Control of Listeria monocytogenes in mushroom growing & packing environments

Mushroom News, Nov, 2006 by Luke F. LaBorde

In recent years, awareness of Listeria monocytogenes as a potential microbial contaminant in ready-to-eat foods has increased. Several highly publicized outbreaks attributed to Listeria contamination have been traced to consumption of soft-ripened cheese, under-pasteurized milk, and pre-cooked meat and poultry products. This pathogen has also been found in fresh produce including broccoli, carrots, celery, coleslaw, cauliflower, cucumber, green onions, radishes, tomatoes, lettuce and pre-packed salads.

To date, there have been no reported cases of foodborne illness attributed to mushrooms contaminated with Listeria. However, growers and packers would be wise to understand the unique characteristics of this microorganism and take steps to prevent contamination from occurring.

Listeriosis, the name given to a range of symptoms caused by L. monocytogenes, is potentially life-threatening to fetuses of pregnant women, newborns, the elderly and people with weakened immune systems caused by chronic illnesses, such as AIDS or cancer. Less severe cases of listeriosis may result in flu-like symptoms that can include fever, muscle aches, nausea or diarrhea. More severe forms of the disease cause blood infection, meningitis and encephalitis. It is estimated that approximately 20 to 30 percent of severe infections result in death. Infected pregnant women may show little or no evidence of the disease. However, severe cases can lead to premature delivery, spontaneous abortion or stillbirth.

The amount of bacterial cells that must be ingested in order to cause listeriosis has not been clearly established, although it is thought to be relatively low among susceptible individuals. For this reason and because of the potentially severe consequences of the disease, U.S. regulatory agencies have adopted a "Zero Tolerance" policy toward L. monocytogenes in ready-to-eat foods. Foods contaminated with Listeria monocytogenes are thus considered "adulterated" under the law and can be subject to a recall.

Listeria monocytogenes is found almost everywhere in agricultural environments and has been isolated from 37 species of mammals, both domestic and wild, and 17 species of birds. It is estimated that up to 10 percent of humans may be intestinal carriers. The bacterium is likely to be found in soil, equipment or bins, cull piles and in packinghouses and fresh-cut processing areas. The pathogen is constantly being reintroduced into the plant by employees and by incoming products, soil, vegetation and equipment. For this reason, prevention of growing and packing facility contamination, which can be an effective control for some other human pathogens, is not reliable with Listeria. Faced with this reality, well-planned and implemented sanitation programs are essential. Mushroom substrate materials, especially manure-based ingredients, may contain abundant levels of pathogenic microorganisms including Listeria. Unlike most human pathogens, Listeria grows well under cool, wet conditions often encountered in growing and packing areas. Therefore, produce buyers increasingly require documentation of monitoring of growing and packing facilities for general cleanliness and sanitation as well as for the presence of Listeria.

By controlling the introduction and growth of Listeria monocytogenes, it is possible to minimize or prevent contamination of mushrooms. Because low levels of the pathogen are so widespread in production and packing environments and, if multiplication occurs, the consequences are so severe, growers and packers should implement systematic food safety programs that document adherence to Good Agricultural Practices for mushroom growing and packing.

Guidelines for monitoring and minimizing the presence of L. monocytogenes in mushroom growing and packing operations are provided below. These recommendations were originally published in the article "Guidelines for Controlling Listeria monocytogenes in Small- to Medium-Scale Packing and Fresh-Cut Operations" by Trevor V. Suslow and Linda J. Harris of the University of California--Davis. While the material presented here specifically addresses control of Listeria, many of these concepts can also be applied to controlling other pathogens such as E. coli and Salmonella. Adaptations were made, as necessary, to take into account practices unique to the mushroom industry.

GENERAL CONSIDERATIONS

Control of L. monocytogenes can be achieved by both preventing the introduction of bacteria into growing and packing areas and by implementing a systematic cleaning and sanitizing program to reduce levels on equipment, walls, floors and drains. Because the incidence of Listeria will vary with each individual operation, managers should develop their own set of procedures and monitor effectiveness by testing for indicator bacteria, such as "generic Listeria" to establish a baseline of performance in general sanitation procedures. This monitoring program acts to detect the presence of all Listeria species, with the assumption that if any Listeria species are detected, L. monocytogenes may also be present. The program becomes plant-specific when the frequency of sampling, location of samples and the corrective action taken are tailored to the plant's operation. Common sites for this type of cross-contamination are listed below: