Tip of the Week: What is Listeria Monocytogenes?

Listeria monocytogenes is famous for the well-known name Listeria, which is a serious infection usually caused by eating L. monocytoenes contaminated food. Listeria monocytogenes is ubiquitous in the environment, including soil, water and vegetation. Listeria is noted as a serious infection because an estimated 1,600 people in the US contract it each year and on average 260 die.

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After learning about and researching Listeria for years, AIB’s microbiologist, Dr. Kantha Channaiah created a cheat sheet of facts about this opportunistic pathogen. Here is a sample of facts from Channaiah’s list:

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Following this list of facts, Channaiah provided a deeper explanation of several facts above.

1. Why do we still hear stories about Listeria outbreaks if there is a zero-tolerance policy in the US?

The cold tolerance and ubiquitous nature of the organism makes it difficult to identify the harborage sites. Although L. monocytogenes is not the most heat resistant bacteria, the following advantages over other vegetative bacteria makes it challenging to control in the food manufacturing environment. L. monocytogenes can:

  • survive and grow at refrigeration temperatures, and survives frozen storage for extended periods.
  • tolerate high salt concentrations (such as in non-chlorinated brine chiller solutions).
  • survive under low oxygen and acid conditions, and is more resistant to heat (although it can be killed by pasteurization temperature) than many other non-spore forming foodborne pathogens.
  • persist within biofilms on equipment, utensils, drains, floors, walls, etc. ultimately reaching final products via cross-contamination.
  • trace the source of L. monocytogenes in a food processing environment is a challenge.
  • re-contaminate the finished product after the kill-step during handling, distribution and transportation. 

2. Why listeriosis is a dangerous disease?

Although, the estimated incidence of listeriosis is lower than other enteric illnesses, the infection caused by L. monocytogenes require hospital care and about one in five people with the infection die. Pregnant women and immunocompromised people are at greater risk due to listeriosis.

3. How do we control L. monocytogenes in the food processing establishment?

Despite improvements in production, processing, handling, and distribution of food products in recent years, protecting consumers from L. monocytogenes food poisoning still remains a challenge. The success of any Listeria control program depends on an effective HACCP/HARPC plan, a good hygienic design, cGMPs, sanitation program, employee hygiene practices, pest control program, environmental monitoring program, and good hygiene post-process handling procedures.

Next week our two part series continues as we explain scenarios that could lead to contamination of ready-to-eat foods with L. monocytogenes