Induction of pneumonia in multidrug-resistant Acinetobacter baumannii infected immunocompetent BALB/c mice

Assessing the efficacy and safety of potential therapeutics for multidrug-resistant (MDR) Acinetobacter baumannii infections necessitates the use of in vivo models, typically involving mice and highly virulent isolates of the bacterium. In this study, we investigated the clinical isolate Ab35 of MDR Acinetobacter baumannii to determine its ability to infect and induce pneumonia in a mouse infection model. Immunocompetent BALB/c mice were infected through the oropharyngeal aspiration route. Enlarged spleen germinal center, reduced lung air space, and infiltration of immune cells within the lungs of infected mice were observed. Notably, there were no significant changes in body weight among the infected mice. Clinical scores were elevated from days 5 to 10 post-infection in groups administered with 1×10⁸ CFU/ml Ab35 (score: 3) and 1×10¹⁰ CFU/ml Ab35 (score: 6). In contrast, immunosuppressed mice exhibited clinical scores as early as 5 minutes after inoculation with 1×10¹⁰ CFU/ml Ab35, with observations beginning on day 2. Furthermore, a lung burden of 1.32 log10 CFU/ml (21 CFU/ml) was recorded in immunocompetent mice inoculated with 1×10¹⁰ CFU/ml Ab35. These findings suggest that infection with clinical isolates of A. baumannii in BALB/c mice through oropharyngeal aspiration can lead to symptomatic infections, including pneumonia. Thus, this study supports the feasibility of utilizing an in vivo mouse infection model with immunocompetent mice and clinical isolates of A. baumannii for future therapeutic evaluations.