Listeria bacterial culture (white) growing in a petri dish on agar (red). Listeria bacteria can cause food poisoning (listeriosis) if people eat food contaminated by Listeria bacteria. This can lead to influenza-like symptoms, septicaemia and brain inflammation. These last two symptoms can be fatal. Listeria bacteria are normally cultured on blood agar, an agar that has been enriched with blood. The agar is a growing medium that provides the bacteria with nutrients.