Listeria Preventive Controls Fact Sheet

June 20,2018 12:05 PM By James Flynn

A 3iVerify fact sheet to help understand preventive controls

Our preventive controls fact sheet series is compiled for you in an easy to follow format that helps inform you on the facts about food safety risks. In this fact sheet we cover everything you need to know about Listeria. Feel free to ask questions in the comments below.

About Listeria

In the summer and fall of 2011, cantaloupes contaminated with Listeria sickened 147 people in 28 states, killing 33 of them. This was the deadliest Listeria outbreak in a decade and was unusual because listeria is rarely associated with fresh produce, according to a new report from the U.S. Centers for Disease Control and Prevention (CDC).

Although rare, listeria is an important public-health issue because of its severity, said report co-author Benjamin Silk, a CDC epidemiologist in the division of foodborne, waterborne and environmental diseases. “Nearly all people who have listeria infections diagnosed are hospitalized, and about one in five die.”

A 2011 CDC report cited above estimated that L. monocytogenes causes 1,591 cases annually and about 800 laboratory-confirmed cases of Listeria infection are reported each year in the U.S. and typically 3 or 4 outbreaks are identified. The foods that typically cause these outbreaks have been Mexican-style soft cheeses made with unpasteurized milk, deli meats and hot dogs. In the past, produce was not often identified as a source, but sprouts caused an outbreak in 2009, and more recently in 2014, which resulted in the closure of a food processing firm, and pre-cut celery caused an outbreak in 2010.

Susceptible Populations

The main people impacted by listeriosis are:

  • Pregnant women, fetuses and new born children with the infection often passed from mother to child;
  • Immunocompromised people, for example, corticosteroids, anticancer drugs, graft suppression therapy, HIV and AIDS;
  • Cancer patients, particularly leukemic;
  • People with underlying health problems such as diabetic, cirrhotic, asthmatic and ulcerative colitis patients;
  • Elderly people;

Listeria is often deadly with a very high mortality rate.

Microbiological Profile

Of the 6 species in the genus Listeria – L. monocytogenes, L. grayi, L. innocua. L. ivanovii, L. seeligeri and L. welshimeri – the only species of real significance for humans is L. monocytogenes.  L. ivanovii is considered pathogenic, and mainly in ruminants, rather than in humans.

Listeria monocytogenes is a Gram-positive, rod-shaped, facultative bacterium, motile by means of flagella, that is among the leading causes of death from foodborne illness.

It has 13 serotypes, among these, serotypes 1/2a, 1/2b, and 4b have been associated with the vast majority of foodborne infections.

Listeria monocytogenes is hardy; it is salt-tolerant and not only can survive in temperatures below 1°C, but also grow in these conditions, unlike many other pathogens. This means refrigeration is not sufficient to control its growth and is a core reason why it is so dangerous, given that most of the foods it infects tend to be refrigerated for long periods.

It is also notable for its persistence in food-manufacturing environments, inhabiting drains and floor areas and continually re-contaminating the factory surfaces through aerosolization with the use of high pressure wash down equipment.

Listeria is ubiquitous in the environment and can be found in moist environments, soil, and decaying vegetation.

Source: FDA Bad Bug Book

Listeriosis Illness

The table below gives an overview of the organism, its impact on humans, along with sources and the controls usually employed to prevent illness. This information may be used in growing and food processing operations as part of a HACCP food safety plan or preventive controls food safety plan.

