Shiga toxin producing Escherichia coli (STEC) phages enhance innate immune functions of macrophages.

Shiga toxin (STX) producing Escherichia coli (STEC) inhabit the ruminant gastrointestinal system without any clinical manifestation and serves as a source of STEC for human infections. Human infections are associated with severe clinical manifestations. Although antibiotics are effective against many bacterial infections, antibiotic treatment of STEC infection in humans leads to the aggravated clinical manifestations. In search of novel STEC control measures, bacteriophage therapy is gaining increasing attention as a biocontrol method. Although, bacteriophages target specifically the bacteria leading to bacterial killing, the bacteriophages can cross the mucosal barriers and encounter immune cells. One such immune population encountered by the bacteriophages is the professional antigen presenting cells (APCs) such as macrophages. The interaction of bacteriophages and macrophages culminate phagocytosing of bacteriophages by the macrophages rather than an infection. This interaction has many implications due to mounting of host responses. The information is scarce on bacteriophage-macrophage interactions and their consequences. The study investigated bacteriophage-macrophage interactions with the objectives of 1) evaluating if STEC phages are able to induce innate host responses, particularly nitric oxide (NO) production and 2) establishing if STEC phages are able to alter the phagocytic ability of macrophages. First, we evaluated a range of multiplicity of infection (MOI) of Tequinatavirus (T5) phage, AKFV33 and Tequatrovirus (T4) phage, wV7 in murine macrophage cells (J744.1) via cell viability assay. To achieve the objective 1, the optimum MOI of AKFV33 and wV7 phages with minimal impact on cell viability was used along with positive (lipopolysaccharide) and negative controls to evaluate the NO production via total nitric oxide assay kit.  To achieve the objective 2, the optimum MOI of AKFV33 and wV7 phages with minimal impact on cell viability was used in murine macrophage cells and phagocytic function was evaluated following inoculation using latex beads via immunofluorescent and flowcytometry assays. We discovered that AKFV33 and wV7 phages could induce NO production and enhance the phagocytic functions in murine macrophages. Our observations imply that STEC phages are immunostimulatory and this can be an added advantage when these phages are used as biocontrol agents.