Perils and Pearls of Acinetobacter baumannii: an emerging gram-negative pathogen

Abstract Acinetobacter baumannii has emerged as a highly troublesome pathogen for many institutions globally. As a consequence of its immense ability to acquire or up-regulate antibiotic drug resistance determinants, and cause outbreaks in healthcare institutions, it has justifiably been propelled into 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 new at-risk populations, including military personnel injured in the Iraq/Afghanistan conflict. The organism thrives within the harsh environment of a hospital setting, thus promoting its dissemination via cross-transmission. Relative to other hospital-acquired gram-negative pathogens, such as Pseudomonas aeruginosa, very little is known about the pathogenesis of this captivating organism, A. baumannii. The broad objectives of my PhD are to understand the significance of A. baumannii infection to hospitalized patients and develop in vivo model systems to study its pathogenesis. More specifically, the aims of my PhD include, 1) to assess for independent predictors of mortality among patients with A. baumannii bloodstream infection, including an assessment of carbapenem resistance on patient mortality, 2) to determine the mechanism by which A. baumannii develops resistance to the most contemporary of antimicrobials, tigecycline, 3) to utilize the non-mammalian model host, Caenorhabditis elegans, to study A. baumannii pathogenesis, and more specifically, the molecular interactions between A. baumannii and a common eukaryotic species found to co-inhabit hospitalized patients, Candida albicans, and 4) to develop and utilize a second, complimentary non-mammalian model system, known as Galleria mellonella, to study host-pathogen interactions of A. baumannii and explore the utility of the model to study therapeutics. Through the development of a consortium of healthcare institutions around the world (16 institutions across 5 continents), an observational series of patients with Acinetobacter bloodstream infection was performed. By use of Cox proportional regression analysis, independent predictors of mortality from A. baumannii bloodstream infection were determined. There is a concerning lack of new antimicrobials with activity toward Acinetobacter. Most recently, a tetracycline-like antibiotic, known as tigecycline was introduced, and given its in vitro activity, has provided some hope for the treatment of Acinetobacter infections. After observing several patients with tigecycline-resistant Acinetobacter bloodstream infection, we characterized, for the first time, the mechanism of resistance to this new antimicrobial, and found it to be due to overexpression of a resistance-nodulation division (RND)-type multidrug efflux pump (AdeABC). Given the life-threatening nature of some Acinetobacter infections and the paucity of data on A. baumannii pathogenesis, a major focus of my PhD was to develop skills in the use of non-mammalian model systems, such as C. elegans (round worm) and G. mellonella (caterpillars of the greater moth), for the study of A. baumannii. Furthermore, given the fact that A. baumannii often co-inhabits the patient and hospital environment with the human fungal pathogen, C. albicans, I extended the current paradigms of studying monomicrobial infection in C. elegans by studying A. baumannii-C. albicans co-infection within the worm. Through the development of a C. elegans polymicrobial infection model, I was able to study the in vivo molecular interactions between these diverse, yet clinically important pathogens. This work identified that A. baumannii uses both cell-cell contact and a secretary factor to antagonize several key virulence determinants of C. albicans, notably filamentation and biofilm formation. This process is at least partly controlled by a two-component regulatory system mediated through the A. baumannii gacS-like sensor kinase. This work advances our understanding of microbial pathogenesis within a more realistic environment of a polymicrobial setting, and by exploiting the antagonistic interactions between bacteria and fungi, novel therapeutic targets, which are desperately required for pathogens such as A. baumannii, may be uncovered. Finally, I confirm the use of a second, complimentary model system, known as G. mellonella, to study A. baumannii pathogenesis and demonstrate its utility in studying antimicrobial therapeutics, a potentially exciting approach for screening new compounds in vivo before mammalian experimentation