 General:Although not a leading cause of foodborne illness, L. monocytogenes is among the leading causes of death from foodborne illness. A recent report by the Centers for Disease Control and Prevention (CDC) estimated that domestically acquired foodborne L. monocytogenes causes 255 deaths in the U.S. annually putting it in the top five domestically acquired causes of food death as a result of foodborne illness.
 Caused by: Listeria monocytogenes.
 Mortality: Listeriosis: 15-30%, Listerial meningitis: 70-80%
 Onset: Non-invasive gastroenteritis: A few hours to 2 or 3 days.
Invasive infection: 3 days to 3 months.
 Infective dose:<1000 viable cells.
 Symptoms: Otherwise healthy people: Mild symptoms or no symptoms if infected with L. monocytogenes. Others may develop fever, muscle aches, nausea and vomiting and, sometimes, diarrhea. When the more severe form of the infection develops and spreads to the nervous system, symptoms may include headache, stiff neck, confusion, loss of balance and convulsions.
 Duration: Symptoms generally last 4 to 7 days.
Acute symptoms usually lasting 1 to 2 days or longer, depending on host factors, the dose ingested and strain characteristics.The duration of symptoms generally depends on the health status of the infected person. The symptoms can last from days to several weeks.
 Complications:L. monocytogenes infection causes two forms of disease in humans:

1) Non-invasive gastrointestinal illness, which generally resolves in otherwise healthy people.

2) the much more serious, invasive form of the illness, which may cause septicemia and meningitis.

Manifestations of L. monocytogenes infection tend to be host-dependent. In people with intact immune systems, it may cause acute febrile gastroenteritis, the less severe form of the disease. In vulnerable populations, however, the more severe form of the disease may result in Sepsis and spread to the nervous system, potentially causing meningitis. In elderly and immunocompromised people who develop the severe form, it usually manifests in this manner. Pregnant women, who are disproportionately infected with L. monocytogenes, may experience mild, flu-like symptoms; however, their offspring do not fare as well – they may abort or be stillborn and those born alive may have bacteremia and meningitis. One third of confirmed cases of maternal-fetal L. monocytogenes infections lead to abortion or stillbirth.

 Route of Entry: Oral (e.g., ingestion of contaminated food).
 Pathway:The pathogenesis of L. monocytogenes is unique, because the organism is able to spread directly from cell to cell in the host, rather than having to “travel” interstitially to reach other cells. Once the bacterium enters the host’s monocytes, macrophages, or polymorphonuclear leukocytes, it can reproduce, and it is bloodborne. Groups of proteins on the L. monocytogenes cell surface enable it to survive in phagocytic cells and enhance its cell-to-cell spread. 
 Implicated Foods: Raw milk, inadequately pasteurized milk, chocolate milk, cheeses (particularly soft cheeses), ice cream, raw vegetables, raw poultry and meats (all types), fermented raw-meat sausages, hot dogs and deli meats, and raw and smoked fish and other seafood.

Listeria is a major health problem to humans and, although rare, its high mortality rate makes it one of the deadliest organisms in food and causes very significant suffering and death, particularly in pregnant women and the elderly.

It has a significant cost to industry, health services and damages high profile food company brands, many of whom struggle to recover from outbreaks. The advent of social media means that food companies have nowhere to hide and are easily exposed for the lack of controls they have in place. This means that food businesses must be extra vigilant, with respect to the preventive controls in place in their food processing and storage operations. 

Increased oversight over the preventive controls in their supply chains is critical to prevent illness and operate a profitable and sustainable food business.


Many foods have been associated with L. monocytogenes. Examples include dairy products, particularly unpasteurized dairy products, raw vegetables, raw poultry and meats, raw and smoked fish and other seafood.

L. monocytogenes can grow in refrigerated temperatures, which makes this organism a particular problem for the food industry. Potential contamination sources include food workers, incoming air, raw materials, and food processing environments. Among those, post-processing contamination at food-contact surfaces poses the greatest threat to product contamination.

In many cases the use of high pressure wash down equipment when cleaning floors and drains can spread Listeria into the air and then land on food surfaces, thus continually re-contaminating food surfaces and then food.

Implicated Foods

Although Listeria is traditionally associated with dairy and meat products, fresh produce has also been implicated regularly in recent years. The organism also survives well on many refrigerated foods and in foods high in moisture and protein.

Foods linked to Listeria: Raw milk, inadequately pasteurized milk, chocolate milk, cheeses (particularly soft cheeses), ice cream, raw vegetables, raw poultry and meats (all types), fermented raw-meat sausages, hot dogs and deli meats and raw and smoked fish and other seafood.

