Mouse pneumonia model by Acinetobacter baumannii multidrug resistant strains: Comparison between intranasal inoculation, intratracheal instillation and oropharyngeal aspiration techniques

Infectious pneumonia induced by multidrug resistant (MDR) Acinetobacter baumannii strains is among the most common and deadly forms of healthcare acquired infections. Over the years, different strategies have been put in place to increase host susceptibility to MDR A. baumannii, since only a self-limiting pneumonia with no or limited local bacterial replication was frequently obtained in mouse models. Direct instillation into the trachea or intranasal inoculation of the bacterial suspension are the techniques used to induce the infection in most of the preclinical models of pneumonia developed to date. More recently, the oropharyngeal aspiration procedure has been widely described in the literature for a variety of purposes including pathogens administration. Aim of this study was to compare the oropharyngeal aspiration technique to the intranasal inoculation and intratracheal instillation in the ability of inducing a consistent lung infection with two MDR A. baumannii clinical isolates in immunocompromised mice. Moreover, pneumonia obtained by bacteria administration with two out of three techniques, intratracheal and oropharyngeal, was characterised in terms of histopathology of pulmonary lesions, biomarkers of inflammation level and leukocytes cells infiltration extent after mice treatment with either vehicle or the antibiotic tigecycline. The data generated clearly showed that both strains were not able to colonize the lungs when inoculated by intranasal route. By contrast, the bacterial load in lungs of mice intratracheally or oropharyngeally infected significantly increased during 26 hours of monitoring, thus highlighting the ability of these strains to generate the infection when directly instilled into the lower respiratory airways. Furthermore, the intragroup variability of mice was significantly reduced with respect to those intranasally administered. Tigecycline was efficacious in lung bacterial load and cytokines release reduction. Findings were supported by semi-quantitative histopathological evaluation of the pulmonary lesions and by inflammatory biomarkers analysis. To conclude, both intratracheal instillation and oropharyngeal aspiration techniques showed to be suitable methods for inducing a robust and consistent pneumonia infection in mice when difficult MDR A. baumannii clinical isolates were used. Noteworthy, oropharyngeal aspiration not requiring specific technical skills and dedicated equipment, was proven to be a safer, easier and faster technique in comparison to the intratracheal instillation.