Dynamics of intracellular bacterial replication at the single cell level

Area of Science:

• Microbiology
• Cell Biology
• Infectious Diseases

Background:

• Pathogens cause disease by surviving and replicating within host cells.
• Distinguishing bacterial replication from killing is challenging, often relying on net bacterial load.
• Heterogeneity in pathogen populations, especially non-growing bacteria in persistent infections, is poorly understood.

Purpose of the Study:

• To develop a reporter system for quantifying bacterial replication dynamics.
• To investigate the role of Salmonella pathogenicity island 2 (SPI-2) type III secretion system (T3SS) in Salmonella Typhimurium (S. Typhimurium) replication and killing.
• To characterize bacterial behavior within host macrophages, including non-replicating states.

Main Methods:

• Development of a fluorescence dilution reporter system.
• Quantification of S. Typhimurium replication dynamics in murine macrophages at population and single-cell levels.
• Assessment of the impact of SPI-2 T3SS on bacterial replication and resistance to killing.

Main Results:

• The reporter system enabled direct quantification of bacterial replication dynamics.
• SPI-2 T3SS is crucial for S. Typhimurium replication but not significantly for resistance to killing.
• A substantial proportion of bacteria enter a dormant-like state upon macrophage entry, potentially forming a reservoir for persistence.

Conclusions:

• The developed reporter system allows precise measurement of bacterial replication and killing.
• SPI-2 T3SS primarily influences bacterial replication, not host-mediated killing.
• Non-replicating, dormant bacteria within macrophages are a significant finding with implications for chronic infections.
• This approach can be applied to other pathogens to study virulence and host-pathogen interactions.