The dynamics of microbiota in Atlantic salmon GI tract might be associated with salmon gut dysfunction. Diets given salmon currently contain a mix of fish and legume-based meals and oil either from poultry or fish. The desire is to improve diets to be cost effective and to reduce dysbiosis. This study aimed to investigate digestive enzymes produced by Atlantic salmon GI tract digesta (hind gut faecal contents) and microbial communities.
Predominant Atlantic salmon gut microbiota strains (Aliivibrio, Vibrio, Photobacterium) from digesta were qualitatively screened for extracellular enzyme production. Digesta bacteria community structures were also determined based on 16S rRNA gene Illumina MiSeq data.
We observed strong alkaline phosphatase activity for all digesta Vibrionaceae isolates while activity was observed for C4 esterase, leucine arylamidase, valine arylamidase, acid phosphatase and naphthol-AS-BI-phosphohydrolase for 29-98% strains. Surprisingly, most strains tested did not form extracellular proteases (substrates trypsin, α-chymotrypsin, casein) but trypsin activity was clearly detected in salmon digesta. Both digesta samples and strains showed negative activity for lipase production. Only 5% of strains were positive for β-glucosidase activity in contrast to all digesta samples which tested positive. Most strains were able to degrade starch and pectin while digesta had detectable α-amylase activity but no pectinase activity. No degradation of other plant polysaccharides was detected in strains or digesta. Overall enzymes profiles were homogenous across digesta.
Based on the results the Vibrionaceae, which can account for up to 98% of the digesta community likely grow on predigested diet components and are not primarily involved in digestion since they were protease negative. The profile of enzymes found for salmon digesta was consistent with a largely carnivorous diet and suggests the contribution of bacteria to the digestion process could be limited, instead growing on smaller molecules produced during digestion. The possibility that dybiosis involves a competitive relationship is consistent with improved feeding and weight gain when salmon receive antimicrobial treatment and will be studied further.