Phylogenetic Analysis of Intestinal Microflora Indicates a Novel Mycoplasma Phylotype in Farmed and Wild Salmon

All studies of the microbial community of the gastrointestinal tract of salmon to date have employed culture-based approaches, typically on pond- or tank-raised, freshwater animals. We present a phylogenetic survey of the bacterial populations present in the distal intestine of salmon from three different marine locations in Europe. This was accomplished through PCR amplification, cloning, and sequencing of partial 16S rDNA genes from microbial community DNA isolated from the contents of the GI tract distal to the pyloric ceca. Using this approach, the intestinal microbial communities of wild salmon from Scotland and pen-raised salmon from Scotland and Norway were compared. The predominating bacterial populations detected were Acinetobacter junii and a novel Mycoplasma phylotype. This Mycoplasma phylotype apparently comprised ~96% of the total microbes in the distal intestine of wild salmon. Substantial differences in intestinal microbial community composition and diversity were observed between the two groups of pen-raised salmon, which, in addition to geographical separation, were raised on different feeds. The microbial profiles found in this study were substantially different from those indicated in earlier culture-based studies for several species of fish, presumably because of the culture-independent techniques employed. Further, analysis of short-chain fatty acids in the digestive tract indicated that the decreasing redox gradient from proximal to distal reaches common to homeothermic animals was absent in salmon, and that the bacterial fermentation levels were much lower than are reported in homeothermic animals.