The organism is a rapidly emerging pathogen in the healthcare setting, where it causes healthcare-associated infections including bacteremia, pneumonia, meningitis, urinary tract infections,
and wound infections.
Properties of A. baumannii, such as its ability to survive under a wide range of environmental conditions and to persist for extended periods of time on environmental surfaces, make it a frequent cause of outbreaks as well as an endemic pathogen in the healthcare setting.
Risk factors for colonization or infection with A. baumannii include prolonged length of hospital stay,exposure to intensive care units (ICUs) and mechanical ventilation, antimicrobial use, recent surgery, invasive procedures, and underlying severity of illness.
Whereas prior to the 1990s Acinetobacter baumannii were almost universally susceptible to broad spectrum antibiotics, during
this decade they became increasingly resistant to penicillins, cephalosporins, fluoroquinolones and aminoglycosides (Bergogne-Berezin and Towner, 1996). Thus in recent years, many of these antimicrobials are no longer reliable for treatment of infections caused by this organism. Most notable is the increase in resistance to the carbapenems which are caused by a variety of Beta- lactamases and changes in penicillin-binding proteins (PBPs) (Nordmann and Poirel, 2002). There are now reports of multidrug resistant A. baumannii strains that are susceptible only to polymixin B and colistin (Levin et al., 1998).
U.S. Military Experience
An increased number of A. baumannii infections have been noted in U.S. military personnel wounded during the conflicts in Iraq and Afghanistan. These infections are generally extremely resistant to a variety of antimicrobial agents, are commonly associated with traumatic injuries, and include deep wound infections, osteomyelitis, respiratory infections, and bacteremia.
-The first is that soldiers are previously colonized with the organism, perhaps on their skin or in their nares, and are auto-inoculated during a penetrating traumatic injury. However, cultures taken prior to deployment from healthy soldiers' skin and nares found no Acinetobacter in the nares, and though skin colonization was common,strain types did not match the strains that infect returning wounded soldiers. Therefore, it is less likely that prior colonization explains Acinetobacter infected war wounds.There are three main hypotheses for how wounded soldiers acquire Acinetobacter
-The second hypothesis is that Acinetobacter from the local environment (soil or water) is introduced during traumatic injury. Two studies showed that cultures of war wounds taken in Iraq soon after injuries grew primarily low virulence Gram-positive organisms. The few Gram-negative organisms recovered were susceptible to many antimicrobial agents, which does not support the hypothesis of environmental contamination at the time of injury.
-The third hypothesis is that Acinetobacter is healthcare-associated, acquired by soldiers in medical facilities during the process of stabilization, emergency treatment,and evacuation through the military medical system.
Acinetobacter baumannii Infections Among Patients at Military Medical Facilities Treating Injured U.S. Service Members, 2002--2004
Definitions of MDR Acinetobacter
Two of the most common criteria for MDR are:
-carbapenem resistance
-resistance to three or more antimicrobial classes
Carbapenem-resistant Acinetobacter has been reported by tertiary-care hospitals, usually in the setting of nosocomial outbreaks. Some strains of MDR Acinetobacter are susceptible only to
colistin or polymyxin B, peptide antibiotics not routinely used due to earlier reports of toxicities (sometimes these isolates are referred to as polymyxin-only-susceptible or POS Acinetobacter).
Strains that demonstrate resistance to all antimicrobial agents including polymyxin (sometimes referred to as pan-drug-resistant, or PDR, Acinetobacter) have also been reported making treatment of these infections extremely difficult and in some cases impossible to treat.
Mechanisms of resistance
The mechanisms of resistance generally fall into three categories:
(i) antimicrobial inactivating enzymes
(ii) reduced access to bacterial targets
(iii) mutations that change targets or cellular functions
Given the lack of good therapeutic options, the development or discovery of new therapies and greater emphasis on the prevention of healthcareassociated transmission of MDR Acinetobacter are essential.
