Listeria monocytogenes in the mushroom production environment: biofilm formation and control measures

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Dygico, Lionel K.
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University College Cork
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Mushrooms are Ireland’s most valuable horticultural crop and as such, proactive steps must be taken to prevent association with foodborne pathogens which can cause reputational and economic loss to the industry. Listeria monocytogenes is one of the major foodborne pathogens and while it has a relatively low incidence rate, it is one of the deadliest, especially for the immunocompromised individuals. This pathogen has been found to be present within the mushroom production environment which is of concern to the industry. The persistence of L. monocytogenes in different environments has been attributed to its ability to form biofilms. However, prior to this work, the biofilm forming ability of L. monocytogenes on mushroom industry relevant surfaces was undetermined. Furthermore, the mushroom industry generally employs the use of biocides that are specifically marketed for disease control within the mushroom production environment, but their efficacy against L. monocytogenes biofilms was unknown. In this work, it was established that mushroom industry isolates of L. monocytogenes can form biofilms on most of the surfaces that can be encountered within the mushroom production environment, especially on the concrete floors. In addition, the biocidal products that are approved for use within the mushroom production environment were found to be effective against L. monocytogenes biofilm. It was also found that biocidal products with more environmentally friendly active ingredients such as peracetic acid or plant-derived compounds could be effective alternative biocides for the mushroom industry. Interestingly, a reduction in efficacy was observed from the biocidal products when tested on concrete floors within the mushroom production environment. Lab-scale experiments revealed that nisin-producing Lactococcus lactis strains derived from the mushroom production environment had the potential to be utilised as a biocontrol agent in the form of a competitive exclusion organism against L. monocytogenes. However, in a similar pattern to the biocidal products, this efficacy did not translate well in pilot-scale trials as it was found to be ineffective when applied during a standard mushroom cropping process. Despite the ineffectiveness of the biocontrol agents tested in pilot-scale conditions, the L. monocytogenes numbers decreased naturally over the duration of the cropping cycle. Due to the rich microbial diversity present, this decrease in L. monocytogenes numbers may have been caused by other species producing anti-listerial agents. In order to better understand the microbial community dynamics during cropping and to identify other potential biocontrol agents, a shotgun metagenomic sequencing study was carried out which revealed other potential bacteriocin producing bacteria and clearly illustrated the changes in species dominance which occur over time. All in all, this body of work has provided beneficial information for the mushroom industry in relation to L. monocytogenes, as it identified surfaces of specific concern and determined the efficacy of biocides against the biofilm form of L. monocytogenes. Moreover, alternative biocontrol options were tested and while shown to be ineffective in this case, their use is promising as the application method tested did not have a negative effect on the crop yield. The sequencing dataset provides valuable information on the indigenous microflora present during production and how they may influence product quality and safety.
Listeria monocytogenes , Competitive exclusion , Biocides , Biofilm , Mushroom , Lactococcus lactis
Dygico, L. K. 2020. Listeria monocytogenes in the mushroom production environment: biofilm formation and control measures. PhD Thesis, University College Cork.
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