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INTRODUCTION

The Pseudomonas and Acinetobacter species are widely distributed in soil and in 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. Burkholderia consists of many species, but only B. cepacia complex, B. pseudomallei, B. mallei, and B. gladioli are notable human or animal pathogens. Like pseudomonads, Bukholderia are typically environmental organisms and opportunistic pathogens. Stenotrophomonas maltophilia is typically not pathogenic for healthy people; however, the organism is a well-known opportunistic and nosocomial pathogen.

THE PSEUDOMONAD GROUP

The pseudomonads are ubiquitous Gram-negative, motile, aerobic rods, some of which produce water-soluble pigments. The pseudomonads inhabit a variety of diverse environments, such as soil, water, plants, and animals. Data from the Human Microbiome Project demonstrated that P. aeruginosa is almost completely absent from the skin and nares of healthy humans but some other pseudomonads may be present in small numbers in the normal intestinal flora and oral cavity. Changes in the microbiome, however, can lead to decrease in colonization resistance, and subsequent colonization of specific body sites (eg, skin, mucous membranes, and GI tract) with P. aeruginosa. The clinically relevant Pseudomonas species can be divided into two distinct groups based on their ability to produce certain fluorescent pigments. The fluorescent group of pseudomonads includes P. aeruginosa, P. fluorescens, P. putida, P. monteilii, P. veronii, and P. mosselii. These organisms produce a water-soluble, yellow-green pigment (pyoverdine) that fluoresces blue-green under UV light. Many of the strains of P. aeruginosa also produce pyocyanin, a blue pigment that, when combined with pyoverdine, results in the bright green color that is characteristic of this organism. The following pseudomonads belong to the non-fluorescent group: P. stutzeri, P. mendocina, P. alcaligenes, P. pseudoalcaligenes, P. luteola, and P. oryzihabitans. While P. aeruginosa is considered the major pathogen in the group of pseudomonads, other pseudomonads may be less frequently isolated in clinical laboratories as a cause of disease, as well. The classification of pseudomonads is based on rRNA/DNA homology and common culture characteristics.

PSEUDOMONAS AERUGINOSA

P. aeruginosa is widely distributed in nature and is commonly present in moist environments in hospitals. While not a part of the normal human microbiome, P. aeruginosa is capable of colonization of various body sites (eg, mucous membrane, respiratory tract, and GI tract). It is known to cause disease in humans, especially in people with altered and decreased host defenses (eg, neutropenia, chemotherapy, and burn wounds). The acquisition of the organism and subsequent infection can be either endogenous or exogenous. ...

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