Temporal dynamics of Escherichia coli O157:H7 transcriptomes on the surface of shredded lettuce

Escherichia coli (E. coli) O157:H7 is one of the major foodborne pathogens associated with lettuce-related outbreaks over recent decades. Our study illustrates that this pathogen not only survives on the lettuce surface but also undergoes replication and metabolic alterations. Conventional culture methods showed approximately 2.5 log cfu/g increase in the E. coli O157:H7 population on lettuce, from 5.27 log cfu/g to 7.71 log cfu/g without any external energy source. Our transcriptome study revealed 1003 differentially expressed genes (DEGs) (|log2FoldChange|>2 and P_adj <0.05) after 1 hour storage and 355 DEGs after 18 hours of storage. The majority of upregulated genes observed at the 1-hour storage lettuce compared with pure culture, were associated with metabolism, biosynthesis of major amino acids (such as methionine, tryptophan, arginine, and branched-chain amino acids), and stress response. Conversely, genes upregulated in the 18 hours of storage lettuce samples, compared with the 1-hour samples, were linked to anaerobic respiration, biosynthesis of alternative amino acids, and surface attachment. The downregulation of metabolism and amino acid biosynthesis genes was observed in these samples. Additional analysis mapped the DEGs to metabolic pathways using the DAVID database, and showed most genes were clustered with metabolism and ion transport pathways. This investigation identified DEGs involved in the initial adaptation phase and the growth of E. coli O157:H7 during storage on the lettuce surface, shedding light on the molecular mechanisms underlying its survival and proliferation in this environment.