Seasonal diet shifts alter the gut microbiome and resistome of captive geriatric giant pandas (Ailuropoda melanoleuca)

The nutritional changes of giant pandas (Ailuropoda melanoleuca) in response to the seasonal variations from bamboo shoots (rich in proteins) to fibrous leaves trigger significant alterations in the structure and functions of the gut microbiome. However, the effect these dietary changes have on the gut resistome, especially in older adults, is not well characterized. In this study, shotgun metagenomic sequencing and quantitative PCR (qPCR) were used to investigate the microbial composition, functional potential, and profiles of antibiotic- and metal-resistance genes (ARG and MRG) in feces of adult (n = 11) and geriatric captive pandas (n = 11) that were fed on bamboo shoots or leaves. The microbes varied considerably among diet and age groups, with diet becoming the main source of taxonomic and functional disparity (P < 0.05). Shoot-fed pandas exhibited higher alpha diversity at the genus level and distinct clustering in principal coordinate analyses, whereas leaf-fed groups showed enrichment of taxa associated with fiber degradation and stress tolerance (P < 0.05). Functional annotation of bacterial responses to diet showed changes in carbohydrate processing pathway, carbohydrate transport, and cellular process pathways by changes in the KEGG pathway (P < 0.05). Changes depending on diet were also identified with significant changes in carbohydrate-active enzyme (CAZy) family during changes in the composition of the bamboo parts. Metagenomics and qPCR revealed that several antibiotic resistance genes, such as aac(3)-Xa, bcrA, tet44, sul2 and macB, were highly interacting between diet and age and the most diverse resistome was found in geriatric pandas (P < 0.05). Correlation analysis demonstrated that there is a positive co-occurrence pattern of Enterobacteriaceae and several ARGs. Collectively, our findings demonstrate that seasonal dietary shifts and host aging jointly restructure the gut microbiome and resistome of giant pandas, suggesting diet-mediated modulation of microbial adaptation, resistance dissemination, and ecological resilience in captivity.