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Infections with bacteria of the genus Acinetobacter are established as a significant problem worldwide. Acinetobacter baumannii is particularly formidable because of its propensity to acquire antibiotic resistance determinants. Endemic infections caused by strains of A. baumannii resistant to multiple antibiotic classes, including carbapenems, are a serious concern in many specialized hospital units, especially intensive care units (ICUs). The foremost implication of infection with carbapenem-resistant A. baumannii is the need to use “last-line” antibiotics such as colistin, polymyxin B, or tigecycline; these options have the potential to render these bacteria resistant to all available antibiotics.
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Acinetobacter species are oxidase-negative, nonfermenting, short, gram-negative bacilli. They were traditionally thought of as nonmotile—a characteristic from which the genus name was derived (from the Greek akineto, meaning “nonmotile”). However, recent work has shown that Acinetobacter organisms demonstrate motility under certain growth conditions. The bacteria grow well at 37°C in aerobic conditions on a range of laboratory media (e.g., blood agar). Some species may not grow on MacConkey agar. Differentiation of Acinetobacter species is difficult with the means typically available to most clinical microbiology laboratories, including commercial semiautomated identification systems. The commonly used matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) systems are undergoing evaluation for species-level identification of Acinetobacter. DNA–DNA hybridization is a method used for speciation in reference laboratories. Naturally occurring oxacillinase genes (blaOXA) have been identified in several Acinetobacter species, and their detection by polymerase chain reaction can aid in species identification.
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Widely distributed in nature, Acinetobacter species can be found in water, in soil, and on vegetables. Acinetobacter is a component of the human skin flora and is sometimes identified as a contaminant in blood samples collected for culture. Fecal carriage can be detected in both healthy and hospitalized individuals. However, despite the ubiquity of some Acinetobacter species, the natural habitat of the most clinically important species, A. baumannii, remains to be fully defined.
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A. baumannii infections have been diagnosed in patients on all inhabited continents. The vast majority of infections occur in hospitalized patients and other patients with significant health-care contact. Outbreaks of carbapenem-resistant A. baumannii are particularly problematic. A significant issue is the introduction of carbapenem-resistant A. baumannii into hospitals as a result of medical transfers, especially from hospitals where the organism is highly endemic.
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In 1991 and 1992, outbreaks of carbapenem-resistant A. baumannii infection occurred in a hospital in New York City. Subsequently, numerous other hospitals in the United States and South America have had outbreaks of carbapenem-resistant A. baumannii. Infections with A. baumannii among military personnel from the United States and Canada injured in Iraq or Afghanistan were widely observed beginning in 2002. Acinetobacter was one of the most common causes of bloodstream infections and bone and soft tissue ...