Listeria spp. vs Listeria Monocytogenes

By: Joseph Nicholl

When submitting a sample for testing which option should you choose: Listeria spp. or Listeria monocytogenes?

That is a good question. One is very broad (Listeria spp.) and will give you a lot of information about the bacteria in the genera and the other is specific (L. monocytogenes) one species in the genera. Think about the tests like this: one is all the different types of apples and the other is a granny smith apple.

Most would choose the more specific granny smith or Listeria monocytogenes. But what does that mean for your operation? Let me explain.

Listeria monocytogenes causes one of the most serious and severe foodborne diseases called Listeriosis. The pathogen is very specific and the leading cause for major Listeria recalls. If you obtain a negative result for L. monocytogenes, this indicates there is no presence of contamination in your sample that may cause illness. This negative result is what we want.

When the laboratory performs Listeria spp. testing, they are looking for both the pathogenic and non-pathogenic species of Listeria. If you obtain a confirmed positive result for Listeria, the analytical report will indicate the species that has been found.

If your analytical report states:

• Listeria monocytogenes – this means you have contamination with the pathogenic Listeria.
• A Species of Listeria – such as Listeria innocua, this means you have contamination with non-pathogenic Listeria.

Testing for Listeria spp. is a good way to measure your sanitation processes. It is recommended to test for Listeria spp for your environmental monitoring program. The primary reason is if a non-pathogenic Listeria spp can be found, the pathogenic Listeria monocytogenes may also be present.

Listeria Explained

Today there are 20 identified species of Listeria. The pathogen received its name from British surgeon Joseph Lister in 1940 but wasn’t first recorded as disease-causing until 1924 with the death of a young rabbit. Listeria is found living ubiquitously in soil, water, and vegetation which often leads to food contamination.

L. monocytogenes is contracted by eating food contaminated with the bacteria. This specific pathogen is also very hardy and can grow in a wide range of environments; temperatures from 39°F (4°C) (the temperature of your refrigerator) to 98.6°F (37°C) (your bodies internal temperature), wet damp areas such as drains, and has varying incubation periods of three to 70 days.

Due to the large incubation period, it makes it difficult to investigate and determine where the contaminated food originated. For this reason, Listeria statistically is one of the most fatal bacteria, with death rates near 25%, this in comparison to Salmonella which has mortality rates of less than 1%. In the US it is estimated 1,600 people are infected each year.

Listeria in the News

Cantaloupes 2011: 147 people infected in 28 states leading to 33 deaths.

Packaged Salads 2016: 19 people infected in 9 states leading to 1 death.

Avocados 2019: Possible contamination leading to a voluntary recall of avocados in 6 states after environmental samples taken at the packing facility were positive.

How To Detect The Pathogen

When evaluating appropriate testing methods, it is always important to determine which testing method will work best for your operation. Listeria detection methods in the food industry can be cultural or based on alternative automatized technologies like PCR or ELFA. The test methods below have different advantages and disadvantages.

Polymerase Chain Reaction (PCR):

This method is a molecule technique where the DNA is extracted from the bacteria and amplified, creating millions of copies of the bacterium’s DNA. These copies are labeled with a dye or isotope and are detected in a thermocycler by a laser. As time passes the number of copies increase and the DNA is detected or not detected, giving a result in just a few hours.

• Advantages: Time – this method is fast, specific, and produces results rapidly, with an average turn-around time of 24 to 48 hours. The DNA is a molecule “fingerprint” which is specific to the Listeria spp. Sensitivity – PCR detection systems can detect as little as 1 Colony Forming Unit (CFU) in a sample.
• Disadvantages: Reporting – Typically PCR testing is not quantifiable (number results). It is considered a qualitative test, which provides negative and positive results.

Enzyme-Linked Fluorescent Assay (ELFA):

The VIDAS instrument is a widely used ELFA technology platform in the food testing industry. It uses the proteins on the surface of the bacteria for the detection. Typically, a VIDAS commercial kit has antibody-coated wells that correspond to the antigen of the target bacterium. If the target pathogen is present, it will bind to the well and give a fluorescence indicating a negative or positive result.

• Advantages: Time – this method is rapid, with an average turn-around time of 24 to 48 hours. FDA Approved – The VIDAS is an acceptable FDA official pathogen detection system and can be used for FDA detention samples.
• Disadvantages: Time – while effective as a rapid method, larger sample sizes may require an additional 24 hours when compared to the other methods. It is considered a qualitative test, which provides negative and positive results and requires a confirmation.

The VIDAS method at Safe Food Alliance

At Safe Food Alliance, we use an approved PCR and ELFA detection platform that is an Official Method of Analysis (OMA). It has proven to be robust, accurate and reliable and we have the capability to detect both non-pathogenic and pathogenic Listeria. We are ISO 17025 accredited for this analysis for various commodities.

Petrifilm (PF):

Petrifilms are used to grow the colonies of Listeria spp. It is a direct plating technique that uses cold-water-soluble gelling agents, nutrients and indicators for activity and enumeration.

• Advantages: Quantifiable Results – Similar to conventional microbiology in that you are able to detect and enumerate (number result) the Listeria spp. Results are fast and quantifiable results are given in colony forming units (cfu)/g. This is a major difference compared to methods such as PCR and VIDAS which will give you a qualitative, positive/ negative result.
• Disadvantages: Sample Volume – A smaller aliquot is tested compared to PCR and VIDAS. Very Short Enrichment – The sample has not been incubated in an enrichment broth; it is a sample surface wash method.

Choosing the Method That Works for You

All in all, the testing choices you make will vary based on the goal you are trying to accomplish. Best practices suggest using Listeria spp. for your Environmental Monitoring Program. However, you may need to adhere to customer specifications or your certification body requirements. We recommend following these requirements to ensure you are incompliance. If you need help bridging the gap, our team of food safety experts is here to help.

The various methods for detections are constantly changing and evolving to make the process easier, more accurate, and quicker. As we move further into the age of genetics, PCR or whole-genome sequencing will play a very vital role in the detection of microorganisms. Genome sequencing will allow for traceability, helping with recalls and pinpointing the epidemiological impact of a specific illness-causing bacteria. These exciting developments go to show that we all have the same goal in mind, to create food that is safe for our friends and families.