No abstract

This study provides understanding of farmed salmon GI bacterial communities and describes the relative impact of diet, environmental and farm factors.

Sterilized seawater was used to assess the effects of temperature and salinity on the survival of Vibrio vulnificus. In the temperature range of 13 to 22°C, numbers of V. vulnificus increased during the 6-day incubation. Temperatures outside this range reduced the time of V. vulnificus survival in sterile 10-ppt seawater. At these restrictive temperatures, V. vulnificus numbers were reduced by 90%o after 6 days of incubation. Incubation between 0.5 and 10.5°C demonstrated that V. vulnificus survives poorly below 8.5°C. At salinities between 5 and 25 ppt and at 14°C, V. vulnificus numbers actually increased or remained unchanged after 6 days of incubation. At salinities of 30, 35, and 38 ppt, numbers of V. vulnificus decreased 58, 88, and 83%, respectively. V. vulnificus could not be recovered from deionized water, indicating lysis. When a rifampin-resistant strain of V. vulnificus was used to inoculate sterilized and unsterilized seawater (20 ppt, 20°C), numbers increased in sterile seawater but decreased to undetectable levels in 14 days in the unsterilized seawater, indicating that biological factors may play a role in the survival of V. vulnificus in the environment. Since our studies demonstrated sensitivity to low temperatures, the survival of V. vulnificus in naturally contaminated oysters at temperatures of 0, 2, and 4°C was also determined. Numbers of endogenous V. vulnificus in oyster shellstock increased by more than 100-fold in shellstock stored at 30°C but were reduced approximately 10-and 100-fold after 14 days at 2 to 4°C and 0°C, respectively. We conclude that both biological and physicochemical factors are important to the survival of V. vulnificus in the environment and that temperature is critical to controlling its growth in oyster shellstock.

To better understand salmon GI tract microbial community dynamics in relation to diet, a feeding trial was performed utilising diets with different proportions of fish meal, protein, lipid and energy levels. Salmon gut dysfunction has been associated with the occurrence of casts, or an empty hind gut. A categorical scoring system describing expressed digesta consistency was evaluated in relation to GI tract community structure. Faster growing fish generally had lower faecal scores while the diet cohorts showed minor differences in faecal score though the overall lowest scores were observed with a low protein, low energy diet.The GI tract bacterial communities were highly dynamic over time with the low protein, low energy diet associated with the most divergent community structure. This included transiently increased abundance of anaerobic (Bacteroidia and Clostridia) during January and February and facultatively anaerobic (lactic acid bacteria) taxa from February onwards. The digesta had enriched populations of these groups in relation to faecal cast samples. The majority of samples (60-86%) across all diet cohorts were eventually dominated by the genus Aliivibrio.The results suggest that an interaction between time of sampling and diet is most strongly related to community structure. Digesta categorization revealed microbes involved with metabolism of diet components change progressively over time and could be a useful system to assess feeding responses.

Vibrio cholerae serogroup 01, the causative agent of cholera, is capable of surviving in aquatic environments for extended periods and is considered an autochthonous species in estuarine and brackish waters. These environments contain numerous elements that may affect its ecology. The studies reported here examined physical interactions between V. cholerae 01 and natural plankton populations of a geographical region in Bangladesh where cholera is an endemic disease. Results showed that four of five clinical V. cholerae 01 strains and endogenous bacterial flora were attached preferentially to zooplankton molts (exuviae) rather than to whole specimens. One strain attached in approximately equal numbers to both exuviae and whole specimens. V. cholerae 01 also attached to several phytoplankton species. The results show that V. cholerae 01 can bind to diverse plankton species collected from an area where cholera is an endemic disease, with potentially significant effects on its ecology.