Microscopic and transcriptomic characterization of Listeria monocytogenes aggregation and biofilm formation on stainless steel surfaces in the presence of cantaloupe juice extract
Listeria monocytogenes (Lm) cells can attach to both cantaloupe surface and food contact surfaces and promote biofilm growth. This study was to understand the impact of cantaloupe juice on the physiology and transcriptome of Lm planktonic cells and biofilm cells grown on stainless steel coupons using confocal laser scanning microscopy (CLSM), Cryo-Scanning Electron Microscopy (Cryo-SEM) and RNA Seq technology. Lm showed a strong autoaggregation phenotype when grown in cantaloupe juice at room temperature. It is interesting to note that Lm formed significantly more biofilms on stainless steel (SS) coupons when grown in cantaloupe juice than in TSB. SEM images revealed a different attachment profile of Lm on SS coupons. In TSB, Lm cells were mainly found in scratches/groves of the metal surface, whereas, in cantaloupe juice they attached to the smooth surface as well. Interestingly, Lm planktonic and biofilm cells in cantaloupe juice showed an elongated cell shape which might be a stress-induced phenotype in cantaloupe juice. Cantaloupe juice induced a distinct transcriptional profile of biofilm and planktonic cells of Lm from TSB. Functional annotation indicated that the significantly differentially expressed genes (DEGs, Padj < 0.05, log2foldchange = 1) from the comparison mainly participated in metabolism, signaling and stress response. Notably, certain pathways downregulated for planktonic cells were significantly upregulated for biofilm cells in cantaloupe juice compared to TSB, including ABC transporters, two-component system, quorum sensing, chemotaxis, and flagellar assembly.  These data highlighted the interaction of Lm with food matrix (i.e., cantaloupe) and the role of food matrix on Lm survival and adaptation. These results provided the basis for future functional characterization of genes with potential roles in biofilm formation and persistence of Lm in cantaloupe juice, as well as for development of mitigation practices for Lm biofilms on produce and food contact surfaces. Overall design: We investigated the effect of food matrix (i.e., cantaloupe juice) vs. lab medium on Lm physiology by microscopic analysis (CLSM & SEM) and on Lm gene expression by RNA-Seq. Lm planktonic cells were collected after 48h static growth at RT in 1X cantaloupe juice or TSB and Lm biofilm cells were collected from stainless steel coupons after 3 days static growth in 0.5X media, all samples were from 3 independent experiments.