Chronic Heat Stress Can Induce Conjugation of a Novel <i>ermB-</i>Containing ICE_FZMF, Increasing Resistance to Erythromycin Among <i>Enterococcus </i>Strains in Diverse Intestinal Segments in Mouse Model

The impact of heat stress on intestinal bacterial antimicrobial resistance (AMR) and its mechanisms is not fully understood. In this study, SPF mouse model were used and divided into a control group (25°C) and a heat stress group (42°C for 1 hour, twice daily for 55 days). Intestinal tissues of mice were analyzed for intestinal function and bacterial resistance. RT-qPCR and histopathology showed intestinal damage and significant upregulation of stress, integrity, and inflammation-related genes, indicating the damage of intestinal function due to the heat stress and the successful establishment of the mouse heat stress model. Antibiotic susceptibility testing revealed increased resistance to erythromycin, chloramphenicol, and tetracycline among Enterococcus strains. Clonal analysis showed that resistant strains were clonally unrelated. Sequencing identified a novel ermB-carrying integrative and conjugative element (ICEFZMF) among 4 erythromycin-resistant strains, capable of transferring resistance within and between species. The rectum harbored a higher proportion of erythromycin-resistant Enterococcus strains, with higher minimum inhibitory concentrations (MICs) after 25 days of heat stress. Colonization assays confirmed that heat stress led to the accumulation of erythromycin-resistant Enterococcus in the rectum, suggesting the colonization preference of erythromycin-resistant Enterococci in the rectal environment after heat stress. Metagenomic sequencing revealed significant changes in microbial composition, favoring anaerobic metabolism. This study suggests that chronic heat stress can promote the emergence of antibiotic-resistant strains through ICE transfer, providing insight for environmental safety.