The affect of diet on the microbial community of the gut of both freshwater and marine fish.

A review of the paper: Silva, C.D.P.F. ,Nicoli, J.R., Zambonino-Infante, J.L., Kaushik, S., Gatesoupe, F.J., (2011), Influence of the diet on the microbial diversity of faecal and gastrointestinal contents in gilthead sea bream (Sparus aurata) and intestinal contents in goldfish (Carassius auratus), FEMS Microbiology Ecology, 78, 285-296

The gastrointestinal microbiota is known to have many roles within organisms including maintaining the gut integrity, strengthening immunity, increasing disease resistance and contributing to digestion. This microbiota has been much less studied in fish than in homeothermic animals despite the growing importance in relation to fish farming. There have been an increased proportion of plant proteins being introduced to fish feed, despite not knowing the consequences. The aim of this study was to investigate this further using both marine fish (sea bream) and freshwater fish that lack in a stomach (goldfish).

The sea bream were reared in 18-20°C seawater and the goldfish in 23-25°C freshwater. Three feeding treatments were used: 1) diet F – composing of fish meal as the sole protein source, 2) diet L – a mix between fish meal and Lupin meal and 3) diet R – a mix between fish meal and rapeseed meal. The fish were fed twice a day for 30 days. Throughout the experiment, the faeces was collected. At the end of the treatments, 6 of each species of fish were sampled and the stomach and intestine were removed by dissection. Microbial counts were carried out on the faeces and the stomach and intestine tissues. Samples were also incubated and cultures were grown. Microbes were classified on the basis of cell morphology, mortality, catalase and oxidase reactions. Further identification was carried out using amplified ribosomal DNA restriction analysis (ARDRA) and DGGE.

Results from the sea bream found 1152 bacterial strains from the faeces and 576 strains in the stomach and intestines. Ten-fold more cultivable bacteria were found in the faeces of the fish with diet L than the control diet F. No significant differences were found in the stomach or intestines between treatments. Originally, the bacterial community of sea bream faeces was dominated by the family Vibrionaceae however, this family was not detected by day 17. The results from DGGE showed a shift in community between the stomach and intestines.

Results from the goldfish showed 555 strains in the intestines. Bacteria of the taxa Aeromonas were found in all samples and were particularly dominant in fish with diet R. The communities were much less variable within samples from goldfish than sea bream but each dietary group had distinct profiles.

I found the conclusion of this paper really hard to understand and was very confusing. After trying very hard to interpret it, I found the conclusion was that there was none! They found that there was a different affect of diet on goldfish and sea bream and that the sea bream had a much higher variability between samples. From this information, they decided that it was not possible to conclude on the dietary influence! The paper showed much potential at the start and the method seemed solid, yet the results and discussion were hard to understand and no conclusion was made.