Recall of food products contaminated by Listeria monocytogenes are on the rise, due in part to federal regulatory agencies’ increased scrutiny of food processing plants. Each year in the United States, L. monocytogenes causes approximately 2,500 cases of illness, which result in 500 deaths (Mead, 1999). Susceptible persons include pregnant women and those with immune system compromise due to cancer, organ transplant therapy, kidney disease, diabetes, aging and AIDS.
Persons with compromised immune systems account for approximately 20 percent of the United States population, and this percentage continues to rise. The key reason for the heightened regulatory focus on L. monocytogenes is that it causes a higher rate of hospitalization than any other foodborne pathogen: 95 percent of the individuals who acquire Listeriosis are hospitalized, and this disease is the leading cause of death from a foodborne pathogen (Mead, 1999).
An excellent surveillance system is operational in the United States, and the Centers for Disease Control and Prevention (CDC) has achieved tremendous advances in disease surveillance through PulseNet and FoodNet. This enhanced surveillance, however, means an increased likelihood of tracing sources of infection to food product manufacturers. The good news for the food industry is that cases of Listeriosis are stable or on the decline due largely to food manufacturer intervention and prevention efforts.
When comparing data from 1996 to 1998, the incidence of Listeriosis in the United States declined by 40 percent in 2004 (CDC, 2005). Control of L. monocytogenes requires a focused commitment to maintaining the highest levels of plant sanitation and contamination prevention of food being processed. Risk assessment by FDA and USDA has identified those foods at highest risk for transmission of L. monocytogenes to susceptible persons (Health and Human Services [HHS]/USDA, 2003). Particularly for those manufacturers of ready-to-eat (RTE) foods that support high level growth of L. monocytogenes, there is a critical need for environmental control, which is best achieved by frequent environmental monitoring.
Species of Listeria, L. monocytogenes, Listeria innocua, and Listeria welshimeri, are very common and can be found almost anywhere in the environment. As indicator organisms, Listeria is useful in assessing the potential presence of L. monocytogenes in the processing plant environment. During food processing and manufacturing, there is the potential for L. monocytogenes to be continually introduced into that environment. The challenge for food manufacturers is to direct efforts to prevent the growth and establishment of L. monocytogenes within the plant environment through appropriate controls. These include good manufacturing practices (GMPs), sanitation and employee training. It is also critical to have in place a system that verifies these control procedures are functioning. This is most effectively accomplished through an environmental monitoring program. The contamination of food contact surfaces poses the greatest threat to product contamination. Contamination from other areas of the plant can serve as indirect sources of product contamination. Frequent testing is imperative to ensure that routine cleaning and sanitation procedures are working. Absence of Listeria is the desired outcome of this verification.
Failure to control L. monocytogenes may result in the establishment of niches, including biofilms, after which routine cleaning and sanitizing efforts become ineffective. Only through sampling and testing can identification of niche areas be accomplished. If these areas are left unchecked, they will serve as a source of product contamination (Tompkin, 2000).
Numerous studies have shown that colonization of L. monocytogenes in food processing plants and establishment of niches can lead to continuous contamination of food during processing. Investigation of a multistate outbreak of Listeriosis, occurring in 2000 and linked to delicatessen turkey meat, revealed contamination by a strain of L. monocytogenes, which may have persisted in the incriminated processing plant for at least 12 years, and caused intermittent contamination during that time period (Olsen, et. al. 2005). Focus sampling and testing efforts to niche areas where resident strains of L. monocytogenes may persist, such as drains, floors and mats (Tompkin, 2002).