“We’re managing to keep the fry bacteria-free for up to 12 weeks after the eggs hatch,” said Ingrid Bakke, a professor at NTNU’s Department of Biotechnology and Food Science, in a news release from NTNU.
This step has now helped researchers on the trail to figuring out how bacteria and fish affect each other. Understanding their interaction could one day also lead to a method for improving disease resistance.
The researchers have studied how bacteria affect the growth, genes and mucous membranes of the fish.
Model systems based on bacteria-free animals are called gnotobiotic experimental systems, in which researchers can control which bacteria are present.
By comparing bacteria-free animals with "normal" animals colonised by bacteria, researchers can determine how bacteria affect the host's development and health.
For example, experiments with bacteria-free zebrafish and mice showed that some of the responses in the host to bacterial colonisation are the same in fish and mammals. Several of these bacteria involve the development of the immune and digestive system.
In the alevin stage juvenile salmon have a yolk sac attached that supplies their nutrition.
Fish are normally bacteria-free in the egg phase, but are colonised by bacteria as soon as they hatch. In contrast to all other salmon, these fry have no natural bacterial community.
“We’ve come up with a model system where we can keep the yolk sac of the salmon fry bacteria free throughout the 12-week yolk sac phase,” said Bakke.
The researchers breed the fish in a protected, germ-free environment, a standard method for making bacteria-free salmon fry. The research group has come up with an efficient and effective method that works for salmon eggs and fry.
“We surface treat the fish eggs to keep them bacteria free and keep the eggs, and later the fry, in bacteria-free water,” said Bakke.
Knowing how to create bacteria-free fry is necessary for the group to research them afterwards.
The bacteria-free fry become almost like a kind of blank slate where the researchers can add the bacteria they want and then see exactly what happens, without interference from unknown bacteria.
“Bacteria-free model systems are generally important for understanding interactions between the bacteria and host,” said Bakke. “An example would be understanding how gut microbiota affect development and health in humans and other mammals.”
The microbiota consist of all the microorganisms found in our whole body or parts of our body.
“We can use bacteria and bacterial communities that we define, and investigate how both the host and bacteria are affected by living together,” said Bakke.
For example, the researchers can investigate which factors control the composition of bacterial flora in the fry. The researchers may then be able to influence the bacterial composition in the fish to avoid negative effects, or they can introduce good effects instead.
Zebrafish have been widely used as a model system in this context. But salmon fry have some characteristics that make them particularly suitable.
“We have large and well-developed fry, which makes them easier to study,” said Bakke.
The fry phase is long enough for the researchers to carry out several types of experiments. Since the fry obtain their nutrition from the yolk sac, the researchers don’t need to add fish feed that could contain microorganisms that disturb the research results.
How bacteria impact the mucous membrane
To date, the researchers have published one article about their findings, but there are more to come. In the first article, they show that bacteria affect the protective skin mucous layer in the fish.
“The salmon have a protective mucous layer on the surface of their body. It appears that the composition of bacteria might affect the properties of this mucous layer,” said Bakke.
The fry that were not exposed to bacteria developed a thinner mucous layer on the outside of their bodies than the fry that were exposed to the researchers’ specially selected bacteria, or bacteria from a lake.
The bacteria can also affect the fat reserves of the fish. The fry that received bacteria from a lake developed greater fat reserves.
“We needed interdisciplinary expertise to study the effect of bacteria on the fish’s mucous layer. Researcher Sol Gómez de la Torre Canny was key in developing the germ-free model system with yolk sac fry,” said Bakke.
Researcher Catherine Taylor Nordgård, who is an expert in rheology, characterised the properties of the mucous layer that covers the fish.
The goal of the researchers is to understand which mechanisms affect the composition of the bacterial communities that colonise the fish immediately after hatching.
“We’re looking at how the bacterial communities possibly protect against bacterial infections, and whether it’s possible to influence the early bacterial colonisation of fry,” Bakke said.
Enabling such probiotic treatment would mean that researchers could add live microorganisms to the fish to achieve beneficial effects, such as better health and growth.
“But probiotic treatment on a large scale is still a long way off,” said Bakke.
The Norwegian product Stembiont is already available. This is a probiotic product intended for larger fish. More research is needed for probiotic use on a larger scale. The research is being financed by the Research Council of Norway through FRIPRO funding.