Acinetobacter Pneumonia: A Review

Joshua D. Hartzell, MD, Andrew S. Kim, MD, Mark G. Kortepeter, MD, MPH, Kimberly A. Moran, MD

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Taxonomy/Microbiology

Acinetobacter as a genus was not definitively established until 1971, although today more than 25 species have been identified via DNA-DNA hybridization.[11,17,18] To date, only 10 of these species have been named and identification of specific species in the laboratory using phenotypic tests can be very difficult. In most circumstances, species identification requires advanced molecular techniques. The term Acinetobacter calcoaceticus-A baumannii complex (Abc) has been given to 4 phenotypically similar strains (strains 1, 2, 3, and 13) that account for 80% of clinical infections. Consequently, most laboratories will report isolation of Abc, which is sufficient to direct clinical decision-making. Other species, however, can cause disease and should not be considered contaminants, especially in the setting of repeatedly positive cultures.[18]

Acinetobacter species are gram-negative, nonfermentative, non-spore-forming, nonmotile, aerobic coccobacillary organisms. However, they can be gram-variable and occasionally gram-positive on initial gram stain (Figure). Morphologic characteristics may change depending on the growth phase, resulting in a rod-shaped appearance during rapid growth but a coccobacillary appearance during the stationary phase. Acinetobacter species are oxidase-negative, which helps distinguish them from other gram-negative organisms, such as Pseudomonas, Neisseria, and Moraxella.[18]

A. Gram-variable cocci in sputum. B. Gram-variable rods in blood culture (arrow = gram-positive; arrowhead = gram-negative).

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