Volume 19 Part 1 Article 1
Title: Incidence and fate of Listeria monocytogenes during mushroom growing, packing, and processing
Author: Luke F. LaBorde
Listeria bacteria are widely found in agricultural environments including soils, surface water, decaying vegetation, and animal feces. Only one species, Listeria monocytogenes, is capable of causing human illness. L. monocytogenes is of particular concern in ready-to-eat fresh and minimally processed produce because it is able to grow in continuously wet and cool growing, packing, and processing environments and it is difficult to eradicate once it becomes established. Fresh commercially grown button mushrooms (Agaricus bisporus) have not been implicated in any cases of listeriosis to date. However, in North America, a succession of recalls of sliced mushrooms found to contain L. monocytogenes has occurred. Our food safety research program has demonstrated that Listeria spp. can be found in mushroom production and processing environments at levels similar to other food types. However, standard industry production practices include steps that decrease contamination risks. In a microbial survey of a small-scale mushroom research farm on the Penn State University campus, 29 of 184 samples (15.8%) were positive for non-pathogenic species of Listeria. L. monocytogenes was found at 3 sites (1.6%), each within the phase 1 ingredient receiving and compost preparation area. However, composted substrate is not a likely source of mushroom contamination since earlier work we conducted demonstrated that an industry standard 6-day phase 2 pasteurization and conditioning protocol was capable of achieving complete inactivation of inoculated (7 log/cfu/gm) L. monocytogenes, Salmonella spp., and E. coli O157:H7. The microbial ecology of casing soil, which is typically not heat treated, plays in a role in minimizing mushroom contamination risks. For instance, we showed that up to 4 log reductions of L. monocytogenes and Salmonella spp. inoculated into un-treated light peat-based casing soil occurred between casing and pinning, while no reductions occurred in previously autoclaved soil. Furthermore, when light peat soils were supplemented with up to 40% dark peat, which has lower levels of background microorganisms, the rate of pathogen destruction decreased slightly. In a year-long post-harvest microbial survey of 255 non-food contact surfaces in a mushroom packing and slicing facility, we showed that floors, walls, drains, and equipment surfaces in continuously wet areas can become harborage sites for persistent strains of L. monocytogenes. However, our mid-study recommendations for sanitation improvements dramatically decreased the incidence from a maximum of 31% of collected samples to 8.5%. Continuing education on cleaning, sanitizing, and facilities maintenance is therefore critical for maintaining the industry’s excellent record for producing safe products.