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The pseudomonads and acinetobacters are widely distributed in soil and water. Pseudomonas aeruginosa sometimes colonizes humans and is the major human pathogen of the group. P aeruginosa is invasive and toxigenic, produces infections in patients with abnormal host defenses, and is an important nosocomial pathogen.

Gram-negative bacteria that rarely cause disease in humans are included in this chapter. Some of these bacteria (eg, chromobacteria and chryseobacteria) are found in soil or water and are opportunistic pathogens for humans. Other gram-negative bacteria (eg, Capnocytophaga, Eikenella corrodens, Kingella, and Moraxella) are normal microbiota of humans and occur in a wide variety of infections; often they are unexpected causes of disease.

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. The medically important pseudomonads are listed in Table 16-1.

Table 16–1 Classification of Some of the Medically Important Pseudomonadsa

Pseudomonas aeruginosa

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.

Morphology and Identification

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.

Figure 16–1

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


P aeruginosa is an obligate aerobe that grows readily on many types of culture media, sometimes producing a sweet or grapelike 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 ...

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