Drug-Resistant Acinetobacter baumannii-calcoaceticus Complex: An Emerging Nosocomial Pathogen With Few Treatment Options

Acinetobacter baumannii-calcoaceticus complex (ABC) organisms are aerobic gramnegative coccobacillary rods with a natural reservoir in soil and water sources around the world. Clinically, ABC organisms cause health careYassociated infections, such as ventilatorassociated pneumonias, urinary tract infections, surgical wound infections, and bloodstream infections. These bacteria are also well-known causes of health careYassociated outbreaks, particularly in intensive care units and among immunocompromised patients. Over the past decade, the challenge of managing infections caused by ABC organisms has been complicated by an increase in antimicrobial resistance, especially multidrug resistance, with some strains now resistant to all or almost all commonly used antimicrobial agents. ABC species have emerged as health careYassociated pathogens, in part, because they are resilient bacteria with a diverse natural habitat. Not only can they survive in moist environments, but they can also survive for weeks on dry surfaces; these organisms can live for an average of 20 days at a relative humidity of 31%. Outbreak investigations have demonstrated that environmental contamination with ABC can be widespread and serve as sources of infection. In addition, ABC organisms can colonize the human skin, throat, and respiratory tract. Thus, in health care facilities, these bacteria can contaminate the environment and spread between colonized or infected patients directly from hospital equipment or via the hands of health care workers, especially when there are lapses in infection control practices. Aside from their capacity to survive in the hospital environment, ABC organisms have a remarkable ability to develop antimicrobial resistance. These bacteria possess a variety of intrinsic resistance mechanisms that can be expressed constitutively or in response to antimicrobial pressure. Changes in outer membrane proteins can reduce the access of antimicrobial agents to penicillin-binding proteins in the cell membrane of the bacteria. Mutations in topoisomerase genes can provide fluoroquinolone resistance. Efflux pumps can expel various classes of antimicrobial agents including "-lactams, quinolones, tetracyclines, and aminoglycosides. ABC organisms also have several types of aminoglycoside-modifying enzymes (acetylating, adenylating, and phosphorylating). Although various "-lactamases have been described in ABC, the AmpC cephalosporinases are common to most, if not all, strains. In certain circumstances, production of this cephalosporinase is up-regulated and confers resistance to ceftazidime and other thirdgeneration cephalosporins. Another intrinsic "-lactamase, whose genes have been identified in all ABC isolates, is an oxacillinase (specifically, OXA-69). This oxacillinase has carbapenemase properties, although, in many circumstances, its expression is low. In addition to these myriad intrinsic resistance mechanisms, ABC organisms also have the ability to acquire resistance genes. These bacteria can readily acquire resistance from Editorial Comment