Vibriostatic Agent-nonsensitive Vibrio anguillarum Isolated from Ayu

Abstract

Vibrio anguillarum is well known as the causative agent of vibriosis in ayu (Plecoglossus altivelis) in Japan.
During the diagnostic works on the vibriosis in cultured ayu in Tokushima Prefecture from 1976 to 1977, many strains were obtained. Among them, 52 strains were submitted to characterization tests, and these strains were found to be identified as Vibrio anguillarum. The problem is, however, that 27 strains out of 52 strains were not sensitive to the vibriostatic agent 0/129 (VSA).
In ayu culture, many therapeutic agents such as chloramphenicol, oxolinic acid, and nalidixic acid have been used, and recently, trimethoprim mixed with sulfadoxine (TS) has been used. Among those drugs, the new one, TS, was suspected of producing VSA-nonsensitive strains.
Then, the relationship between the sensitivity of the isolates to VSA and that to TS was examined. As a result, minimal inhibitory concentration of TS for the four VSA-sensitive strains and the four VSA-nonsensitive strains were found to be 1.2μg/ml and 24.0μg/ml, respectively. Furthermore, it was confirmed that the VSA-sensitive strains lost their sensitivity to VSA when they became resistant to TS by passing through TS containing media.
In consequence, it was concluded that some strains of V. anguillarum lost their sensitivity to VSA by obtaining resistance to TS administered to diseased ayu in ponds.