Formate Promotes Shigella Intercellular Spread and Virulence Gene Expression.

Area of Science:

• Microbiology
• Pathogen-host interactions
• Bacterial metabolism

Background:

• Shigella flexneri is an intracellular pathogen that invades the colon epithelium.
• Upon host cell invasion, S. flexneri metabolizes host carbon sources, producing formate as a byproduct.
• The mechanisms by which S. flexneri senses and adapts to the intracellular environment are not fully understood.

Purpose of the Study:

• To investigate the role of formate in S. flexneri virulence and intracellular lifestyle.
• To determine if formate acts as an intracellular signal for S. flexneri.

Main Methods:

• Genetic manipulation of S. flexneri to generate mutants lacking pyruvate formate lyase (pflB) or formate dehydrogenase (fdnG).
• Assessing plaque formation in epithelial cell monolayers.
• Supplementing cell cultures with exogenous formate.
• Measuring the expression of S. flexneri virulence genes (icsA, ipaJ).
• Analyzing host gene expression (interferon, TNF-stimulated genes).

Main Results:

• Loss of PFL in S. flexneri eliminated formate production and impaired cell-to-cell spread, but not intracellular growth.
• Supplying exogenous formate complemented the plaque defect of the ΔpflB mutant and increased plaque size of wild-type S. flexneri.
• Deletion of fdnG increased host cell formate accumulation and S. flexneri plaque size.
• Formate significantly increased the expression of S. flexneri virulence genes icsA and ipaJ.
• Formate altered host interferon and TNF-stimulated gene expression.

Conclusions:

• Formate is a critical byproduct of S. flexneri intracellular metabolism.
• Formate acts as an intracellular signal, modulating S. flexneri virulence gene expression and cell-to-cell spread.
• S. flexneri senses host cell formate accumulation to regulate virulence factors, potentially controlling spatiotemporal spread and immune evasion.