Characterization of Yersinia ruckeri Isolates in Japan

Yersinia ruckeri is the causative agent for enteric redmouth disease of salmonids. Although the bacterium (NBRC102019) was isolated from the sediment of a river before 2007 in Japan, the disease first occurred in a different region in March 2015 in a chum salmon Oncorhynchus keta hatchery. In the present study, we characterized the isolate from the river sediment and 9 isolates from the disease outbreak. The serotype of NBRC102019 was classified as O-6, whereas the serotypes of the 9 isolates from the outbreak were all O-7. Multilocus sequence typing revealed that the 9 isolates from the outbreak belong to the same genotype and that the genotype of NBRC102019 was distinct from that of the 9 isolates. In challenge tests using chum salmon with one of the isolates from the outbreak and NBRC102019, substantial mortality was observed (the average cumulative mortality of the duplicated test was 43.3%) with the outbreak isolate but not with NBRC102019, although both of these isolates did not cause any deaths in rainbow trout O. mykiss. These results suggest that the Y. ruckeri that caused the outbreak in chum salmon belongs to a distinct lineage from that of the bacterium isolated from the river sediment.

Automated summary

This study characterized a strain (NBRC102019) isolated from river sediment and 9 isolates from a disease outbreak. The serotype of NBRC102019 was classified as O-6, while the serotypes of the 9 isolates from the outbreak were all O-7. Multilocus sequence typing revealed that the 9 isolates belonged to the same genotype, distinct from NBRC102019. Challenge tests showed that the outbreak isolates caused substantial mortality in chum salmon, while NBRC102019 did not, indicating a distinct lineage for the Y. ruckeri causing the outbreak.