Bacteria could make salmon healthier

Newswise —

Researchers, including from NTNU, are breeding bacteria-free fish fry. This pursuit is more important than you might think.

"According to Ingrid Bakke, a professor at NTNU's Department of Biotechnology and Food Science, we are successfully maintaining the fry's freedom from bacteria for a maximum of 12 weeks post-egg hatching."

This measure has aided researchers in their quest to comprehend the interplay between bacteria and fish. Grasping this relationship may eventually yield a means of safeguarding fish health, which, while distant, bodes well for both the fishing sector and our future food sources - not to mention the well-being of the fish.

The researchers have studied how bacteria affect the growth, genes and mucous membranes of the fish.

But first a little about the bacteria in your body.

Trillions of bacteria

Bacteria obviously affect our health, but not only in a negative way.

Whilst residing within our mother's womb, we exist in a shielded environment, potentially free of germs, however, this changes the moment we are born. The human body typically accommodates numerous trillions of bacteria - a figure succeeded by 15 zeroes - and the same principle applies to other living beings.

Bakke states, "A multitude of our bacteria are indispensable for the proper functioning of the human body. They play a crucial role in the development of our immune system, aid in digestion, enhance the energy yield from our diet, safeguard against pathogenic bacteria, and even synthesize essential vitamins."

All of these functions, and more, emphasize the significance of delving deeper into the workings of our bacterial allies.

So how do researchers go about doing this research?

Knowledge from model systems

Bakke notes, "Many of our insights into the influence of bacteria on the host organism are derived from experiments conducted on model systems."

What does that actually mean?

Model systems refer to living beings that are convenient to utilize in the study of biological processes. Typically, these species are easy to breed, cost-effective to sustain, possess a relatively long lifespan, and exhibit genetic traits that are readily modifiable, alongside other advantageous attributes.

The particular traits that researchers seek in model systems largely hinge on the nature of their research. Some of the most commonly employed species in this regard include zebrafish, fruit flies, and various types of mice and rats.

Bakke and her colleagues have chosen a different species this time: Atlantic salmon.

Bacteria-free salmon fry

During a certain stage of their development, salmon fry inhabit a pouch referred to as a yolk sac, which furnishes them with vital nutrients.

Bakke remarks, "We have devised a model system that enables us to maintain the yolk sac of salmon fry free of bacteria during the entire 12-week yolk sac stage."

Typically, fish are devoid of bacteria during their embryonic phase but become colonized by bacteria shortly after hatching. In contrast to all other salmon, these cultivated fry lack a natural bacterial community.

The researchers propagate the fish in a safeguarded, germ-free setting, which is a conventional approach for producing bacteria-free salmon fry. The research team has developed a highly efficient and effective method that is applicable to both salmon eggs and fry.

Bakke explains, "We apply a surface treatment to the fish eggs to preserve their bacteria-free state and then maintain the eggs and subsequently the fry in water that is devoid of bacteria."

Having the knowledge to produce bacteria-free fry is crucial for the research team to investigate and study them afterward.

Salmon are like blank slates

The bacteria-free fry serve as a sort of clean canvas on which the researchers can introduce specific bacteria and observe the resulting effects, without any interference from unknown bacteria.

Bakke states, "Model systems that are free of bacteria are generally essential for comprehending the interactions between bacteria and the host organism. For instance, it could aid in comprehending how gut microbiota influence the growth and well-being of humans and other mammals."

Microbiota refers to the collection of microorganisms present in our entire body or specific body parts.

Bakke adds, "We can employ bacteria and bacterial communities that we identify, and examine how coexisting with the host and bacteria affects both parties.

As an illustration, the researchers can scrutinize the factors that regulate the makeup of the bacterial community in the fry. By doing so, they can potentially manipulate the bacterial composition in the fish to prevent any detrimental outcomes or even introduce favorable effects.

Salmon fry well suited for research

Although zebrafish have been widely utilized as a model system in this area of research, salmon fry possess certain qualities that make them particularly appropriate for this purpose.

According to Bakke, "We have relatively large and well-developed fry, which makes them more manageable for study."

The fry phase is long enough for the researchers to conduct various types of experiments. As the fry are nourished by the yolk sac, the researchers do not have to add fish feed that could potentially introduce microorganisms that could disrupt the research findings. Additionally, the fry have an aesthetically pleasing appearance, which is a bonus.

Bacteria found to affect skin mucus layer in salmon

So far, the researchers have published one article detailing their findings, with more expected to come. The first article highlights the impact of bacteria on the fish's protective skin mucus layer.

"Salmon possess a defensive mucus layer on their body surface. The bacterial composition seems to influence the characteristics of this mucosal layer," stated Bakke.

The fry devoid of bacteria showed a decrease in the thickness of the protective skin mucus layer compared to the fry exposed to either the researchers' chosen bacteria or bacteria from a lake, according to Bakke.

The bacteria can also affect the fat reserves of the fish. The fry that received bacteria from a lake developed greater fat reserves.

According to Bakke, the interdisciplinary expertise was necessary to study the effect of bacteria on the fish's mucus layer, and researcher Sol Gómez de la Torre Canny played a key role in developing the germ-free model system with yolk sac fry.

Researcher Catherine Taylor Nordgård, an expert in rheology, analyzed and described the properties of the mucus layer that covers the fish.

Opens the door to treat fish

The researchers aim to comprehend the mechanisms that influence the composition of bacterial communities that inhabit the fish soon after they hatch.

Bakke explains that the researchers aim to investigate whether the bacterial communities can protect against bacterial infections, and if it is feasible to manipulate the early bacterial colonization of fry.

By enabling probiotic treatment, researchers could introduce live microorganisms to the fish, leading to positive effects like improved health and growth.

“But probiotic treatment on a large scale is still a long way off,” says Bakke.

There is already a probiotic product called Stembiont available in Norway, which is designed for larger fish.

Additional research is required to scale up the use of probiotics. The study is being supported by the Research Council of Norway through funding from FRIPRO.

Reference:
Gómez de la Torre Canny Sol, Nordgård Catherine Taylor, Mathisen Amalie Johanne Horn, Degré Lorentsen Eirik, Vadstein Olav, Bakke Ingrid. A novel gnotobiotic experimental system for Atlantic salmon (Salmo salar L.) reveals a microbial influence on mucosal barrier function and adipose tissue accumulation during the yolk sac stage. Frontiers in Cellular and Infection Microbiology, 12 2023 DOI:10.3389/fcimb.2022.1068302