A note on challenge trials to determine the growth of Listeria monocytogenes on mushrooms (Agaricus bisporus)

Abstract

In the EU, food is considered safe with regard to Listeria monocytogenes if the number of micro-organisms does not exceed 100 colony forming units (cfu)/g throughout its shelf-life. Therefore, it is important to determine if a food supports growth of L. monocytogenes. Guidelines for conducting challenge tests for growth assessment of L. monocytogenes on foods were published by the European Union Reference Laboratory (EURL) in 2014. The aim of this study was to use these guidelines to determine if refrigerated, fresh, whole, closed-cap, prepackaged mushrooms (Agaricus bisporus) support the growth of L. monocytogenes. Three batches of mushrooms were artificially inoculated at approximately 100 cfu/g with a three-strain mix of L. monocytogenes and incubated for 2 days at 8°C followed by 4 days at 12°C. L. monocytogenes numbers were determined (in triplicate for each batch) on days 0, 2 and 6. Water activity, pH and total bacterial counts were also determined. There was no increase in the number of L. monocytogenes above the threshold of 0.5 log cfu/g in any of the replicates. In 8 of 9 replicates, the numbers decreased indicating that A. bisporus do not support the growth of L. monocytogenes. As the EU regulations allow < 100 cfu/g if the food cannot support growth of L. monocytogenes, the significance of this study is that mushrooms with < 100 cfu/g may be within the regulations and therefore, quantitative rather than qualitative determination may be required.

The challenge of challenge testing to monitor Listeria monocytogenes growth on ready-to-eat foods in Europe by following the European Commission (2014) Technical Guidance document

“…They demonstrated that both sliced and whole mushrooms supported growth of L. monocytogenes. In a subsequent experiment, Leong, Alvarez-Ordóñez and Jordan (2015) have shown that, using a different inoculation procedure (surface spread rather than dipping), whole mushrooms did not support growth of L. monocytogenes.…”

Diversity ofListeria monocytogenesstrains isolated fromAgaricus bisporusmushroom production

“…A subsequent study showed that A. bisporus does not support the growth of L. monocytogenes (Leong et al . ).…”

Inhibition ofListeria monocytogenesbiofilms by bacteriocin-producing bacteria isolated from mushroom substrate

“…Therefore, further measures to help control L. monocytogenes in the growth substrates and environment are needed in order to reduce the risk of mushroom contamination (Leong et al . , ; Viswanath et al . ; Akata et al .…”

“…Thus, while there are some inactivation/control procedures in place to control L. monocytogenes, there is also ample opportunity for recontamination of the growing substrates and contamination of the growing environment once the substrate preparation procedure is complete. Therefore, further measures to help control L. monocytogenes in the growth substrates and environment are needed in order to reduce the risk of mushroom contamination (Leong et al 2013(Leong et al , 2015Viswanath et al 2013;Akata et al 2015;Murugesan et al 2015).…”