Researchers found Listeria monocytogenes in half of the Northern Irish food businesses involved in a study to monitor the pathogen in food and environments. More than seven in 10 of the isolates discovered carry a gene that gives them resistance to commonly used sanitizers.

Positive samples came from 12 of the 24 participants, said Safefood, an organization that promotes awareness and knowledge of food safety issues in Ireland. The potentially deadly Listeria monocytogenes was found in 18 food samples (4.6 percent) and 76 environmental swab samples (6.3 percent).

The study monitored the occurrence and persistence of Listeria monocytogenes in foods and environments of food-processing facilities. It complements similar work in the Republic of Ireland.

The number of microbiological incidents in food in Ireland each year involving Listeria are the second most frequent after Salmonella.

The study involved 24 food business operators in Northern Ireland. Most were classed as small to medium-sized enterprises. Participants swabbed six sites in their premises. Sampling took place from July 2015 until November 2016. Overall, 1,594 samples were submitted for analysis — 1,197 environmental swabs and 397 food samples.

Listeria monocytogenes isolates were subjected to pulsed-field gel electrophoresis at Teagasc, Moorepark and whole genome sequencing at the London School of Hygiene and Tropical Medicine.

About 30 percent of incidents involving Listeria reported between 2005 and 2011 involved ready-to-eat sliced meats. The pathogen can be introduced into meat- and fish-processing environments by several routes, including being present on contaminated ingredients such as raw meat, fish and packing materials.

European regulation states levels of Listeria monocytogenes in a ready-to-eat food must not exceed 100 bacterial colony-forming units per gram during its shelf life. In the study two meat products exceeded the legal limit of 100 CFU per gram in July 2015.

The pathogen was mainly found in sites producing processed mushrooms, cooked meats or sandwiches. None of the four companies producing dairy products submitted any positive samples.

In the food business facilities, floors, drains, trolley wheels, boots and chill surfaces yielded 81 percent of the Listeria monocytogenes isolates. Significantly more environmental samples were positive in warmer months of May, July and September, but no underlying cause was found.

More than seven in 10 Listeria monocytogenes isolates carried qacH. This gene gives the organism resistance to quaternary ammonium compounds, commonly used in sanitizers, which may help explain persistence of some strains. A wide range of products were used illustrating the lack of consistency in approaches to cleaning.

The same sequence type was isolated from environmental swabs and food products in some businesses indicating cross contamination. Of the isolates found, 98 percent shared a sequence type with Listeria monocytogenes strains that have been isolated from clinical cases in the UK.

A total of 130 food samples were analysed to measure their pH and aw. Only 19 of these could not support the growth of Listeria monocytogenes. Based on their pH and aw, 85 percent of food samples submitted would support growth, as estimated using the ComBase mathematical model.

Sequence types ST8, ST20 and ST21 recurred in one food production plant each, ST 121 in two sites, ST 6 in three plants and ST204 in four factories.

To assess procedures for the control of Listeria monocytogenes a questionnaire was submitted to all participating FBOs. Statistical analysis of responses found no significant differences between the five companies which yielded no Listeria monocytogenes and the eight where it was found. This suggests the FBOs all have appropriate procedures but that implementation may differ.

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