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A number of opportunistic Gram-negative rods of several genera not considered in other chapters are included here. With the exception of Pseudomonas aeruginosa, they rarely cause disease, and all are frequently encountered as contaminants and superficial colonizers. The significance of their isolation from clinical material thus depends on the circumstance and site of culture and on the clinical situation of the patient.


There are a large number of Pseudomonas species, the most important of which is P aeruginosa. The number of human infections produced by the other species together is far lower than that produced by P aeruginosa alone. Pseudomonas species are most frequently seen as colonizers and contaminants, but are able to cause opportunistic infections. The assignment of species names has little clinical importance beyond differentiation from P aeruginosa. Reports vary regarding the frequency of their isolation from cases of bacteremia, arthritis, abscesses, wounds, conjunctivitis, and urinary tract infections. In general, unless isolated in pure culture from a high-quality (direct) specimen, it is difficult to attach pathogenic significance to any of the miscellaneous Pseudomonas species.

P aeruginosa most important

Other Pseudomonas species cause opportunistic infection

Pseudomonas aeruginosa


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

Pigment-producing rod is resistant to many antimicrobics

Pseudomonas aeruginosa is sufficiently versatile in its growth and energy requirements to use simple molecules such as ammonia and carbon dioxide as sole nitrogen and carbon sources. Thus, it does not require enriched media for growth and can survive and multiply over a wide temperature range (20–42°C) in almost any environment, including one with a high salt content. The organism uses oxidative energy-producing mechanisms and has high levels 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.

Grows aerobically with minimal requirements

Colonies are oxidase-positive

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 oxidase 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 aeruginosa. Fluorescin, a yellow pigment that fluoresces under ultraviolet light is produced by P aeruginosa and other free-living less pathogenic Pseudomonas species. Pyocyanin and fluorescin ...

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