Diseases Caused by Acinetobacter and Stenotrophomonas Species

The Pathogen

Acinetobacter spp are gram-negative, aerobic, coccobacillary bacteria, which are generally non–lactose-fermenting, nonfastidious, catalase positive, oxidase negative, and nonmotile, though their description of being “nonmotile” is questioned. The Gram stain appearance of Acinetobacter spp is highly dependent on the life cycle. In the early growth phases, Acinetobacter spp appear rod shaped. In the stationary phase, they acquire a coccobacillary morphology. A. baumannii and other members of the A. calcoaceticus-baumannii (Acb) complex are the most common human pathogens among the more than 70 validated Acinetobacter spp (see E-Table 283-1 for 30 representative species) and more than 20 species that are reported but unvalidated and unnamed.

Pneumonia

The two distinct syndromes associated with respiratory tract infection due to Acinetobacter are community-acquired pneumonia and health care–associated pneumonia ( Chapter 85 ). Community-acquired pneumonia due to Acinetobacter species is associated with a high incidence of bacteremia, acute respiratory distress syndrome ( Chapter 90 ), sepsis, and death, with mortality rates sometimes exceeding 50%. Rarely, community-acquired pneumonia due to Acinetobacter species can be manifested with consolidation and multiple lung abscesses.

More frequently, Acinetobacter spp are a cause of pneumonia in patients who have risk factors for antimicrobial resistant organisms, often manifested as ventilator-associated pneumonia ( Chapter 85 ). Health care–associated pneumonia due to Acinetobacte r largely resembles the clinical spectrum of gram-negative pneumonias (bilateral infiltrates, pleural effusion, cavitations, hypoxemia, and bacteremia). Most cases are in patients on ventilators.

Bacteremia

Blood stream infection due to Acinetobacter spp is often a consequence of infection of intravenous catheters (i.e., central line–associated blood stream infection) or is secondary to health care–associated pneumonia. Less commonly, wound infections can cause bacteremia. Infective endocarditis ( Chapter 61 ) is uncommon, with about 40% of cases involving a prosthetic valve.

Urinary Tract Infection

Urinary tract infections are only rarely caused by Acinetobacter spp. An indwelling bladder catheter is the major risk factor.

Wound, Burn, and Skin and Soft Tissue Infections

Traumatic or postoperative wounds, burns ( Chapter 97 ), and skin and soft tissue infections are among the most common sites of Acinetobacter infections. Prior antibiotic use or devitalized tissues are predisposing factors. The spectrum of infection can range from cellulitis to necrotizing fasciitis.

Necrotizing skin and soft tissue infections with A. baumannii occur in hosts with underlying comorbidities (e.g., trauma, cirrhosis), especially associated with the use of central venous catheters and parenteral nutrition, Acinetobacter spp–associated skin infections can be manifested with a peau d’orange appearance, with overlying tiny vesicles ( Fig. 283-1 ), and, when untreated, can progress to necrotizing infection with bullae (hemorrhagic and nonhemorrhagic). Concomitant bacteremia is common.