Shiga toxin-producing Escherichia coli (STEC) are notorious foodborne pathogens capable of causing severe diarrhea and other life-threatening complications in humans. Cattle remain a major reservoir and asymptomatic carriers of STEC. In reservoir animals, the rectoanal junction (RAJ) serves as the primary colonization site of STEC playing a critical role in its transmission to humans through contaminated food sources. However, the molecular mechanisms behind the adaptation of STEC in the RAJ of the asymptomatic reservoir host and its subsequent infection of human colonic epithelial cells in an event of a foodborne transmission to cause diarrhea remain largely unexplored. This study aimed to unravel the intricate dynamics of STEC O157:H7 serotype in these two distinct host environments: bovine RAJ cells and human colonic epithelial cells, during initial colonization. In this context, comparative transcriptomics analysis was employed to investigate the differential gene expression profiles of STEC O157:H7 during its interaction with these cell types. The bacterial cells were cultured under controlled conditions to simulate the microenvironments of both bovine RAJ and human colonic epithelial cells. Using high-throughput RNA sequencing, we identified key bacterial genes and regulatory pathways that were significantly modulated in response to each specific host environment. Our findings reveal distinct expression patterns of bacterial genes involved in virulence, including adherence, metal iron homeostasis and stress response in STEC O157:H7 grown in bovine RAJ cells as op-posed to human colonic epithelial cells six hours post-infection. Additionally, the comparative analysis highlights the potential role of certain genes in host adaptation and tissue-specific pathogenicity. Furthermore, this study sheds light on the potential factors contributing to the survival and persistence of STEC O157:H7 in the bovine reservoir, and its ability to colonize in-testinal epithelium and cause disease in humans.