Replicative Acinetobacter baumannii strains interfere with phagosomal maturation by modulating the vacuolar pH

Area of Science:

• Immunology
• Microbiology
• Pathogenesis

Background:

• Multidrug-resistant Acinetobacter baumannii causes dangerous nosocomial pneumonias.
• Alveolar macrophages are crucial for fighting respiratory infections.
• Clinical A. baumannii strains, not lab strains, persist in macrophages within Acinetobacter Containing Vacuoles (ACV).

Purpose of the Study:

• To investigate the in vivo survival of a clinical A. baumannii isolate (398) versus a lab strain (19606) in alveolar macrophages during pneumonia.
• To elucidate the mechanisms by which A. baumannii persists within macrophages.

Main Methods:

• Infection of alveolar macrophages with A. baumannii strains 398 and 19606 in a murine pneumonia model.
• Analysis of bacterial fate within macrophages using endocytic pathway markers (EEA1, LAMP1) and autophagy.
• Measurement of phagosome acidification and ammonia secretion by A. baumannii.

Main Results:

• Clinical isolate 398, but not lab strain 19606, infected alveolar macrophages and formed ACVs in vivo.
• Strain 19606 was degraded via autophagy, while strain 398 replicated within ACVs.
• Strain 398 prevented phagosome acidification by secreting ammonia, a byproduct of amino acid catabolism.

Conclusions:

• Clinical A. baumannii isolates possess a unique mechanism to survive within alveolar macrophages.
• Ammonia secretion allows A. baumannii to evade phagosome acidification and degradation.
• Macrophage survival is a critical factor for the lung persistence of clinical A. baumannii strains in pneumonia.