Isolation and characterization of a Vibrio sp. strain MA3 associated with mass mortalities of the pearl oyster Pinctada fucata.

In the summers of 2019 and 2020, a previously undescribed disease occurred in both juvenile and adult shellfish, causing mass mortalities in cultured pearl production, characterized by the major symptom of extreme atrophy of the soft tissues, including the mantle. However, the causative organism was uncertain. We isolated Vibrio sp. strain MA3 from the mantles of diseased pearl oysters Pinctada fucata. Analyses of 16S rRNA gene and DNA gyrase sequence homologies and its biochemical and morphological characteristics suggested that strain MA3 is a new strain of Vibrio alginolyticus. In addition, a hemolysin gene (Vhe1) of strain MA3 was detected as one of the virulence factors, and the complete sequence was determined. BLAST searches showed that Vhe1 shares 99.8% nucleotide sequence identity with Vibrio alginolyticus strain A056 lecithin-dependent hemolysin (ldh) gene, complete cds. Experimental infection of healthy oysters via injection with strain MA3 indicated it could cause high mortalities of the typically affected oysters from which the strain was isolated. These results suggest that the newly isolated Vibrio sp. strain MA3 is a putative causal agent of the recent disease outbreaks in Akoya pearl oysters.