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The pseudomonads and Acinetobacter species are widely distributed in soil and water. Pseudomonas aeruginosa sometimes colonizes humans and is the major human pathogen of the pseudomonads. P aeruginosa is invasive and toxigenic, produces infections in patients with abnormal host defenses, and is an important nosocomial pathogen. Of the Acinetobacter species, Acinetobacter baumannii is responsible for most human infections. It is a significant nosocomial pathogen, especially in critical or intensive care units, and is frequently resistant to multiple antibiotics.


The pseudomonads are Gram-negative, motile, aerobic rods, some of which produce water-soluble pigments. The pseudomonads occur widely in soil, water, plants, and animals. P aeruginosa is frequently present in small numbers in the normal intestinal flora and on the skin of humans, and is the major pathogen of the group. Other pseudomonads infrequently cause disease. The classification of pseudomonads is based on rRNA/DNA homology and common culture characteristics.


P aeruginosa is widely distributed in nature and is commonly present in moist environments in hospitals. It can colonize normal humans, in whom it is a saprophyte. It causes disease in humans with abnormal host defenses, especially in individuals with neutropenia.

Morphology and Identification

A. Typical Organisms

P aeruginosa is motile and rod shaped, measuring about 0.6 × 2 μm (Figure 16-1). It is Gram-negative and occurs as single bacteria, in pairs, and occasionally in short chains.


Gram stain of Pseudomonas aeruginosa, which are about 0.6 × 2 μm. Original magnification ×1000. (Courtesy of H Reyes.)

B. Culture

P aeruginosa is an obligate aerobe that grows readily on many types of culture media, sometimes producing a sweet or grape-like or corn taco–like odor. Some strains hemolyze blood. P aeruginosa forms smooth round colonies with a fluorescent greenish color. It often produces the nonfluorescent bluish pigment pyocyanin, which diffuses into the agar. Other Pseudomonas species do not produce pyocyanin. Many strains of P aeruginosa also produce the fluorescent pigment pyoverdin, which gives a greenish color to the agar (Figure 16-2). Some strains produce the dark red pigment pyorubin or the black pigment pyomelanin.


Pseudomonas aeruginosa on a 10-cm Mueller-Hinton agar plate. Individual colonies are 3–4 mm in diameter. The organism produces pyocyanin, which is blue, and pyoverdin, which is green. Together these pigments produce the blue-green color that is seen in the agar around the pseudomonas growth. (Courtesy of S Lowe.)

P aeruginosa in a culture can produce multiple colony types (Figure 16-3). P aeruginosa from different colony types may ...

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