A live, genetically modified probiotic bacteria Bifidobacterium longum (B. longum) engineered to deliver genetic material encoding the human pro-inflammatory transgene interleukin-12 (IL-12), with potential immunoactivating activity. Upon administration of genetically modified IL-12 transgene-encoding B. longum, the bacteria selectively colonize hypoxic tumor tissues and deliver plasmid DNA encoding the IL-12 transgene within the tumor microenvironment (TME). IL-12 is translated in cells and activates the immune system by promoting the activation of natural killer cells (NK cells), inducing secretion of interferon-gamma and promoting cytotoxic T-cell responses against tumor cells. This may result in both immune-mediated tumor cell death and the inhibition of tumor cell proliferation.