Most gram-negative infections are caused by organisms in the Enterobacteriaceae or Pseudomonadaceae families; how ever, a few are caused by a heterogeneous group of gramnegative organisms. These organisms do not fit conveniently into a single genera and have undergone frequent taxonomic changes, making understanding them even more difficult for clinicians. The clinical presentation varies widely, affecting different types of hosts and requiring a variety of antibiotics for therapy (Table 160.1). Varied predisposing environmental and host factors are outlined in Table 160.2.

ACINETOBACTER

Acinetobacter is a member of the family Moraxellaceae, with at least 25 genospecies and two commonly recognized clinical species, Acinetobacter calcoaceticus (formerly A. calcoaceticus var. Iwoffii) and Acinetobacter baumannii (formerly A. calcoaceticus var. anitratus). Because of problems in separating these two strains using phenotypic tests, some laboratories have chosen to report them as “A. calcoaceticus-baumannii complex.” They are nonmotile, oxidase-negative, gram-negative coccobacilli often appearing as diplococci and thus are easily confused with Neisseria or Haemophilus spp. They differ from Enterobacteriaceae in that they do not grow anaerobically or reduce nitrates. They are distinguished from Neisseria and Moraxella by their negative oxidase reaction. Virulence factors include a polysaccharide capsule that may prevent phagocytosis and fimbriae that potentiate adherence to epithelial cells.