Rat Pneumonia and Soft-Tissue Infection Models for the Study of Acinetobacter baumannii Biology

Abstract

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.

Role of Acinetobactin-Mediated Iron Acquisition Functions in the Interaction of Acinetobacter baumannii Strain ATCC 19606 T with Human Lung Epithelial Cells, Galleria mellonella Caterpillars, and Mice

“…This surface-exposed protein plays a clear role in the abilities of A. baumannii to attach to, invade, and cause the apoptotic death of A549 human alveolar epithelial cell monolayers. A. baumannii clinical isolates, including the ATCC 19606 T type strain, also cause deadly acute sepsis, pneumonia, and soft tissue infections when tested in murine experimental infections (30,38) as well as the death of infected larvae of the greater wax moth Galleria mellonella (35). Taken together, these studies and observations indicate that A. baumannii is able to persist within an infected host by acquiring essential nutrients.…”

Recent Advances in the Immunopathogenesis of Acinetobacter baumannii Infection

“…Blood contains a number of innate immune components that can restrict bacterial growth and even kill a large proportion of infecting microorganisms. Human serum is bactericidal or bacteriostatic to most strains of A. baumannii and this was shown to be mediated by complement (29,45). The alternative complement pathway is responsible for killing the bacteria (45,51).…”

“…As expected, treatment of mice with immunosuppressive drugs (such as cyclophosphamide) greatly exacerbate the infection and can convert an otherwise self-limiting infection into a lethal one (27). In addition, a rat model has been established and used to study both pneumonia and soft tissue injury (29). Human studies are so far limited to bactericidal assays using serum or ascites fluid and the use of human peripheral blood mononuclear cells and various epithelial cell lines (29)(30)(31)(32)(33).…”

“…In addition, a rat model has been established and used to study both pneumonia and soft tissue injury (29). Human studies are so far limited to bactericidal assays using serum or ascites fluid and the use of human peripheral blood mononuclear cells and various epithelial cell lines (29)(30)(31)(32)(33). More basic in vivo models involving inhibition of Caenorhabditis elegans and Dictyostelium discoideum were employed for screening the virulence of multiple A. baumannii transposon insertional mutants (34).…”

Two bacterial infection models in tree shrew for evaluating the efficacy of antimicrobial agents

“…Testing the therapeutic potential of new antimicrobial drugs in animal models is a very important stage of that process (Craig, 1993). To date, bacterial infection models have been established in mice, rats, and rabbits, but no models have been developed for small primate species (McCormick et al, 2008;Retsema et al, 1993;Russo et al, 2008). The mechanisms of infectious disease in non-primates are not similar with humans and although some antimicrobials have been found useful in non-primate animal models, they lost their activity during clinical trials (Druilhe et al, 2002).…”