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 fruit 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 operations? Let me explain.
Listeria monocytogenes is the most common foodborne pathogen of Listeria. The pathogen is very specific and the leading cause for major Listeria recalls. If you obtain a negative result for L. monocytogenes, you do not have contamination with the specific species that causes illness.
If you choose to test for Listeria spp. the lab is going to test for every species of Listeria, pathogenic or nonpathogenic. If you obtain a positive result for Listeria spp. it means you have contamination but it may or may not be the pathogenic species. This is a good way to measure sanitation processes.
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 which often leads to fruit and vegetable 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.
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.
Testing Methods (PCR vs VIDAS vs Petrifilm)
When testing, it is always important to determine which testing method will work best for your operation. The test methods have differing advantages and disadvantages.
Polymerase Chain Reaction (PCR):
This method is a molecule technique where the DNA is extracted from the bacteria. Copies of the bacterial DNA are made exponentially (see figure below). These copies are labeled with a dye or isotope and are detected in a themocycler by a laser. As time passes the number of copies increases and the DNA is detected or not detected giving the final results.
- Advantages: Time, this method is very fast, with an average turn-around time of 24 to 48 hours. This method is also very specific. The DNA is a molecule “fingerprint” which is specific to the Listeria spp.
- Disadvantages: This method is currently used to detect all Listeria spp. It will not distinguish between pathogenic and nonpathogenic strains.
This method uses the proteins on the outside of the bacteria for detection. The detection method that is specific for L. monocytogenes is completed by detecting a virulence antigen specific to the pathogenic strain.
- Advantages: Time, this method is rapid, with an average turn-around time of 24 to 48 hours. It can be utilized to distinguish L. monocytogenes verses Listeria spp.
- Disadvantages: While effective, this method may require an additional 24 hours when compared to the other methods.
The VIDAS method at Safe Food Alliance
We currently use LPT VIDAS, which is a 24 to 26 hours analysis with a similar turnaround time (TAT) as the PCR method. The method currently being used is approved by the Association of Official Agricultural Chemists (AOAC), and has been peer reviewed, validated and verified under our ISO scope for various commodities. Safe Food Alliance is currently working to complete validation of the LMX VIDAS method which has a shorter 24-hour turnaround time.
Petrifilms are used to grow the colonies of Listeria spp.
- Advantages: Similar to conventional microbiology in that you are able to detect the Listeria spp by actually growing it. It will give you confidence that the pathogen you are looking for is living or dead. Results are fast and usually will be 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 only give you a qualitative, positive/negative result.)
- Disadvantages: This method is currently used to detect all Listeria spp. it will not distinguish between pathogenic and nonpathogenic strains. It is a quantitative method compared to the more qualitative methods of PCR and VIDAS.
Choosing the Method That Works for You
All in all, the testing you require may vary based on your facility, and your overall goal of the testing. If you want to make sure you have proper sanitation processes in order, selecting the Listeria spp testing might be the best option for your business. If you are more worried about a possible recall, think you might have contamination issues with Listeria monocytogenes or even to ensure your sanitation practices are adequate, selecting for L. mono testing would be your best option.
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 detection of microorganisms. Genome sequencing will allow for traceability, helping with recalls and pinpointing the epidemiological impact of a specific illness causing bacteria. The cost of the technology is going down as more and more models are becoming available making it easier for all labs to utilize these tools.