Occurrence of Antimicrobial-Resistant Escherichia coli in the Pond in Kochi Prefecture, Japan and the Influence of the Feces of Migratory Birds

The spread of antimicrobial-resistant bacteria (ARB) has become one of the major threats to human health worldwide, and migratory birds are potentially contributing to the spread of ARB in the environment. This study investigated Escherichia coli and antibiotic-resistant E. coli in Ishizuchi Pond and Tochi River in Kochi Prefecture, Japan, where migratory birds congregate, as well as in mallard (Anas platyrhynchos) feces. We examined the number of E. coli and antibiotic resistance patterns and estimated the source of E. coli isolates. At Ishizuchi Pond, E. coli counts increased during the bird migratory period, suggesting that migratory birds may serve as a source of E. coli contamination. In contrast, Tochi River, which receives septic tank effluent, showed no significant seasonal variation in E. coli counts throughout the year. Antibiotic-resistant E. coli counts in mallard feces were approximately 1/100 to 1/300 of total E. coli counts, indicating high E. coli abundance but relatively low antibiotic-resistant E. coli presence. Phylogenetic analysis revealed similar trends between E. coli isolates from Ishizuchi Pond and mallards, with comparable positive rates for chicken-specific markers, supporting the potential contribution of migratory birds to E. coli loads in the pond.