Acinetobacter baumannii is a pleomorphic aerobic gram-negative bacillus (similar in appearance to Haemophilus influenzae on Gram stain) commonly isolated from the hospital environment and hospitalized patients. A baumannii is a water organism and preferentially colonizes aquatic environments. This organism is often cultured from hospitalized patients' sputum or respiratory secretions, wounds, and urine. In a hospital setting, Acinetobacter commonly colonizes irrigating solutions and intravenous solutions.
Acinetobacter species have low virulence but are capable of causing infection. Most Acinetobacter isolates recovered from hospitalized patients, particularly those recovered from respiratory secretions and urine, represent colonization rather than infection.
Acinetobacter infections are uncommon but, when they occur, usually involve organ systems that have a high fluid content (eg, respiratory tract, CSF, peritoneal fluid, urinary tract), manifesting as nosocomial pneumonia, infections associated with continuous ambulatory peritoneal dialysis (CAPD), or catheter-associated bacteruria. The presence of Acinetobacter isolates in respiratory secretions in intubated patients nearly always represents colonization. Acinetobacter pneumonias occur in outbreaks and are usually associated with colonized respiratory-support equipment or fluids. Nosocomial meningitis may occur in colonized neurosurgical patients with external ventricular drainage tubes.
A baumannii is a multiresistant aerobic gram-negative bacillus sensitive to relatively few antibiotics. Multidrug-resistant Acinetobacter is not a new or emerging phenomenon, but A baumannii has always been an organism inherently resistant to multiple antibiotics.
In the uncommon situations in which Acinetobacter causes actual infection, the pathological changes that occur depend on the organ system involved. The pathological changes, as observed in patients with pneumonia, are indistinguishable from those caused by other noncavitating aerobic gram-negative bacilli that cause nosocomial pneumonias. Similarly, Acinetobacter urinary tract infections are clinically indistinguishable from catheter-associated bacteremias caused by other aerobic gram-negative bacilli.
Acinetobacter commonly colonizes patients in the intensive care setting. Acinetobacter colonization is particularly common in patients who are intubated and in those who have multiple intravenous lines or monitoring devices, surgical drains, or indwelling urinary catheters. Acinetobacter infections are uncommon and occur almost exclusively in hospitalized patients.
• Although Acinetobacter is primarily a colonizer in the hospital environment, it occasionally causes infection. Mortality and morbidity resulting from A baumannii infection relate to the underlying cardiopulmonary immune status of the host rather than the inherent virulence of the organism. • Mortality and morbidity rates in patients who are very ill with multisystem disease are increased because of their underlying illness rather than the superimposed infection with Acinetobacter. Race
Acinetobacter infection has no known racial predilection.
Acinetobacter infection has no known sexual predilection.
Acinetobacter infection has no known predilection for age.
Other Problems to Be Considered
The main differential diagnostic problem presented by Acinetobacter is to differentiate colonization from infection.
In the presence of pulmonary infiltrates in ICU patients, CAPD-associated peritonitis, meningitis, wound infection, or catheter-associated bacteruria, the differential diagnoses include other aerobic gram-negative bacilli that colonize or infect these fluids, ie, Enterobacter species, Stenotrophomonas maltophilia, Burkholderia cepacia, Pseudomonas aeruginosa, Flavobacterium meningosepticum, and Serratia marcescens.