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Acinetobacter species were first described in 1911 and named Micrococcus calcoaceticus. Thereafter, the genus was renamed multiple times; since 1950, it has been known as Acinetobacter. Acinetobacter species are gram-negative, oxidase-negative, nonmotile, nonfermenting coccobacilli that are easily recovered on standard culture media. Differentiation among Acinetobacter species on the basis of phenotypic characteristics alone is very difficult. Molecular-based methods such as matrix-assisted laser desorption–ionization–time-of-flight mass spectrometry (MALDI-TOF-MS) and quantitative real-time polymerase chain reaction (PCR) are usually necessary to identify Acinetobacter baumannii, the most clinically relevant species of the genus.
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ETIOLOGY AND EPIDEMIOLOGY
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Acinetobacter species are naturally encountered in water and soil and have also been recovered from fruits and vegetables. In humans, Acinetobacter can be found on the skin and in the respiratory and gastrointestinal tracts. A. baumannii is capable of surviving environmental desiccation for weeks; this characteristic is important from an infection-control perspective as it allows this organism to persist in the hospital environment and on equipment.
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Acinetobacter was historically considered a pathogen of hot and humid climates. In recent years, however, hospital outbreaks caused by A. baumannii have been reported worldwide, even in temperate climates. In the United States, the Centers for Disease Control and Prevention (CDC) estimates that 12,000 Acinetobacter infections occur every year, 7300 of which are caused by multidrug-resistant strains, with 500 attributable deaths. The increase in the number of infections with A. baumannii is suspected to be due to the rapid spread of certain genetically distinct lineages; of the three international clonal lineages (ICLs), ICL I and ICL II are multidrug resistant. The predominance of these lineages remains unexplained, although it has been proposed that this population structure is the result of two waves of expansion. The first wave followed a bottleneck (possibly linked to a restricted ecologic niche) that occurred in the distant past. The second wave is ongoing and is being driven by the rapid expansion of a limited number of multidrug-resistant clones.
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Analysis of the A. baumannii pangenome (the sum of the core and dispensable genomes) has shown that its organization is characterized by a small core genome and a large accessory or disposable genome. This organization reflects A. baumannii’s high plasticity, which enables it to acquire exogenous genetic material. With few exceptions, gene functions associated with virulence are found in the core genome; this observation suggests a limited role for the acquisition of new virulence traits in the recent nosocomial expansion of A. baumannii clones. Genes associated with resistance to antimicrobial agents are found in both the species core genome and the accessory genome. In the accessory genome, these genes have been found in alien islands, often flanked by integrases, transposases, or insertion sequences. This pattern suggests possible acquisition by horizontal gene transfer from other Acinetobacter strains or even from different bacterial species present in the immediate environment. Acquisition of these antimicrobial resistance genes is hypothesized to ...