There are six different species of the bacterium Listeria. Only one of these species Listeria monocytogenes (L. monocytogenes) causes human illness – the others are usually harmless for humans. Listeria can be found almost everywhere and has been isolated from a number of sources, including soil, water and foods. It has evolved the ability to survive in a variety of different environmental conditions, including moist environments like refrigerators, and under a variety of stress conditions.
Within L. monocytogenes, a multitude of different strains (e.g. more than 300 strains identified in one study) have been documented. Strains can be defined by a variety of methods, including a process known as pulsed-field gel electrophoresis (PFGE) which gives different “genetic fingerprint” patterns.