The reason that so many foods can become contaminated or adulterated with Listeria is down to the fact that it continues to grow at very low temperatures. This allows the organism to easily grow to ineffective levels on cooked and ready to eat foods in commercial and domestic refrigerated conditions. This means that the two main controls are prevention of cross contamination in the first place and proper cooking to destroy any viable cells in contaminated food.

Main Root Causes

Cross contamination is the main root cause and this occurs when Listeria is spread from a contaminated source, such as contaminated foods or contaminated water in drains or floors, to other foods or objects in the environment.

An example of how this may occur is when potentially contaminated dairy products, fresh produce or seafood are not kept separate from each other during preparation or cooking, or when a food handler does not adequately clean utensils, surfaces, equipment and hands after they have come into contact with these products.

Contamination can spread to factory and equipment surfaces, as well as kitchen surfaces and utensils. Cross contamination may occur at any point in the food process including from handling contaminated foods, then handling food, food-preparation utensils, or other objects in the environment.

Limiting Conditions (including Kill Temperatures)

The following table gives the conditions that can be used to limit the growth of Listeria.

Environmental Condition MinimumOptimumMaximum
Growth Temperature31oF / -0.4oC99oF / (37oC)13oF / 45oC
Water activity (aw) 0.92 N/A N/A
Max % water phase salt N/A N/A 10

Kill temperatures for Listeria include heating to:

  • 145oF (63oC) for 17 minutes
  • 153 oF (67oC) for 5 minutes
  • 162 oF (72oC) for 1 minutes

The above kill times assume a food with high water activity and pH near neutral.

When it comes to killing microorganisms, both temperature and time determine the effectiveness of the result. In addition, the kill temperatures must be measured in the center of the food product to ensure the minimum core temperature required for organism death is achieved.

Preventive Controls

When reviewing a preventive plan for any of the affected foods above (or any food that could come into contact with Listeria contaminated sources) the key controls you must look for may include the following:

  • Water Sources: Prevention of use of sewage contaminated water sources for irrigation, washing or processing, only potable water sources are to be used.
  • Other Food Sources: Prevention of cross contamination from potential Listeria sources (such as water, unpasteurized dairy products, raw meat, raw fish and fresh produce sources) to food, ingredients and food processing surfaces or equipment.
  • Cleaning Program: Ensure that cleaning procedures that involve high pressure cleaning techniques are properly risk assessed and scheduled to minimize aerosolization. For example, floors and drains are cleaned using sanitizers and low-pressure techniques and that food contact surfaces are sanitized after all floors and drains have been cleaned or food room fogging sanitation takes place after all other cleaning.
  • Food handlers: Exclusion of food handlers who have recently been sick.
  • Medical Testing: Medical testing of food handlers who have been sick before allowing return to work to prevent human cross contamination.
  • Kill Temperatures: For cooked and ready to eat foods, that have a thermal processing, ensure that adequate temperature controls are in place and that kill temperatures are being achieved.
  • Preventive Controls: For ready to eat foods that do not have a thermal processing step, ensure that rigorous prevention controls are in place and that regular Listeria testing takes place.
  • Packaging Integrity: For raw foods, intended to be cooked by the consumer or further processed, ensure that packaging is secure and leak proof so that other foods are not contaminated.
  • Cooking Instructions: For raw foods, intended to be cooked by the consumer or further processed, ensure that clear cooking instructions are given to consumers, including, in the case of meats, the instruction not to wash the meat before cooking (this spreads bacteria in their kitchen / processing area). Also, ensure that the correct cooking times and temperatures are clearly displayed on the product label cooking instructions.

We hope this Listeria Prevention Fact Sheet has been informative and can be used to help you assess the safety of foods that you purchase from suppliers or process. The team at 3iVerify are highly experienced and qualified food safety consultants, please get in touch if you would like further help and advice related to Listeria control.

Our 3iVerify solution can help take the time, hassle and risk out of risk assessing your suppliers and reduce the risk of your company falling foul of Salmonella and other food safety risks. If you would like to know more about how 3iVerify can help you reduce risks, save time, prevent food borne illness and brand damage, or you have any questions regarding the above, please feel free to contact us.