Newly isolated bacterium Tenacibaculum sp. strain Pbs-1 from diseased pearl oysters is associated with black-spot shell disease

In recent years, black-spot shell disease has appeared on the shells of Akoya pearl oysters, Pinctada fucata, causing serious problems in cultured pearl production. This disease was believed to be caused by boring of Polydora ciliata into calcareous substrate, and a control measure against P. ciliata was developed. As a result, boring activity of P. ciliate decreased, but black-spot shell disease remains a serious problem. We detected the genus Tenacibaculum specifically from the shells of diseased oysters using PCR-DGGE, and isolated Tenacibaculum sp. strain Pbs-1 from the diseased oyster shells. Analyses of 16S rRNA gene sequence homology and its biochemical and morphological characteristics suggested that strain Pbs-1 was a new species of Tenacibaculum. We conducted four infectivity experiments by different infection methods: smear, injection, immersion, or a combination of wound and immersion. The highest infectivity (100%) in the shell was observed in the test group by the smear method, which was accompanied by severe clinical signs that included a focal blackish discoloration of the shell. The results of the infectivity experiments indicated that the newly isolated Tenacibaculum sp. strain Pbs-1 is one of the putative causative agents of black-spot shell disease in Akoya pearl oysters.