ABSTRACT. The present study describes the in vitro and in vivo survival of Lactococcus garvieae bacteriophages and the potential of the phage for controlling experimental L. garvieae infection in yellowtail. Anti-L. garvieae phages persisted well in various physicochemical (water temperature, salinity, pH) and biological (feed, serum and alimentary tract extracts of yellowtail) conditions, except for low acidity. In the in vivo, the phage PLgY-16 was detected in the spleens of yellowtail until 24 h after intraperitoneal (i.p.) injection, or the phage was recovered from the intestine of yellowtail 3 h after the oral administration of phage-impregnated feed but undetectable 10 h later. Simultaneous administration of Live L. garvieae and phage enhanced recovery of the phage from the spleen or intestine. The survival rate was much higher in yellowtail that received 1.p. injection of the phage after i.p. challenge with L. garvieae, compared with that of control fish without phage injection. When fish were i.p.-injected with phage at different hours after L. garvieae challenge, higher protective effects were demonstrated in fish that received phage treatment at the earlier time. Protection was also obtained in yellowtail receiving phage-impregnated feed, in which fish were challenged by an anal intubation with L. garvieae. Anal-intubated L. garvieae were detected constantly In the spleens of the control fish, while they were detected sporadically and disappeared from the phage-treated fish 48 h later. On the other hand, orally administered phage was detected at high plaque-forming units from the intestines and spleens of the phage-treated fish until 48 h later. These results indicate that intraperitoneally or orally administered anti-L. garvieae phage prevented fish from experimental L, garvieae infection, suggesting potential use of the phage for controlling the disease.

ABSTRACT. A virulent bacteriophage, designated PLgY, was detected from cultures of Lactococcus garvieae (formerly Enterococcus seriolicida) Isolated from diseased yellowtail Sel-iola quinqueradiata. The phage had an isometric head measuring 50 to 60 nm, a thin flexible tail of 7 x 140-180 nm, and the genome consisting of double stranded DNA, indicating that PLgY is a member of the family Siphoviridae. Of 26 strains of L. garvieae examined, 24 were sensitive to the phage but 2 strains of L. garvieae and another 22 strains of bacteria including fish and shellfish-pathogenic bacteria were not. Lysis of L. garvieae cells d u e to the phage infection was dependent on culture temperature and occurred at between 17 and 29°C. Klthough an infection experiment of young yellowtail revealed that the 2 phage-insensitive L. garvieae stralns were less virulent than 2 phage-sensitive strains, there was no correlation between phage sensitivity and antigenic variation.