Preventing disease outbreaks in cultured turbot Psetta maxima L. caused by Aeromonas salmonicida subsp. salmonicida (ASS) requires a better understanding of how this pathogen colonizes its host. Distribution of 1 virulent and 2 avirulent ASS strains in turbot tissues was investigated during early and late stages of infection following an immersion challenge. To track bacteria within the turbot, the ASS strains were tagged with green fluorescent protein (GFP). Both virulent and avirulent strains colonized the epidermal mucus, gills, and intestine within the first 12 h post challenge, suggesting that these sites may serve as points of entry into turbot. Although the avirulent strains colonized these initial sites in the turbot tissues, they were rarely found in the internal organs and were cleared from the host 4 d post challenge. In contrast, the virulent ASS strain was found in the liver and kidney as early as 12 h post challenge and was found in the muscle tissue at very late stages of infection. The virulent strain persisted in all tested host tissues until death occurred 7 d post challenge, suggesting that ASS must colonize and survive within the turbot tissues for an infection to result in death of the fish. Comparisons of the distribution profiles of both virulent and avirulent strains during early and late stages of an infection in turbot has provided important information on the route and persistence of an ASS infection in this host.KEY WORDS: Aeromonas salmonicida · Persistence · Turbot · Green fluorescent protein · GFP · Immersion challenge Resale or republication not permitted without written consent of the publisherDis Aquat Org 95: [167][168][169][170][171][172][173] 2011 for ASS in other internal organs and did not analyze the transmission of the bacteria through the internal tissues. Interestingly, Hodgkinson et al. (1987) compared infectivity of virulent and avirulent strains in salmonids but only during the first 24 h post challenge, showing inconclusive results since the virulent strain showed an avirulent behavior. Thus, additional studies are needed to investigate the fate of both virulent and avirulent ASS strains during the progression of early and late stages of disease. In particular, studies using non-salmonid fish such as turbot will give a broader understanding on how ASS colonizes its host and will provide information for the design of new therapeutic strategies to protect against this pathogen in aquaculture.The green fluorescent protein (GFP) from Aequorea victoria has been used successfully as a genetic marker for the detection of bacterial pathogens during the infection of fish (Ling et al. 2000, 2001, O'Toole et al. 2004, Welch & Wiens 2005, Chu & Lu 2008. In most cases, expression of GFP in the bacterial cell does not interfere with growth or virulence (Chalfie et al. 1994, Valdivia et al. 1996, Ling et al. 2000, Chu & Lu 2008. In addition, GFP fluorescence, which is induced by UV light, facilitates visualization of bacteria in fish tissues and the quantification o...

Preventing vibriosis in juvenile cultured Turbot Scophthalmus maximus caused by Vibrio anguillarum frequently requires the use of feed supplemented with antibiotics in addition to vaccines. Whether the use of probiotics instead of antibiotics in juvenile Turbot is a safer strategy requires more study. The antibacterial potential of 148 Vibrio spp. strains (mostly isolated from cultures of healthy oysters, clams, and Turbot) was analyzed in vitro against V. anguillarum and other pathogens by means of an agar diffusion assay. A wide spectrum of inhibitory activity was shown by 9 strains. Based on their easy phenotypic differentiation from V. anguillarum, we selected two strains (S1 and S2, both isolated from the European flat oyster Ostrea edulis) for testing in juvenile Turbot (3 g). None of the strains were virulent by intraperitoneal or bath challenges, and all were susceptible to the antibiotics most frequently used in aquaculture. Three different stocks of Turbot, which were assayed separately, were significantly protected from infection with V. anguillarum. The final survival rates of fish treated in mixed challenges with S1 or S2 and V. anguillarum were 44% and 66%, respectively, whereas only 17% of the fish treated with only the pathogenic strain survived. The application of probiotic strains also increased the survival time of juvenile Turbot after infection with V. anguillarum. Both strains persisted in the epidermal mucus layer of the fish for 30 d, and they were not displaced by the pathogen. These data prove the efficacy of using bacteria well adapted to the dynamics of culture production as a way to provide juvenile Turbot immediate protection against infection by V. anguillarum. Moreover, the epidermal mucus sampling was useful for investigating the persistence of both probiotic strains when exposed to the pathogen.