A Listeria bacterium is disclosed which comprises a first mutation in the multidrug resistance transporter M (MdrM) gene which causes a decrease in interferon-β production in macrophages as compared to wild-type Listeria bacterium and a second mutation in the multidrug resistance transporter T (MdrT) gene which causes a decrease in interferon-β production in macrophages as compared to the wild-type Listeria bacterium.
