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Chapter: Microbiology and Immunology: Bacteriology: Pseudomonas, Burkholderia, and Moraxell

Pseudomonas aeruginosa: Properties of the Bacteria

P. aeruginosa is the most important species associated withhuman infection. It is a most common human saprophyte, but it rarely causes disease in healthy individuals. P. aeruginosa causes most of human infections in immunocompromised human host.

Pseudomonas aeruginosa

P. aeruginosa is the most important species associated withhuman infection. It is a most common human saprophyte, but it rarely causes disease in healthy individuals. P. aeruginosa causes most of human infections in immunocompromised human host.

Properties of the Bacteria

 Morphology

P. aeruginosa shows the following morphological features:

·           P. aeruginosa is a straight or slightly curved, Gram-negativebacillus measuring 0.5–1.0 3 1.5–5.0 mm in size arranged singly, in pairs, or in short chains.

·           It is motile by the presence of a polar flagellum. Occasion-ally, strains may possess two or three polar flagella.

·           Pseudomonas spp. is noncapsulated. Although the bacteria arenoncapsulated, many strains appear mucoid by production of an abundant of extracellular polysaccharide composed of alginate polymers. This slime layer forms a loose capsule or glycocalyx around the bacillus. These strains are particularly isolated from patients with cystic fibrosis.

·           Pseudomonas spp. is nonsporing and fimbriated.

 Culture

Pseudomonas spp. are strictly aerobic bacteria. P. aeruginosa growsover a wide range of temperatures (5–32°C), the optimum tem-perature being 37°C. P. aeruginosa grows on commonly used routine media including nutrient agar, blood agar, MacConkey agar, and deoxycholate citrate agar (DCA).

1.        Nutrient agar: P. aeruginosaafter incubation for 24 hoursat 37°C on nutrient agar produces large (2–3 mm in diam-eter), opaque, translucent, and irregularly round colonies. These colonies emit a characteristic musty to fruity odor due to production of aminoacetophenone from the amino acid tryptophan. It produces hemolytic colonies on blood agar.

 

2.        MacConkey agar: The organism produces colorless non–lactose-fermenting colonies on MacConkey media.

 

3.        Cetrimide agar: Cetrimide agar is a selective medium forculture of P. aeruginosa.

 

4.        Nutrient broth: In nutrient broth, it produces a denseturbidity with surface pellicle.

 

aeruginosa produces different types of pigments, such as (a)pyocyanin, (b) pyoverdin, (c) pyorubin, and (d) pyomelanin. However, some strains of Pseudomonas are not pigmented.

·             Pyocyanin is specifically produced byP. aeruginosa,whichdiffuses into the surrounding medium (Fig. 37-1). The pigment is soluble in chloroform and water.

 


·           Pyoverdin, or fluorescein, is produced byP. aeruginosaas well as by many other Pseudomonas species. These pigments give a yellow tinge to the colonies of bacteria and are best demonstrated in microscope using ultravi-olet source of light. Fluorescein is soluble in water, but insoluble in chloroform.

·           Pyorubin is a red pigment, which is soluble in water butinsoluble in alcohol.

·           Pyomelanin is a brown pigment.

 Biochemical reactions

·             They are oxidase positive. Oxidase test is an important test for identification of P. aeruginosa. All the strains are oxidase positive within 30 seconds of performing the test.

·      They are nonfermentative bacteria. They utilize sugars by an oxidase metabolism, with oxygen as the termi-nal electron acceptor. Special media, such as oxidation-fermentation (OF) media, are used to demonstrate the low quantity of acid produced during oxidative breakdown of sugars. P. aeruginosa utilizes glucose, forming acid only (Color Photo 38).

 

·      They do not utilize lactose and maltose.

 

·      They reduce nitrates to nitrite, which is further broken down to gaseous nitrogens.

 

·      They are catalase positive.

 

·      They are arginine dihydrolase positive.

 

·           They are indole, MR, VP, and H2S negative.


 Other properties

Susceptibility to physical and chemicals agents: P. aeruginosais heat-labile bacterium, readily killed at 55°C in 1 hour. It is also highly susceptible to acid, silver salts, 2% alkaline, glutaralde-hyde, and disinfectants (such as Dettol and cetrimide). However, P. aeruginosa is very strongly resistant to common antisepticsand disinfectants, such as chloroxylenol, hexachlorophene, and quaternary ammonium compounds


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