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Summary: The infamous Typhoid Mary was a carrier of Salmonella typhi but surprisingly never appeared sick because 1-6% of infected people are asymptomatic, chronic carriers. A mouse model was created to demonstrate the unique processes behind asymptomatic infection of Salmonella: 

1. Pathogens secrete inflammatory substances which activates the body’s macrophages to violently engulf the invaders. 

2. If the pathogen can withstand the inflammation (like Salmonella can), the body secretes anti-inflammatory factors (i.e. for wound healing) which tames the macrophages.

3. Salmonella associate/infect these calmer post-inflammatory M2 macrophages.

4. PPARδ, which is a transcription factor in fatty acid metabolism, helps provide Salmonella with the glucose necessary to replicate. Therefore, Salmonella could not survive in PPARδ null mice.

“These data suggest that M2 macrophages represent a unique niche for long-term intracellular bacterial survival and link the PPARδ-regulated metabolic state of the host cell to persistent bacterial infection.”

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