Egrets Promote the Transmission and Spread of Plasmid-Mediated Colistin Resistance Gene mcr-1-Bearing Escherichia coli Strains in Crested Ibis.

Colistin resistance genes have attracted increasing global attention due to their plasmid-mediated transmissibility. To clarify the transmission dynamics of colistin resistance genes in the endangered crested ibis (Nipponia nippon) and its environment, surveillance for mcr-1 was conducted at a captive breeding center in Deqing, Zhejiang, China, from 2017 to 2024. Genomic sequencing was used to analyze the phylogenetic relationships among the Enterobacteriaceae isolates harboring these resistance genes. A total of 2660 fecal samples of crested ibises were collected seasonally. There was only one episode of mcr-1 occurrence with a total of 144 mcr-1-positive strains in crested ibises. The prevalence was first detected at 8% in April 2021, surged to 25% in July, and then declined to 10.6% in October. Additionally, mcr-1-harboring Escherichia coli isolates were also found in feces from wild egrets (33%), the staff member's dog (8.3%), and effluent sewage, but not in the staff member, loach samples, or water sources. Phylogenetic analysis showed that the egret-derived isolate BLM86 and the crested ibis-derived isolate ZHM12 belonged to the same ST58-cgST28300 type; no core-genome SNP differences were detected. All 36-deeper analyzed mcr-1-positive isolates displayed multidrug resistance, with four distinct mcr-1-containing plasmid types identified: IncI2 (20/36, 55.6%), IncX4 (19.4%), IncP1 (11.1%), and IncHI2 (13.9%). Phylogenetic analysis indicated that several of these crested ibis-derived isolates were closely related to those from egrets, implying potential environmental transmission routes to both the crested ibis and egrets. Mcr-1 was no longer detected in the crested ibis population after implementation of biosecurity interventions. Our findings demonstrate that wild egrets could act as potential vectors for the environmental transmission of mcr-1 to captive crested ibises, highlighting the importance of interspecies barriers in antimicrobial resistance (AMR) containment.