Dillapiole Dampens the Expression of the Major Virulence Genes of Francisella tularensis

TL;DR

Dillapiole, a compound from fennel, reduces the virulence of Francisella tularensis by dampening key pathogenic genes, offering a new potential treatment for tularemia.

Contribution

Dillapiole is identified as a novel compound that specifically dampens virulence gene expression in Francisella tularensis during infection.

Findings

Dillapiole significantly downregulates MglA- and SspA-controlled virulence genes in Francisella tularensis.

RNA-seq and Western blot analyses confirm reduced expression of IglA and IglC in F. tularensis treated with dillapiole.

Dillapiole limits F. tularensis replication in THP-1 cells without enhancing host immunity.

Abstract

Francisella tularensis is a pathogenic bacterium and the causative agent of the disease tularemia. Because of the virulence of this bacterium and the potential for weaponization, the Centers for Disease Control and Prevention (CDC) has classified F. tularensis as a Category A Bioterrorism Agent. Therefore, the need for new treatments for tularemia is critical. In this work, we screened a cataloged library of natural extracts to identify those that inhibit the growth of F. tularensis only during infection of THP-1 monocyte cells. One of the most promising extracts identified in this screen was derived from Foeniculum vulgare (fennel). Using bioassay-guided fractionation, the fennel extract was fractionated, and the bioactive compound was isolated and structurally elucidated as the phenylpropanoid dillapiole. We subsequently confirmed that dillapiole alone could limit the replication of F. tularensis in infected THP-1 cells, but not outside of this infection model. Investigations on host responses suggested that dillapiole was not substantially augmenting the immunity of these THP-1 cells. We then investigated the potential virulence modulation activity of dillapiole. To test this hypothesis, RNA-seq analysis was carried out on F. tularensis bacteria that were treated with dillapiole. This showed that dillapiole caused a significant downregulation of genes controlled by the transcriptional regulators MglA and SspA, including those encoded in the Francisella pathogenicity island. Western blotting validated these findings as both IglA and IglC expression was diminished in F. tularensis LVS bacteria treated with dillapiole. Because dillapiole dampens the virulence gene expression of F. tularensis, we concluded that this compound has potential to be used as a novel therapeutic for tularemia with a unique mechanism of action.

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