Listeria monocytogenes bacteria are very hardy infectious bacteria and widely distributed in nature, and very difficult to control. Listeria monocytogenes previously known to veterinary science as a pathogen of sheep, first came to light as a major foodborne agent when the largest and most deadly outbreaks in US history occurred in queso fresco cheese manufactured in Los Angeles, California. http://en.wikipedia.org/wiki/List_of_foodborne_illness_outbreaks_by_death_toll
The deadliest foodborne illness outbreak in US history, it claimed the lives of approximately 50 persons and infected 86 known victims. From this time forward the public health response has been to actively track cases identify outbreaks and to put into place initiatives to reduce incidence. USDA was instrumental in developing protocols for the meat industry to follow to control the organism in USDA regulated products. Listeria monocytogenes is considered to be so pathogenic that in the US there is no tolerance for its presence in food. There has been some success in controlling Listeria monocytogenes in USDA commodities, but recently, over the last 3 years or so we have seen this agent get thorough the public health safety net an alarming number of times.
In a just few years, in the US we have seen Listeria monocytogenes in cantaloupe (2011) kill 30 people, and infect 146 known cases, and outbreaks and or recalls involving several brands of ice cream, sliced apples, candy apples and pasta salad.
The Chain of Infection and Controls
Listeria monocytogenes is a uniquely challenging pathogen with novel characteristics. It is psychrotropic, growing at temperatures below 32°F. This important ability gives the bacteria a competitive advantage, as the less hardy spoilage organisms and other competitors are not able to grow, or grow as quickly. Long periods in the cold chain during distribution of a product coupled with a long shelf life are important factors that increase the risk of growth.
Once the organism enters a food production environment, it can create environmental niches that allow for propagation on surfaces such as floors, drains, walls, and equipment. Bio-films are complex substrates of adherent cells frequently embedded within a self-produced matrix of an extracellular polymeric substance. Listeria monocytogenes can establish itself in such substrates further reducing the ability for normal cleaning and sanitizing to remove it. The colonization by Listeria monocytogenes of food processing equipment is an associated factor in many outbreaks. In the cantaloupe outbreak of 2011, investigators found the packinghouse’s packing line to be contaminated, in the 2015 ice cream outbreak in involving the Blue Bell Ice Cream Company; the bacteria were also found in the company’s equipment.
After a food is processed, the bacteria can remain for extended periods in the food and survive to a customer. Ready to eat prepared foods then become the vehicle for one of the most hazardous of all bacteria, with a mortality of around 30%; especially at risk are the elderly, and those with underlying medical conditions. Pregnant women are often included in the victims of outbreaks along with the their unborn, a very tragic consequence. The foods implicated in outbreaks are varied, and include hot dogs, luncheon meat, meat spreads, smoked fish, cantaloupe, and ice cream and candy apples. Attempting to warn at risk persons is made almost impossible, as almost all processed foods and many fruits and vegetables are prone to infection.
Given the growing magnitude of this public health challenge, we need a strong public health response targeted specifically to deal with Listeria monocytogenes. The following recommendations are offered:
A multidisciplinary committee is needed to create a coordinated national response. This group should advise the food processing industry and other at risk points of the food supply chain about controls such as environmental sanitation and verification.
The medical profession must play an increased role in coordinated efforts to better inform consumers and at-risk persons about the prevention of Listeriosis.
A unilateral requirement is needed to require all food processors to perform effective and ongoing testing of equipment and environments. This should be coupled with encouraging the application of in-plant micro-assay methods of detection using QT-PCR and other rapid techniques.
The eventual implementation of the FDA Food Safety Modernization Act and Preventive Controls rules will provide the needed support to strengthen the current public health response.