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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Pseudomonas and Other Opportunistic Gram-negative Bacilli

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Pseudomonas aeruginosa - Bacteriology

Pseudomonas aeruginosa is an aerobic, motile, Gram-negative rod that is slimmer and more pale staining than members of the Enterobacteriaceae.

Pseudomonas aeruginosa

BACTERIOLOGY

Pseudomonas aeruginosa is an aerobic, motile, Gram-negative rod that is slimmer and more pale staining than members of the Enterobacteriaceae. Its most striking bacterio- logic feature is the production of colorful water-soluble pigments. P. aeruginosa also demonstrates the most consistent resistance to antimicrobics of all the medically impor-tant bacteria.

GROWTH AND METABOLISM

P. aeruginosa is sufficiently versatile in its growth and energy requirements to usesimple molecules such as ammonia and carbon dioxide as sole nitrogen and carbon sources. Thus, it does not require enriched media for growth, and it can survive and multiply over a wide temperature range (20 to 42°C) in almost any environment, including one with a high salt content. The organism uses oxidative energy-producing mechanisms and has a high level of cytochrome oxidase (oxidase positive). Although an aerobic atmosphere is necessary for optimal growth and metabolism, most strains multiply slowly in an anaerobic environment if nitrate is present as an electron acceptor.

Growth on all common isolation media is luxurious, and colonies have a delicate, fringed edge. Confluent growth often has a characteristic metallic sheen and emits an intense “fruity” odor. Hemolysis is usually produced on blood agar. The positive oxi-dase reaction of P. aeruginosa differentiates it from the Enterobacteriaceae, and its production of blue, yellow, or rust-colored pigments differentiates it from most other Gram-negative bacteria. The blue pigment, pyocyanin, is produced only by P. aerugi-nosa.Fluorescin,a yellow pigment that fluoresces under ultraviolet light, is producedby P. aeruginosa and other free-living less pathogenic Pseudomonas species. Py-ocyanin and fluorescin combined produce a bright green color that diffuses throughout the medium.

STRUCTURE

Lipopolysaccharide (LPS) is present in the outer membrane, as are porin proteins, which differ from those of the Enterobacteriaceae in offering much less permeability to a wide range of molecules, including antibiotics. Pili composed of repeating monomers of the pilin structural subunit extend from the cell surface. A single polar flagellum rapidly pro-pels the organism.

A mucoid exopolysaccharide slime layer is present outside the cell wall in some strains. This layer is created by secretion of alginate, a copolymer of mannuronic and glucuronic acids. It is created by the action of several enzymes that effectively channel carbohydrate intermediates into the alginate polymer. All P. aeruginosa produce moderate amounts of alginate, but those with mutations in regulatory genes overproduce the poly-mer. These mutants appear as striking mucoid colonies in cultures from the respiratory tract of patients with cystic fibrosis.

EXTRACELLULAR PRODUCTS

Most strains of P. aeruginosa produce multiple extracellular products, including exotoxin A and other enzymes with hemolytic, lecithinase, collagenase, or elastase activity. Exotoxin A enters cells via receptor-mediated endocytosis and is internalized into a low pH vesicle from which it translocates and reaches its target molecule, elongation factor 2 (EF-2). It catalyzes the inactivation of EF-2 by ADP-ribosylation, leading to shutdown of protein synthesis and cell death. Although this action is the same as diphtheria toxin, the two toxins are otherwise unrelated. Expression of exotoxin A is influenced by oxygen, temperature, and iron regu-lated genes.

Exoenzyme S ADP-ribosylates several intracellular proteins, including the cytoskele-ton filament vimentin, and may also function as a surface-bound adhesin. Theelastase acts on a variety of biologically important substrates, including elastin, human IgA and IgG, complement components, and some collagens. P. aeruginosa elastase shows homol-ogy with other proteases, including those produced by Legionella pneumophila and Vib-rio cholerae.

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