UBC Theses and Dissertations
Investigation into the mechanisms behind the differential inhibition of Listeria monocytogenes in soft-ripened cheeses Falardeau, Justin
Soft-ripened cheeses (e.g., Camembert; SRC) are at risk for the growth of the Listeria monocytogenes due to favourable moisture and pH compared to other cheeses; however, the L. monocytogenes growth is not uniform across all SRC. The cheese microbiome, which is influenced by the farm and processing environments, may affect L. monocytogenes growth. Therefore, the purpose of this thesis was to investigate how the taxonomic and functional profiles of SRC microbiomes may affect the growth of L. monocytogenes. Forty-three SRC were inoculated with L. monocytogenes, and pathogen growth was monitored over 12 days at 8°C by select plating. Cheeses were produced from both raw (n=12) and pasteurized (n=31) milk. The taxonomic and functional profiles of the cheeses microbiomes was investigated using 16S rRNA targeted amplicon sequencing, shotgun metagenomic sequencing of 39 and 15 cheeses, respectively. L. monocytogenes growth differed significantly between cheeses (ANOVA; p < 0.001), with increases ranging from 0 to 5.4 log CFU (mean of 2.5 ± 1.2 log CFU) over the incubation period. Raw milk cheeses showed significantly lower L. monocytogenes growth than pasteurized milk cheeses (t-test; p = 0.008), possibly due to an increase in microbial richness. L. monocytogenes growth in cheeses was positively correlated with the relative abundance of Streptococcus thermophilus (Spearman correlation; p < 0.0001), and negatively correlated with the relative abundances of Brevibacterium aurantiacucm (Spearman correlation; p = 0.0002) and two Lactococcus spp. (Spearman correlation; p < 0.01). The inclusion of S. thermophilus as a starter culture is more common in industrialize SRC production, suggesting that industrial production of SRC may increase the risk of L. monocytogenes growth. L. monocytogenes growth was negatively correlated with the relative abundance of two metabolic pathways related to formaldehyde oxidation and β-D-glucuronosides degradation. No mechanism was identified for the antilisterial effects of these pathways, but it may involve the depletion of micronutrients. The presence of bacteriocin-encoding genes did not affect the growth of L. monocytogenes in the cheeses. These results further our understanding of the cheese microbiome’s affect on L. monocytogenes growth, hopefully leading to the development of SRC starter/ripening cultures that can prevent L. monocytogenes growth.
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