The sensu stricto definition includes six species: L. monocytogenes, L. innocua, L. seeligeri, L. welshimeri, L. ivanovii and L. marthii. Though predominantly L. monocytogenes causes food-related illness, the Listeria sensu stricto species are ubiquitous and often used as a hygiene indicator.
Frequently contaminated foods are uncooked products, such as salmon, raw milk, raw sausage or raw cured meat.