For several years, food items other than fresh produce have usually been associated with listeriosis outbreaks.  According to the Centers for Disease Control and Prevention (CDC), listeriosis is an important public health problem in the United States.  Listeriosis is an infection caused by Listeria monocytogenes (L.mono)—one of the two pathogenic species of Listeria, which may present in different ways and primarily affects older adults, pregnant women, newborns and adults with weakened immune systems (known as sensitive populations).

In the last few years, fresh-cut celery, cantaloupes, bean sprouts and caramel apples have been implicated in outbreaks and several other produce commodities have been recalled due to the detection of L.mono.  The recognition that produce commodities may support L.mono, coupled with the knowledge that there is a difference between the detection of listeria and the amount of L.mono that must be present to cause an infection, has raised questions about the United States’ policy related to Listeria in produce.

Some countries such as Canada and Australia have a tolerance level of less than 100 CFU/g in ready-to-eat (RTE) foods that do not support L.mono growth; the United States has a “zero tolerance” for L.mono in RTE foods.  A zero tolerance policy means absence of L.mono in 25 gram samples (i.e., less than 1 in 25 g, or less than 0.04 in 1 g), which is typically reported by microbiology laboratories as <1/25g or <0.04/g.

According to the U.S. Food and Drug Administration (FDA), this standard is currently in effect whether or not a food supports the growth of L.mono.  In 2008, the FDA released draft guidance documents on Listeria for the industry and the FDA field staff that would have altered this zero tolerance approach, but these documents are still under review.

According to Dr. Robert Brackett with the Illinois Institute of Technology, the history of listeriosis in the United States is influencing Listeria policy in the produce sector.  Current policies are based on past risk assessments, raising concerns about the growing demand for ready-to-eat foods by sensitive populations and the increased shelf life of many food items.  The FDA is following a zero tolerance for L.mono and focusing efforts in enforcing Good Manufacturing Practices.  However, the FDA staff is still having internal discussions about this approach and expects to provide greater detail about how industry can comply with the FDA policy on L.mono in foods once their guidance documents are finalized.

According to Dr. Martin Wiedmann with Cornell University, L.mono is a human pathogen that can cause very severe human conditions, but high numbers are required to do so.  Listeria can be found in many environments and persists/survives for long periods of time, up to several years in some cases.  Some organizations such as the Alliance for Listeriosis Prevention and the Institute of Food Technologists have suggested that the zero tolerance approach to dealing with the microorganism was a cautious enforcement policy based on the state of the science during the 1980s.

However, the regulatory community in the United States has not yet supported a regulatory limit for L.mono of 100 CFU/g in RTE foods that do not support the growth of the microorganism.  While research conducted in 2003 by Chen et al. showed evidence that public health could actually be better served by a numeric limit rather than an effort to reduce prevalence, there are also concerns that there is not enough data to ensure that 100 CFU/g is protective of sensitive populations.

The debate continues…

In the meantime, L.mono presence is becoming a challenging issue for the produce sector and questions about a 100 CFU/g standard still remain.  The FDA and CDC are more actively supporting surveillance efforts.  The produce industry is proactive, and companies that may be at risk for listeria are adopting a “search and destroy” strategy for L.mono.  Regardless of a “zero” tolerance or low levels for L.mono, just the presence and persistence of Listeria itself is an indicator of unsanitary conditions and particularly of conditions likely to harbor L.mono.  Proactively controlling its presence is favorable for the industry, government and consumer.

The bottom line is that Listeria is not a good “house pet” as stated by many experts.  For more information from the experts, please visit the WG website at www.wga.com to access a recorded webinar with Dr. Bob Brackett and Dr. Martin Weidman discussing the challenges and policies associated with Listeria monocytogenes and the implications for the produce industry.