins result in antibiotic reistance.

Clinical syndrome

• The organisms produce blue pus and the term aeruginosa means verdigoris which is bluish green in colour and pyocyanea, which means blue pus.
• They are often involved in opportunistic and nosocomial infections.
• It is an important cause of hospital-acquired infections.
• Patients with serious underlying conditions like burns, trauma to the skin or conjunctiva, urinary tract manipulations, cystic fibrosis, malignancy and immune-suppressions are frequently affected.
• Therapeutic procedures like catheterization can also cause infection.
• Previous antibiotic therapy also favours infection with P. aeruginosa.
• The commonest infections caused by P. aeruginosa are:

i) Urinary tract infection which may result after instrumentation or catheterization.

ii) Wound and burns infection.

iii) Chronic otitis media and otitis external.

iv) Eye infection.

v) Acute purulent meningitis which may occur after lumbar puncture or after cranial surgery.

vi) Respiratory infections like necrotizing pneumonia usually occur from contaminated respirators.

vii) Septicemia may develop in newborn baby and old debilitated persons, particularly in those individuals who have already been treated with cytotoxic drug or irradiation.

viii)Acute necrotizing vasculitis which leads to haemorhhagic infarction of skin and of internal organs (liver and kidney).

References:

i) https://www.frontiersin.org/articles/10.3389/fcimb.2017.00039/full

ii) https://www.ehagroup.com/resources/pathogens/pseudomonas-aeruginosa/

iii) https://academic.oup.com/femspd/article/67/3/159/2398791

iv) http://textbookofbacteriology.net/pseudomonas.html

Pseudomonas aeruginosa: Morphology, Biochemical reaction, Pathogenesis