Pseudomonas: Classification, Toxins and Prevention

In this article we will discuss about Pseudomonas which is a Human Pathogen:- 1. Classification of Pseudomonas 2. Toxins and Enzymes of Pseudomonas 3. Resistance 4. Pathogenicity 5. Epidemiology 6. Epidemics 7. Risk Group 8. Vaccine 9.  Control and Prevention.

Contents:

1. Classification of Pseudomonas
2. Toxins and Enzymes of Pseudomonas
3. Resistance of Pseudomonas
4. Pathogenicity of Pseudomonas
5. Epidemiology of Pseudomonas
6. Epidemics of Pseudomonas
7. Risk Group of Pseudomonas
8. Vaccine of Pseudomonas
9. Control and Prevention of Pseudomonas

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1. Classification of Pseudomonas:

On the basis of r RNA/DNA homology, the genus Pseudomonas has been mainly classified into two groups.

Pseudomonas aeruginosa (P. pyocyanea) is the only one human pathogen. It is slender Gram-negative bacillus, 1.5-3 µ x 0.5 µ, actively motile by a polar flagellum, is non- capsulated and non-spore forming. Strains from clinical specimens do possess pili.

It is an obligate aerobe, grows well on ordinary media at 37°C forming large opaque, irregular colonies with a distinctive earthy or fruity smell (Fig. 31.1). On MacConkey and DCA media, it forms non-lactose fermenting (pale) colonies, whereas on blood agar some strains are haemolytic.

Selective medium (Cetrimide agar) can be used for the isolation from the clinical specimens. In broth, it forms a dense turbidity with surface pellicle.

It produces various pigments (pyocyanin, fluorescein). Pyocyanin (Fig. 31.1) is bluish green phenazine pigment soluble in water and chloroform, which is produced by Ps. aeruginosa, fluorescein is greenish yellow pigment soluble in water but not in chloroform, but it is produced by many other species also.

In older culture, it may be oxidised to a yellowish brown pigment. Other pigments are pyoverdin, pyorubrin, and pyomelanin. The role of pigment in the pathogenesis is not yet well-established. Ps. aeruginosa emerges as the dominant bacterium in the mixed infection.

2. Toxins and Enzymes of Pseudomonas:

Ps. aeruginosa produces toxins and enzymes responsible for its virulence:

(a) Extracellular products:

Mitochondrial enzyme in mammalian tissue are inhibited by pyocyanin. Besides, pyocyanin is responsible for the description and cessation of ciliary beat on ciliated nasal epithelium that clears the respiratory mucosa of offending agents, thus the colonisation of organism in the nasal mucosa is favoured.

(b) Extracellular enzymes and haemolysins:

Proteolytic enzymes (protease, elastase), haemolysins (phospholipase C, heat stable rhammolipid) and lipase play an important role in the formation of local lesions.

(c) Exotoxins:

The exotoxins are two extra-cellular ADP-ribosyl transferase, exotoxins A and S. Toxin A is a polypeptide of MW 68000, inhibits protein synthesis.

(d) Endotoxins:

It is a lipopolysaccharide with many biological properties of entero-bacterial lipopolysaccharide.

3. Resistance of Pseudomonas:

Since Ps. aeruginosa is not heat resistant, it is easily killed at 55 C in one hour. It grows in antiseptic lotions used in hospital as they are resistant to most antiseptics and disinfectants (chloroxylenol and hexachlorophene). It is also resistant to many antimicrobial agents due to production of potent chromosomally mediated β-lactamase classification of Ps. aeruginosa.

It can be done by three methods of typing:

1. Serotypes Ps aeruginosa:

It has been divided into 29 serotypes by O agglutination test, though it is not of much use.

2. Phage-types:

It has been proposed that phage typing can be carried out by using specific bacteriophages lysing a particular strain of Ps. aeruginosa, but it is not yet accepted.

3. Pyocin typing:

Ps. aeruginosa is differentiated into 105 types by 13 indicator strains, based on production of particular type of pyocins (bacteriocins).

Metabolism is oxidative but not fermentative. Glucose is utilised oxidatively forming acid only, Indole MR, VP and H₂S are negative. Catalase and oxidase are positive.

Though Ps. aeruginosa is not heat resistant, it is resistant to common antiseptics (Dettol, cetrimide), lotions kept for use in hospitals.

4. Pathogenicity of Pseudomonas:

This bacillus is responsible for nosocomial infection, wound infection, suppurative otitis, bed sore, eye infection, urinary tract infection, infantile diarrhoea. When body resistance is lowered and body tissue is damaged, Ps. aeruginosa produces human infection as opportunistic pathogen, since it is prevalent in water, sewage, soil, air and also as commensals in human intestine.

In the hospital the wound infection is very common due to mixed infection of Ps. aeruginosa and Pyogenic cocci.

The infection takes place from the environment and by cross-infection in the hospital by the use of contaminated syringes, needles, cystoscope, catheters. The hospital acquired infections are mostly due to Ps. aeruginosa (aeruginosa, pyocyanea means “blue pus“). Pathogenic lesion (wound) is covered with blue pus.

Patients with severe burns, malignancy and who have undergone catheterisation of the urinary tract are often infected. Laboratory diagnosis can be done by subjecting the specimens (pus, wound swabs, mid-stream urine, CSF, sputum, blood) to the Cultural test, Biochemical test, Animal pathogenicity test and Typing.

5. Epidemiology of Pseudomonas:

Cross-Infection:

Ps. aeruginosa has the ability to resist and multiply in most environment of hospital wards, bathrooms, kitchens, equipment, antiseptics or disinfectants solution. This ability has a great importance in cross-infection. Though exogenous or endogenous sources may be responsible for many cross-infections, it may also occur in patients with urinary tract infection. Healthy carriers (10%) may harbour Ps. aeruginosa.

6. Epidemics:

In newborns in nurseries and young infants in paediatric wards, epidemics and out-breaks of Ps. aeruginosa have been reported. Sometimes milk-fed babies were infected due to drinking of Ps. aeruginosa contaminated milk. This has been confirmed by further investigation that rubber lungs used to close feeding bottles have been stored in antiseptic solutions.

7. Risk Group:

(1) Burned patients are most vulnerable to Ps. aeruginosa super-infection.

(2) Patients with immuno-suppression, and

(3) Individuals who have undergone cardiac and renal surgery.

8. Vaccine:

It has been claimed that the polyvalent vaccine prepared from the cell surface of 17 internationally recognised serotypes of Ps. aeruginosa stimulated active immunity in man. It is not commercially available at present.

9. Control and Prevention of Pseudomonas:

By constant vigilance and by the use of combined drug therapy, the hospital cross- infection can be prevented.

Treatment:

Polymyxin B, colistin, gentamicin, amikacin, carbenicillin and cefuroxime are effective against most strains.