Host Cytosolic Glutathione Sensing by a Membrane Histidine Kinase Activates the Type VI Secretion System in an Intracellular Bacterium

Area of Science:

• Microbiology
• Bacterial Pathogenesis
• Molecular Biology

Background:

• Type VI secretion systems (T6SSs) are critical virulence factors in Gram-negative bacteria.
• The regulatory signals activating T6SSs, especially during intracellular infection, remain largely unknown.
• T6SS1 of Burkholderia pseudomallei is crucial for virulence and is expressed only within host cells.

Purpose of the Study:

• To identify the host-derived signals that trigger T6SS1 expression in intracellular Burkholderia pseudomallei.
• To elucidate the molecular mechanism by which these signals are sensed and activate the T6SS1.

Main Methods:

• Site-directed mutagenesis of the VirA sensor.
• Biochemical assays to study VirA activity and dimerization.
• Glutathione depletion experiments in host cells.
• Assessment of T6SS1-dependent bacterial virulence phenotypes (multinucleate giant cell formation).

Main Results:

• Host cytosol contains signals that activate bacterial T6SS1.
• Low molecular weight thiols, specifically glutathione, were identified as the activating signal.
• Glutathione binds to and reduces a periplasmic cysteine (C62) on the VirA sensor, converting it from a dimer to a monomer.
• Depletion of host glutathione abolished TSS1 expression and virulence.

Conclusions:

• Intracellular Burkholderia pseudomallei utilizes host glutathione as a specific signal to activate T6SS1.
• This mechanism allows bacteria to precisely time virulence factor expression to the intracellular environment.
• Virulence factor activation is mediated by the VirA sensor's response to glutathione levels.