Researchers find yeast strain that boosts fish immunity

An international team of researchers from Mexico and Spain examined the use of a yeast, sterigmatomyces halphilus​ or strain N16, in the diet of gilthead seabream for a varying length of time when faced with a disease challenge. The group published its results in the journal of Fish and Shellfish Immunology.​​

“The present work describes the isolation, growth and phylogenetic characterization of the yeast S. halophilus strain N16, isolated from an extreme marine environment,”​ the researchers said. “The effects of adding this yeast as a dietary supplement were studied in gilthead seabream (S. aurata L.), which was used as a fish model because it is one of the most important farmed marine fish species.”​

The research team found that feeding the yeast improved levels of trypsin and several genes related to the functioning of the immune system, said the researchers.

“Immune-related genes in intestine and skin were strongly expressed principally in fish fed with 0.55% of S. halophilus for 15 days and 1.1% for 30 days and after infection, respectively,” ​said the researchers. “The present results suggest that the yeast S. halophilus can be considered as a novel fish immuno-stimulant.”​

Why extreme-environment yeast?​

Use of probiotic supplements in aqua feeds have demonstrated abilities as immuno-modulators, improving resistance to disease and pathogens, said the researchers. The positive effects of probiotic use include a production uptick of compounds that resist pathogens, improve nutrient absorption and stimulate local and systemic immune responses.

“Among commonly studied species considered as probiotics in aquaculture are bacteria (e.g. Lactobacillus spp., Bacillus spp., Shewanella spp.) and yeasts (such as Saccharomyces spp. or Debaryomyces spp.,”​ they said. But new challenges and the need for additional economically viable probiotics have created an interest in natural products and the examination of yeasts found in extreme environments, they added.  

Some yeast have been studied for their tolerance for stressors, production of B-glucan, acceptance of high sugar levels, salt concentration or varying pH values, they said. However, little work has been done on the occurrence and biodiversity of yeasts that live in hypersaline water.

Basidiomycetous yeasts have been little studied, but medical interest has examined them for use in human health, said the researchers. Several of them have been found in soils from different geographic regions and in marine water.

Previously, a similar strain isolated from the surface of lemons was examined and found to have beneficial influence on the enzymatic antioxidant status, immune responses and intestinal microbiota of farmed gilthead seabream, they said.

“The excellent potential of marine microorganisms isolated from extreme environments with beneficial properties for fish is discussed, as well as their possible use in finfish aquaculture,”​ they said.

Study methods​

For the study, yeast was extracted from sediment samples collected off the Pacific coast of the Baja California Peninsula, said the researchers. The yeast was found, isolated and analyzed genetically.

Three diets were created for the feeding trials, including a control made from a commercially available fish feed and that feed with 0.55% or 1.1% S. halophilus​ strain N16 added, they said. Fish were fed the diets for four weeks prior to the disease challenge.

In the challenge, the seabream were injected with either the bacteria V. parahaemolyticus ​or a control phosphate buffer solution, said the researchers. Sample fish were collected on weeks two and four of the feeding trial and seven days after the bacterial challenge.

After the challenge, samples of mucus, intestine and skin of infected and non-infected fish were collected, they said. Samples were examined for total immunoglobulin M (IgM), protease activity, anti-proteases, myeloperoxidase activity, lysozyme activity, superoxide dismutase (SOD) and catalase (CAT) and gene expression.

Results​

The trial diets with yeast were eaten by the juvenile fish, and no morality was noted, said the researchers. Healthy gut and liver samples were found, and after 15 and 30 days of feeding, fish getting the supplemented diets had a slight increase in weight gain compared to control fish.

“The present results suggest that the yeast S. halophilus isolated from a marine extreme environment could be considered an interesting immuno-stimulant or probiotic for gilthead seabream,”​ they said. “Several humoral skin innate immune and antioxidant parameters, as well as mucosal immune-related gene expression, were strongly enhanced after the dietary administration of the yeast before and after a bacterial challenge.”​

“It is clear, that the oral delivery of S. halophilus could be regarded as an alternative approach for combating bacterial diseases such as vibriosis in farmed gilthead seabream,”​ they said.

After the disease challenge, mucus samples from fish getting the yeast feed ingredient for 15 days had higher amounts of IgM compared to control fish group, but no difference was found between the control group and fish getting the experimental diets for 30 days, said the researchers. Protease activity was increased for fish getting the 0.55% diet for 15 days and infected fish getting the 1.1% diet.

“Antiprotease activity was statistically enhanced in the mucus from fish fed 0.55 or 1.1% S. halophilus at any time of the experimental trial (including post-infection), compared to the values found in mucus from fish fed the control diet,”​ said the researchers. Myeloperoxidase activity was increased in fish getting the 0.55% diet and lysozyme activity increased linearly with dietary yeast.

SOD increased in fish getting the yeast supplement if they faced a disease challenge and CAT was higher for fish getting the experimental diets, especially at 1.1% after the disease challenge, they said.  

Three patterns were found for trypsin expression, said the researchers. The gene was up-regulated in the intestine of fish getting the supplemented diets; expression was higher for fish getting the 0.55% diet; and the up-regulation tended to decrease with time.

Intestinal IgM gene expression also was up-regulated a week after disease challenge for fish getting the 1.1% diet, they said. Skin levels of Il-1B, TNF-a, IgM, C3 and lysozyme genes also were up-regulated after 30 days in fish getting the yeast supplement and both IL-1B and C3 were increased for fish getting the 0.55% diet after the disease challenge.   

Source: Fish & Shellfish Immunology​

Title: Dietary yeast Sterigmatomyces halophilus enhances mucosal immunity of gilthead seabream (Sparus aurata L.)​

DOI: published online ahead of print: 10.1016/j.fsi.2017.03.027​​

Authors: M. Reyes-Becerril, C. Guluarte, D. Ceballos-Francisco, C. Angulo, M. Ángeles Esteban