Abstract
Francisella tularensis
, the causative agent of tularemia, lacks typical bacterial virulence factors and toxins but still exhibits extreme virulence. The bacterial multidrug efflux systems consist of an inner membrane, a transmembrane membrane fusion protein, and an outer membrane (OM) component that form a contiguous channel for the secretion of a multitude of bacterial products.
Francisella
contains three orthologs of the OM proteins; two of these, termed TolC and FtlC, are important for tularemia pathogenesis. The third OM protein, SilC, is homologous to the silver cation efflux protein of other bacterial pathogens. The
silC
gene (
FTL_0686
) is located on an operon encoding an Emr-type multidrug efflux pump of
F. tularensis
. The role of SilC in tularemia pathogenesis is not known. In this study, we investigated the role of SilC in secretion and virulence of
F. tularensis
by generating a
silC
gene deletion (Δ
silC
) mutant and its transcomplemented strain. Our results demonstrate that the Δ
silC
mutant exhibits increased sensitivity to antibiotics, oxidants, silver, diminished intramacrophage growth, and attenuated virulence in mice compared to wild-type
F. tularensis
. However, the secretion of antioxidant enzymes of
F. tularensis
is not impaired in the Δ
silC
mutant. The virulence of the Δ
silC
mutant is restored in NADPH oxidase-deficient mice, indicating that SilC resists oxidative stress
in vivo
. Collectively, this study demonstrates that the OM component SilC serves a specialized role in virulence of
F. tularensis
by conferring resistance against oxidative stress and silver.
IMPORTANCE
Francisella tularensis
, the causative agent of a fatal human disease known as tularemia, is a category A select agent and a potential bioterror agent. The virulence mechanisms of
Francisella
are not completely understood. This study investigated the role of a unique outer membrane protein, SilC, of a multidrug efflux pump in the virulence of
F. tularensis
. This is the first report demonstrating that the OM component SilC plays an important role in efflux of silver and contributes to the virulence of
F. tularensis
primarily by providing resistance against oxidative stress. Characterization of these unique virulence mechanisms will provide an understanding of the pathogenesis of tularemia and identification of potential targets for the development of effective therapeutics and prophylactics for protection from this lethal disease.