Probiotics for sea cucumbers? More detail on sea cucumber gut bacteria reveals probiotic potential.

Holothurians or sea cucumbers are epi-benthic deposit feeding animals that use their tentacles to pass surface sediments containing organic matter into their mouths. Much of the organic component within sediments, however, is diluted by indigestible matter. The bacteria found within the sediments are an important constituent of the diet of sea cucumbers and are thought to provide a food source as well as a nutritional benefit to the host via provision of nutrients. The bacteria may also provide an additional benefit by degrading and breaking down indigestible components with extracellular enzymes. One particular holothurian species, Apostichopus japonicus inhabits the north-western Pacific Ocean and is an economically important species for aquaculture in China.

Previously, other techniques such as culture-dependent methods and Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) have been used to survey the composition of microorganisms within the digestive tracts of holothurians. This study by Gao et al., (2014) compared the bacterial community composition in the gut contents of sea cucumbers Apostichopus japonicus with that of the surrounding habitat sediment using 16S rRNA gene 454-pyrosequencing. The advantages of using pyrosequencing over more traditional methods is that is reveals a far greater proportion of the total community present. DGGE, for example may typically reveal less than 1% of the total taxa present and culturing methods even less. Pyrosequencing can produce many short sequences from very variable regions in rRNA genes, hence this revolutionary technique can vastly improve sampling effort. This is crucial for revealing more scarce taxa as well as breaking down communities to the lower taxonomic levels, such as genus level.

Sea cucumbers of similar weight were sampled from an area near Qingdao, Eastern China and sediment samples were taken from the same location as the sea-cucumbers. The sea cucumbers were stored on ice within an hour of being collected before being sterilized with 70% ethanol. The hindgut and foregut contents were sampled separately to avoid cross-contamination. DNA was extracted and amplified by PCR, from which amplicon libraries were generated and pyrosequencing conducted.

The results showed that the bacterial diversity of the gut contents of the sea cucumbers was lower that of the habitat sediments. Despite this, the foregut and hindgut contents harboured characteristic bacterial communities that differed from communities in the habitat sediments. Within these communities, the abundances of different groups differed in both the foregut and hindgut contents compared to the sediment. The authors suggest that selective feeding by A. japonicus could be responsible for this, although disappointingly, don’t go into further detail. The abundance of Proteobacteria was lower in the gut contents but Acidobacteria, Actinobacteria, Planctomycetes and Chloroflexi were significantly higher in the foregut contents than the habitat sediments. Furthermore, there were differences between the fore- and hindgut, with Acidobacteria, Actinobacteria and Chloroflexi being much less abundant in the hindgut than the foregut. Interestingly, Sulfate Reducing Bacteria (SRB) in the hindgut and Bacillus sp. were present in all samples. SRB are believed to provide nutrition for the host by nitrogen fixation and acetate production. SRB are also found in marine sediments, particularly anoxic ones. In addition, many of the genera analysed, including Bacillus, Lactobacillus, Lactococcus, Streptococcus, Micrococcus, Alteromonas, Pseudomonas and Vibrio are used as probiotics in aquaculture. These species offer potential as possible biological control agents within the aquaculture industry for an economically important species.

Holothurians are important deposit feeders and their feeding activity likely influences the microbial communities on and within marine sediments. This study was able to identify a greater diversity of different bacteria within the communities associated with the sea cucumber A. japonicus through pyrosequencing. These techniques have revolutionized microbial ecology in terms of exploring bacterial diversity and help enrich the results found by more traditional methods used in previous studies.

Reference: Gao, F., Li, H., Tan, J., Yan, J. and Sun, H. (2014) Bacterial community composition in the gut content and ambient sediment of sea cucumber Apostichopus japonicus revealed by 16S rRNA gene pyrosequencing, PLOS One, 9, (6), 1-10.

http://www.ncbi.nlm.nih.gov/pubmed/24967593