Colonisation of lettuce by Listeria monocytogenes and its biofilm formation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Microbiology at Massey University, Manawatu, New Zealand
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Date
2020
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Massey University
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Abstract
Foodborne outbreaks associated with fresh produce due to pathogenic bacteria such as Listeria monocytogenes (L. monocytogenes) are increasing. A survey conducted as part of this study showed 5% of bagged lettuces sold at retail outlets contained L. monocytogenes. The ability of L. monocytogenes to form biofilms is a concern in terms of fresh produce safety. The use of hydroponics (growing plants in nutrient solution without using soil) in fresh produce production is a more controlled environment than soil, and therefore may be easier to prevent pathogen contamination. The aim of this study was to determine whether hydroponic grown lettuce and UV-C stress can reduce the colonization, growth and biofilm formation of three fresh produce related strains of L. monocytogenes: PFR O8A06 (coleslaw isolate), PFR O8A07 and PFR O8A08 (cabbage isolates) on lettuce.
Evaluation of the nutrient content and indigenous microbiota between hydroponic and soil grown lettuces revealed significant differences. 16S rRNA amplicon sequencing of the bacterial community associated with hydroponically grown lettuce showed that it was more diverse than soil-grown lettuce. Pseudomonas was found to be the dominant bacteria on hydroponically grown lettuce while Bacillus was found to be dominant in soil grown lettuce.
There was no significant difference (p > 0.05) in L. monocytogenes attachment to both hydroponic and soil grown lettuce leaves under minimal exposure times. Exposure of lettuce to 5 log CFU/ml for just one second resulted in at least 0.77 log CFU/cm² attachment. L. monocytogenes was able to survive and grow on both lettuce leaf surfaces at 4 and 10°C. Both hydroponic and soil grown lettuce leaf extracts enhanced the survival, growth and biofilm formation of L. monocytogenes on stainless steel coupons, representing surfaces in lettuce processing plants. The results of this study demonstrate the ability of L. monocytogenes to colonize and form biofilms on lettuce irrespective of the growth system used. Finally, UV-C (1.3 kJm⁻²) on lettuce produced a stress response in the plants that reduced L. monocytogenes attachment, survival and growth at pre-harvest. Further exploration of this technique may enhance the microbial safety of lettuce.
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Keywords
Lettuce, Microbiology, Listeria monocytogenes, Biofilms