Escherichia albertii is a Gram-negative, facultative anaerobic bacillus recognized as a zoonotic foodborne pathogen. Aquatic E. albertii populations likely coexist with protist hosts. Dynamic interactions between pathogenic bacteria and their protist predators may extend beyond shared growth spaces and involve host-directed cytotoxicity. E. albertii carries cytotoxicity-associated genes such as cdt, stx, and astA. This study aimed to elucidate the role of cdtB in E. albertii interaction within the Tetrahymena thermophila CU427 strain. Cytolethal distending toxins (CDTs) are tripartite protein exotoxins; CdtB requires CdtA and CdtC to bind to the glycosphingolipids ganglioside GM1 and GM3 and induce full cytotoxicity in mammalian cells. Ganglioside GM1 was highly distributed in CU427 in our study, whereas GM3 showed no comparable distribution, suggesting that ganglioside GM1 may act as a receptor for CdtB in Tetrahymena cells. However, cholera toxin B subunit treatment failed to inhibit E. albertii cytotoxicity against CU427. A LIVE/DEAD assay using Carboxyfluorescein succinimidyl ester/Propidium Iodide staining was conducted for CU427 infected with E. coli expressing recombinant E. albertii CdtB. CU427 viable cell number decreased half-fold after 24 h in a similar tendency with E. albertii infection. These results indicate that CdtB is involved in E. albertii cytotoxicity against protist hosts, particularly Tetrahymena, and suggest that the cytotoxic mechanism, especially its interaction with host cells, differs from that in mammalian cells. Thus, this study provides deeper insights into the involvement of CdtB in the E. albertii cytotoxicity mechanism.

The Oriental stork, Ciconia boyciana Swinhoe, 1873 (Ciconiiformes: Ciconiidae), once common as a sedentary stork species across Japan, experienced a marked population decline during the last century, reaching extinction in the country in 1971 as a result of anthropogenic alterations to the natural environment. After the reintroduction of five captive descendants of continental pairs in 2005, the naturalized population of Oriental storks has steadily increased in Japan, reaching >500 wild storks by 2025. We recently encountered a case of apparent intestinal fluke infection with Chaunocephalus ferox (Echinostomata; Echinostomatidae) in a 2-month-old stork fledgling within a few days after leaving the nest on 29 June 2025. The parasite was subjected to morphological and molecular identification based on nucleotide sequences of the small- to large-subunit ribosomal RNA gene and mitochondrial cytochrome c oxidase subunit 1 gene and was identified as C. ferox, a parasite exhibiting high host specificity for ciconiid birds worldwide. This finding suggests the re-establishment of its natural transmission cycle in the field in Japan, accompanied by the dispersal of definitive hosts after a gap of at least 30 years. Following the establishment of a reintroduced sedentary Oriental stork population in Japan, this health-threatening chaunocephalosis may, by chance, have a pronounced impact on infection-susceptible stork chicks cared for and fed by parent birds inhabiting fields with dense C. ferox endemicity (hotspots). Another Oriental stork fledgling from the same nest in June 2025 survived and was observed in March 2026 in a paddy field at a distant locality. Parasites are natural components of healthy ecosystems that contribute to biodiversity and trophic balance; therefore, inappropriate anthropogenic habitat modification may create scenarios in which parasites become overly destructive rather than serving merely as ecological regulators.