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Infections with bacteria of the genus Acinetobacter have become a significant problem worldwide. Acinetobacter baumannii is particularly formidable because of its propensity to acquire antibiotic resistance determinants. Outbreaks of infection caused by strains of A. baumannii resistant to multiple antibiotic classes, including carbapenems, are a serious concern in many specialized hospital units, including 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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Definition

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Acinetobacter species are oxidase-negative, nonmotile, nonfermenting, short gram-negative bacilli that 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. DNA-DNA hybridization is a method used for speciation in reference laboratories. Identification of the most clinically relevant species, A. baumannii, by detection of the blaOXA-51-like carbapenemase gene intrinsic to this species has been described.

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Etiology

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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. Despite the ubiquity of some Acinetobacter species, the natural habitat of A. baumannii remains to be fully defined.

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Epidemiology

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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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The Americas

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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. The incidence of infections with A. baumannii among military personnel from the United States and Canada has increased since 2002; 102 patients had bloodstream infections at facilities treating U.S. military personnel injured in Iraq or Afghanistan from January 1, 2002, through August 31, 2004. An epidemiologic investigation revealed that A. baumannii could be grown from environmental sites in field hospitals and that the environmental strains were closely related genotypically to clinical isolates. A. baumannii strains from injured military personnel from the United States and the United Kingdom were also genotypically related; this finding provided further evidence that A. baumannii was being acquired in field hospitals.

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