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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 true disease, and all are frequently encountered as superficial colonizers or contaminants; 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. P aeruginosa produces infection at a wide range of pulmonary, urinary, and soft tissue sites, much like the Enterobacteriaceae. The clinical manifestations of these infections reflect the organ system involved and are not unique for Pseudomonas. However, once established, infections are particularly virulent and difficult to treat, likely because affected patients almost always have some form of debilitation or compromise of immune defenses and the bacteria themselves may be highly resistant to antibiotics.


There is a large number of Pseudomonas species, the most important of which is Pseudomonas aeruginosa. Pseudomonas species are most frequently seen as colonizers and contaminants but are able to cause opportunistic infections; however, the number of human infections produced by the other species together is far lower than that produced by P aeruginosa alone. 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, particularly from a normally sterile site, it is difficult to attach pathogenic significance to any of the miscellaneous Pseudomonas species.

P aeruginosa most important pathogen

❋ Other Pseudomonas species cause opportunistic infection



P 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 vivid and colorful water-soluble pigments. Of all the medically important bacteria, P aeruginosa also demonstrates the most consistent resistance to antimicrobial agents of all the medically important bacteria.

❋ Pigment-producing rod resistant to many antimicrobials

P 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 those with 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 ...

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