Inability to sustain intraphagolysosomal killing of Staphylococcus aureus predisposes to bacterial persistence in...

Area of Science:

• Immunology
• Microbiology
• Cell Biology

Background:

• Macrophages are key immune cells in the innate response to bacterial infections.
• Bacterial clearance relies on phagocytosis and intracellular killing by macrophages.
• The bottleneck in macrophage bacterial clearance, especially with Staphylococcus aureus, is not well understood.

Purpose of the Study:

• To investigate the rate-limiting step in macrophage bacterial clearance when challenged with high loads of Staphylococcus aureus.
• To elucidate the mechanisms underlying bacterial persistence within macrophages.

Main Methods:

• Macrophage challenge with high inocula of Staphylococcus aureus (Newman, SH1000, USA300 strains).
• Assessment of bacterial viability post-phagocytosis.
• Analysis of phagolysosomal acidification, maturation, and cathepsin D activity.
• Investigation of macrophage apoptosis and cell lysis.
• Murine model using alveolar macrophages to study intracellular persistence.

Main Results:

• Macrophages exhibit a finite intracellular killing capacity, failing to match phagocytosis with sustained killing of Staphylococcus aureus.
• Reduced phagolysosomal acidification and maturation lead to the accumulation of viable intracellular bacteria.
• Macrophages do not effectively utilize apoptosis for bacterial killing.
• Cell lysis and subsequent reuptake by other macrophages create a cycle of bacterial persistence.
• Intracellular persistence of Staphylococcus aureus was observed in murine alveolar macrophages.

Conclusions:

• Macrophage intracellular killing capacity is overwhelmed by large Staphylococcus aureus inocula.
• Impaired phagolysosomal function contributes to bacterial survival within macrophages.
• Cycles of lysis and reuptake facilitate the persistence of Staphylococcus aureus within macrophage populations.
• This study reveals a mechanism for intracellular bacterial persistence relevant to Staphylococcus aureus infections.