Background 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.
Pathophysiology 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.
Frequency International 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. Mortality/Morbidity • 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. Sex Acinetobacter infection has no known sexual predilection. Age 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.
Because Acinetobacter is predominantly a colonizing organism, the burden of proof is on the clinician to demonstrate its pathogenic role in a given situation. Workup Laboratory Studies • A CBC count is nonspecific, and leukocytosis, even with a left shift, cannot be used to differentiate infection from noninfection or bacterial infection from nonbacterial infection. The CBC count cannot be used to differentiate infection from colonization. • Culture of the appropriate body fluid that is properly transported, plated, and incubated grows A baumannii. Recovery of the organism from a nonsterile body site (eg, endotracheal secretions, urine in patients with a Foley catheter) does not indicate or imply an infectious pathogenic role. • In outbreaks, Acinetobacter is easily cultured from monitoring devices or biological fluids from multiple patients as part of an epidemiological investigation. Imaging Studies Chest radiography and/or CT scanning or MRI of the chest may be useful in defining the extent of a nosocomial pneumonia caused by any organism. Other Tests Tests are related to the organ system involved. Procedures
CSF culture is necessary if shunt-associated or ventricular drain–associated meningitis is suspected. Histologic Findings Histological changes caused by Acinetobacter infection are indistinguishable from those caused by other aerobic gram-negative bacilli, except those associated with vessel invasion and cavitation, eg, Klebsiella pneumoniae and P aeruginosa. Treatment Medical Care Initiate supportive care, depending on the organ system involved. Surgical Care Colonized or infected lines, drains, shunts, or other devices should be removed or replaced as required.
Consultations A consultation with an infectious disease specialist is advised to differentiate colonization from infection and for antibiotic recommendations if infection is present. Medication A baumannii is intrinsically multidrug resistant. Relatively few antibiotics are active against this organism. While colonization should not be treated, infection should. As summarized by Go and Cunha (1999), medications to which Acinetobacter is usually sensitive include the following:1 • Meropenem • Colistin • Polymyxin B • Amikacin Rifampin • Minocycline • Tigecycline In general, first-, second-, and third-generation cephalosporins, macrolides, and penicillins have little or no anti-Acinetobacter activity, and their use may predispose to Acinetobacter colonization. Antibiotics Empiric antimicrobial therapy should include one of the agents listed below. Colistimethate sodium (Coly-Mycin M) Hydrolyzed to colistin, which acts as cationic detergent that can damage bacterial cytoplasmic membrane, causing leaking of intracellular substances and cell death. Dosing • Interactions • Contraindications • Precautions Adult 2. 5-5 mg/kg/d IV/IM divided bid/qid, depending on severity of infection Pediatric Administer as in adults • Dosing • Interactions • Contraindications • Precautions Concurrent use with aminoglycosides may increase risk of respiratory paralysis and renal dysfunction; concurrent administration with cephalothin may increase risk of renal dysfunction; colistin may enhance neuromuscular blockade of nondepolaizing muscle relaxants • Dosing • Interactions • Contraindications Precautions Documented hypersensitivity; infections caused by Proteus or Neisseria species • Dosing • Interactions • Contraindications • Precautions Pregnancy C – Fetal risk revealed in studies in animals but not established or not studied in humans; may use if benefits outweigh risk to fetus Precautions Respiratory arrest; decreased urine output or increased BUN or serum creatinine levels; paresthesia; tingling of extremities or tongue; generalized itching or urticaria; drug fever; GI upset; vertigo; slurring of speech may occur
Meropenem (Merrem) Bactericidal broad-spectrum carbapenem antibiotic that inhibits cell wall synthesis. Effective against most gram-positive and gram-negative bacteria. Has slightly increased activity against gram-negative bacteria and slightly decreased activity against staphylococci and streptococci compared with imipenem. • Dosing • Interactions • Contraindications • Precautions Adult 1 g IV q8h Pediatric ;10 years: Administer as in adults
