Tracking and controlling Listeria monocytogenes in food processing plants

Listeria monocytogenes is a Gram-positive foodborne bacterium, which cycles between being a saprophyte in the environment and a pathogen in animals including humans.  It causes numerous recalls and outbreaks in various types of ready-to-eat (RTE) foods with resultant high case-fatality rates.  While meat, poultry and dairy products were previously the main food vehicles associated with listeriosis, numerous RTE produce products have also recently been associated with major outbreaks.  Due to its hardy nature and ubiquitous presence in food processing plants, Listeria monocytogenes has proven to be a difficult and costly pathogen for the food industry to control.  Recent sequence-based molecular subtyping methods now enable government, industry and academic laboratories to track and control the main epidemic clones and outbreak clones of this pathogen.  Research using Multi-Virulence-Locus Sequence Typing (MVLST) has shown that seven main epidemic clones cause most cases and outbreaks.  In addition, sequencing of prophages within the whole genome of this pathogen enable the tracking of specific outbreak clones to hard-to-clean reservoirs/harborage sites within food processing plants. The presence of a food “conditioning film” within the harborage site allows bacteria to attach, grow to high levels and form biofilms.  Cells from biofilms can then readily slough off and contaminate foods as they pass over this site on the equipment.  Therefore, eliminating harborage sites, especially those on food-contact surfaces, is of paramount importance for preventing food contamination by L. monocytogenes.  Additional strategies for control include destroying and/or inhibiting the growth of this pathogen if and when it contaminates RTE foods.