Find Listeria before it finds you

The recent Listeria outbreak in cantaloupe has brought this food borne pathogen back into the spotlight for food safety professionals and the fruit-loving public. Listeriosis is an uncommon infection in most segments of our population. The groups most a risk are the elderly (over 60), pregnant women (with consequences for the unborn child) and those who are immuno-suppressed or immuno-deficient. According to the CDC, the tainted cantaloupe has already claimed 16 lives and caused 72 illnesses, making this outbreak the deadliest in a decade.

In the case of fruits and vegetables the potential for Listeria contamination to occur is mainly due to contact with fecal waste or survival of Listeria species in the soil.
If fruit is harvested on the ground (such as cantaloupe) rather than picked off of the plant or tree, the opportunity exists for L. monocytogenes to attach to the outer surface of the fruit. When a fruit has L. monocytogenes attached to its surface it may be difficult to dislodge with simple washing or even scrubbing. When the fruit is cut the L. monocytogenes gains access to the edible fruit and can be ingested.

Microbiology International offers a set of products to enable food testing laboratories to detect and confirm if a food sample has L. monocytogenes or any of the other 5 Listeria species present.

Sample Preparation

During sample preparation a sub-sample of a batch is taken and diluted in enrichment broth. At this point the sample is processed to release the bacteria off the surface of the food matrix to encourage growth. Microbiology International provides the innovative Pulsifier™ to perform this task. The Pulsfier™ uses a novel mode of action to release the attached bacteria into the enrichment broth very quickly, only 15 seconds of processing is required per sample, with minimal breakdown of the food matrix.
The cleaner end product is easier to sample so this minimizes the need to use filter bags for many routine samples. The Pulsifier™ is also the processor of choice for hard and dry foods such as nuts, rice and spices. The Pulsifier is named in the ISO standard for the microbiology of food and animal feed stuffs (ISO 7218:2007(E)).

High Volume Sample Screening for Listeria species

For high volume screening of food samples for Listeria species contamination Microbiology International offers a state of the art real-time PCR method using the RAPID LT Food Security System. This AOAC-RI Performance tested method can be used to deliver a presumptive result within 26 hours of taking the food sample.
Following 24 hours enrichment a small sample of the enrichment culture is mixed with the PCR master mix and introduced into the Rapid LT air thermal cycler. This system performs rapid PCR amplification cycling 45 times in 35 minutes. If the specific Listeria species DNA sequence was present the system will detect the millions of fluorescently labeled copies that have been created in the test tube. To confirm the amplification result the instrument then heats up the bulk fluorescent product until the double stranded DNA melts at a specific temperature. Upon separation the fluorescent signal is lost confirming specificity. Finally the assay incorporates a control PCR reaction which is detected at a different florescent wavelength. The control is included to confirm that a PCR reaction occurred in the tube. This indicates that negative results were not due to a failure of PCR amplification in the test tube.

Culture Confirmation

When positive samples have been detected it is important to go on and confirm that a Listeria species isolate has been grown out and to determine which species is present, L. monocytogenes being the highest risk. To achieve this, a sample of the enrichment broth is grown on a selective agar plate to produce discrete pure colonies for culture confirmation testing. Microbiology International offers the Microgen Listeria ID system to enable laboratories to confirm which Listeria species they have isolated. The Microgen Listeria ID is also AOAC-RI Performance tested and uses biochemical identification to determine the Listeria species present. This novel 12 well strip test also includes an in well haemolysis test which is crucial to differentiating between L. monocytogenes and L. innocua (the two most prevalent species isolated in food samples).

The Food Security Product Range offered by Microbiology International

In this article we have concentrated on Microbiology International’s product range for Listeria in foods. Our full range includes complimentary products for other food borne pathogens such as Salmonella, E. coli O157:H7 and thermophilic Campylobacters.