Pulmonary microbial infection in mice: Comparison of different application methods and correlation of bacterial numbers and histopathology

Many investigations have been performed in characterising experimental bacterial infections in the lung of mice using several pathogens. Robust experimental pulmonary infection models require a reproducible method of application with defined numbers of pathogens to the respiratory tract without contaminating extrapulmonary tissues. At the same time trauma due to the experimental procedure should be kept to a minimum. So far several routes of administration have been used but a systematic comparison of these methods is still missing. Here we provide a comprehensive evaluation of view controlled i.t. instillation, tracheotomy, intranasal application, blind instillation and aerosol infection. An infection dose of up to 5 × 10⁴ bacteria (L. monocytogenes) was applied to a group of ten mice by each technique and the animals were killed after 1h or 24h. The number of viable bacteria was estimated by plating homogenates of the lungs and spleens. In addition, pathological effects on lung tissue were examined by histology 24h after infection. The highest reproducibility was achieved after applying Listeria directly in the trachea under view or by tracheotomy. However, mice were severely affected in their vitality after tracheotomy. Thus, for topical application of bacterial suspension into the lung the view controlled i.t. instillation is most appropriate.