Listeria Testing

The genus Listeria is a group of gram-positive, non-spore forming, rod-shaped, and facultative anaerobic bacteria. The genus currently contains six species in which Listeria monocytogenes (L. monocytogenes) is the most important foodborne pathogens that causes most reported cases of serious illness.

Figure 1. Electron micrograph of Listeria and related contaminated foods

L. monocytogenes is ubiquitous in the natural environment and food processing environments. It can be isolated from plants, animals, soil, and marine fish. L. monocytogenes is commonly associated with ready-to-eat (RTE) foods and chilled processed foods with extended refrigerated shelf life that can support its growth. RTE foods can be contaminated during post processing by the contaminated food-contact surfaces. L. monocytogenes is able to grow at refrigeration temperatures and multiple to dangerous levels in foods. The foodborne outbreaks caused by L. monocytogenes are usually from the contaminated foods including soft cheeses, cooked chicken, delicatessen meats, salads, and smoked seafood. It is very important to monitor and control the proper temperature and appropriate length of the shelf life of foods during storage and distribution to minimize the potential contamination of L. monocytogenes. Listeria species can form biofilms on food processing equipment that are difficult to remove. The presence of Listeria species in the food manufacturing environment is often used as an indicator for the contamination of L. monocytogenes.

L. monocytogenes can cause severe even life-threatening infections to humans. Newborn babies, pregnant women, the elderly, and people with weakened immune systems are particularly susceptible to listeriosis. The symptoms are initially flu-like and then followed by vomiting and diarrhea. When pregnant women get infected, it can lead to miscarriage and 30% of overall mortality rates.

There have been standards and regulations established to control the incidence of contamination of Listeria. In most cases the monitoring of L. monocytogenes is required by regulations. The Microbiological Criteria Regulation in the EU and USA have a "zero tolerance" of L. monocytogenes in 25 g sample for RTE foods at the point of production and up to 100 CFU/g at the end of shelf life. Environmental sampling and testing in the food processing area is also required. The infective dose for vulnerable individuals could be as low as less than 100 CFU/g in some cases.

Listeria Testing of Food at Alfa Chemistry

There are standard methodology and alternative rapid methodologies (EN/ISO, AOAC and FDA BAM) that have been used for the isolation and detection of Listeria from foods and environmental samples. Tradition detection methods are culture-based methods which may require long assay time. The AOAC database contains almost 50 products for the rapid detection of Listeria spp. and L. monocytogenes. Rapid test kits that approved as AOAC with respective enrichment medium can be conditionally used for the screening of the presence of the contamination of Listeria. The rapid test protocols include steps of selective enrichment culture and then apply rapid detection techniques to replace incubation on selective agars. The confirmation test is then performed using biochemical identification technique.

Alfa Chemistry offer accurate and validated testing platforms for the detection and confirmation of Listeria in various food samples to meet customers' needs. We also have experience working on different standards and regulations.

Platform: Rapid ELISA-based assays

Limit of detection: Qualitative test per sample size (negative/positive)

Sample type: Solid and liquid food samples

Sample size: 25 g