Characterising gut microbiome dysbiosis in diarrhoea calves from multiple farms in Inner Mongolia using 16S and metagenomics.

BACKGROUND: The pathogenesis of neonatal calf diarrhoea (NCD), a critical disease that contributes to neonatal mortality in calves, remains nebulous. RESULTS: Inner Mongolia, a key region for cattle farming in China, was selected as a study area to provide a comprehensive overview of the epidemiology and treatment of calf diarrhoea. No significant correlation was found between the incidence of diarrhoea and sampling points or medications. The severity of diarrhoea cases was stratified into five levels based on faecal characteristics. To elucidate the pathogenesis of NCD, 16S rRNA gene and metagenomic sequencing analyses were performed across severity levels. Microbial diversity analyses revealed distinct variations in microbial communities at different severity levels. Employing binning and LEfSe methodologies, two potential bacterial pathogens were identified: Escherichia coli (bin.216), leveraging non-canonical virulence mechanisms; and Streptococcus ruminantium (bin.338), an uncharacterised diarrhoeagenic bacterium. Furthermore, the viral agent Escherichia phage VpaE1_ev108 was significantly associated with disease progression. Gene function enrichment analysis revealed a broad spectrum of antibiotic resistance genes even in farms without direct antibiotic treatment, underscoring the pervasive prevalence of drug resistance. CONCLUSIONS: The findings of this study revealed significant gut microbial dysbiosis in calves with severe diarrhoea, through which two putative NCD-associated pathogens were identified: E. coli (bin.216) and S. ruminantium (bin.338). Marked enrichment of Bacteroides spp. and Methanobrevibacter_A sp. 900313645 was observed in healthy cohorts, suggesting their potential protective roles. Therapeutic strategies employing phage-mediated pathogen targeting combined with probiotic transplantation have demonstrated dual benefits, potentially reducing antimicrobial dependency and preserving microbial homeostasis through ecological network reconstruction. Video Abstract.