Colistin offers prolonged survival in experimental infection by multidrug-resistant Acinetobacter baumannii: the significance of co-administration of rifampicin

Acinetobacter baumannii: emergence of a successful pathogen

Acinetobacter baumannii has emerged as a highly troublesome pathogen for many institutions globally. As a consequence of its immense ability to acquire or upregulate antibiotic drug resistance determinants, it has justifiably been propelled to the forefront of scientific attention. Apart from its predilection for the seriously ill within intensive care units, A. baumannii has more recently caused a range of infectious syndromes in military personnel injured in the Iraq and Afghanistan conflicts. This review details the significant advances that have been made in our understanding of this remarkable organism over the last 10 years, including current taxonomy and species identification, issues with susceptibility testing, mechanisms of antibiotic resistance, global epidemiology, clinical impact of infection, host-pathogen interactions, and infection control and therapeutic considerations.

Acinetobacter baumannii: epidemiology, antimicrobial resistance, and treatment options

Multidrug-resistant Acinetobacter baumannii is recognized to be among the most difficult antimicrobial-resistant gram-negative bacilli to control and treat. Increasing antimicrobial resistance among Acinetobacter isolates has been documented, although definitions of multidrug resistance vary in the literature. A. baumannii survives for prolonged periods under a wide range of environmental conditions. The organism causes outbreaks of infection and health care-associated infections, including bacteremia, pneumonia, meningitis, urinary tract infection, and wound infection. Antimicrobial resistance greatly limits the therapeutic options for patients who are infected with this organism, especially if isolates are resistant to the carbapenem class of antimicrobial agents. Because therapeutic options are limited for multidrug-resistant Acinetobacter infection, the development or discovery of new therapies, well-controlled clinical trials of existing antimicrobial regimens and combinations, and greater emphasis on the prevention of health care-associated transmission of multidrug-resistant Acinetobacter infection are essential.

Acinetobacter baumannii: a universal threat to public health?

Acinetobacter spp. are non-fermentative, strictly aerobic, Gram-negative microorganisms with a confusing taxonomic history. The Acinetobacter baumannii-Acinetobacter calcoaceticus complex is the species most commonly isolated from clinical specimens. It is ubiquitous in nature and has been found as part of the normal skin, throat and rectal flora as well as in food and body lice. It colonises patients in Intensive Care Units and contaminates inanimate hospital surfaces and devices as well as wounds, including war injuries. Although a frequent coloniser, Acinetobacter can be the cause of severe and sometimes lethal infections, mostly of nosocomial origin, predominantly ventilator-associated pneumonia. Bacteraemic infections are rare but may evolve to septic shock. Acinetobacter also emerges as a cause of nosocomial outbreaks and is characterised by increasing antimicrobial multiresistance. Antibiotic use, especially carbapenems and third-generation cephalosporins, is recognised as the most important risk factor for multiresistance. Described resistance mechanisms include hydrolysis by beta-lactamases, alterations in outer membrane proteins and penicillin-binding proteins, and increased activity of efflux pumps. Today, Acinetobacter resistant to carbapenems, aminoglycosides and fluoroquinolones presents a challenge to the clinician. However, sulbactam, tigecycline and colistin represent the current therapeutic approaches, which are associated with satisfactory efficacy.

Rat pneumonia and soft-tissue infection models for the study of Acinetobacter baumannii biology

Acinetobacter baumannii is a bacterial pathogen of increasing medical importance. Little is known about its mechanisms of pathogenesis, and safe reliable agents with predictable activity against A. baumannii are presently nonexistent. The availability of relevant animal infection models will facilitate the study of Acinetobacter biology. In this report we tested the hypothesis that the rat pneumonia and soft-tissue infection models that our laboratory had previously used for studies of extraintestinal pathogenic Escherichia coli were clinically relevant for A. baumannii. Advantages of these models over previously described models were that the animals were not rendered neutropenic and they did not receive porcine mucin with bacterial challenge. Using the A. baumannii model pathogen 307-0294 as the challenge pathogen, the pneumonia model demonstrated all of the features of infection that are critical for a clinically relevant model: namely, bacterial growth/clearance, an ensuing host inflammatory response, acute lung injury, and, following progressive bacterial proliferation, death due to respiratory failure. We were also able to demonstrate growth of 307-0294 in the soft-tissue infection model. Next we tested the hypothesis that the soft-tissue infection model could be used to discriminate between the inherent differences in virulence of various A. baumannii clinical isolates. The ability of A. baumannii to grow and/or be cleared in this model was dependent on the challenge strain. We also hypothesized that complement is an important host factor in protecting against A. baumannii infection in vivo. In support of this hypothesis was the observation that the serum sensitivity of various A. baumannii clinical isolates in vitro roughly paralleled their growth/clearance in the soft-tissue infection model in vivo. Lastly we hypothesized that the soft-tissue infection model would serve as an efficient screening mechanism for identifying gene essentiality for drug discovery. Random mutants of 307-0294 were initially screened for lack of growth in human ascites in vitro. Selected mutants were subsequently used for challenge in the soft-tissue infection model to determine if the disrupted gene was essential for growth in vivo. Using this approach, we have been able to successfully identify a number of genes essential for the growth of 307-0294 in vivo. In summary, these models are clinically relevant and can be used to study the innate virulence of various Acinetobacter clinical isolates and to assess potential virulence factors, vaccine candidates, and drug targets in vivo and can be used for pharmacokinetic and chemotherapeutic investigations.

Acinetobacter infection in the ICU

Acinetobacter is a formidable challenge to managing critically ill patients. This pathogen's ability to rapidly develop antimicrobial resistance to all currently available antimicrobial agents is concerning because increasing data support attributable mortality to these bacteria when associated with hospitalized patients with comorbidities and severe illness. The role of dual therapy is currently unclear and might be associated with increased toxicities without proven synergy or ability to prevent the development of resistance. Infection control and antibiotic control measures might have the greatest impact on these bacteria. Continued efforts are needed to develop new antimicrobial agents against this pathogen and assess the ideal currently available agents.

Therapeutic options for Acinetobacter baumannii infections

Acinetobacter baumannii is an important cause of nosocomial infections, mainly in patients in intensive care units. This microorganism, although with slight differences depending on the country, presents resistance to multiple antimicrobial agents, occasionally including resistance to colistin: hence, it can be considered the paradigm of nosocomial multiresistant bacteria. This review analyzes the evolution of antimicrobial resistance and the molecular bases associated with the increase in antimicrobial resistance, as well as the current treatment of Acinetobacter infections. Although controversy remains, the pooled data suggest that infections by A. baumannii may be associated with considerable attributable mortality. Moreover, in cases of pneumonia and bacteraemia, inappropriate treatment is associated with, among other factors, mortality. Therefore, treatment should be carefully considered.

An increasing threat in hospitals: multidrug-resistant Acinetobacter baumannii

Since the 1970s, the spread of multidrug-resistant (MDR) Acinetobacter strains among critically ill, hospitalized patients, and subsequent epidemics, have become an increasing cause of concern. Reports of community-acquired Acinetobacter infections have also increased over the past decade. A recent manifestation of MDR Acinetobacter that has attracted public attention is its association with infections in severely injured soldiers. Here, we present an overview of the current knowledge of the genus Acinetobacter, with the emphasis on the clinically most important species, Acinetobacter baumannii.