Isolation and Identification of Bacteria with Surface and Antibacterial Activity from the Gut of Mediterranean Grey Mullets

Fish gut represents a peculiar ecological niche where bacteria can transit and reside to play vital roles by producing bio-compounds with nutritional, immunomodulatory and other functions. This complex microbial ecosystem reflects several factors (environment, feeding regimen, fish species, etc.). The objective of the present study was the identification of intestinal microbial strains able to produce molecules called biosurfactants (BSs), which were tested for surface and antibacterial activity in order to select a group of probiotic bacteria for aquaculture use. Forty-two bacterial isolates from the digestive tracts of twenty Mediterranean grey mullets were screened for testing emulsifying (E-24), surface and antibiotic activities. Fifty percent of bacteria, ascribed to Pseudomonas aeruginosa, Pseudomonas sp., P. putida and P. anguilliseptica, P. stutzeri, P. protegens and Enterobacter ludwigii were found to be surfactant producers. Of the tested strains, 26.6% exhibited an antibacterial activity against Staphylococcus aureus (10.0 ± 0.0–14.5 ± 0.7 inhibition zone), and among them, 23.3% of isolates also showed inhibitory activity vs. Proteus mirabilis (10.0 ± 0.0–18.5 ± 0.7 mm inhibition zone) and 6.6% vs. Klebsiella pneumoniae (11.5 ± 0.7–17.5 ± 0.7 mm inhibition zone). According to preliminary chemical analysis, the bioactive compounds are suggested to be ascribed to the class of glycolipids. This works indicated that fish gut is a source of bioactive compounds which deserves to be explored.

Effect of the shrimp farming wastes as co-feed on growth performance and digestibility of juvenile grey mullet, Mugil cephalus

A feeding trial was carried out to evaluate the utilization of residual nutrients from a shrimp farming wastes as co-feed in different proportions in juvenile grey mullets. Four treatments were designed offering shrimp farming wastes at 0, 33, 66, and 100%. The 4 % of feed respecting the total biomass of each experimental tank was offered daily. The total feed ratio was completed with commercial feed for marine fish in the treatments where it was needed. At the end of the feeding trial, final weight, weight gain, relative weight gain, and thermal growth coefficient were reduced in the fish when increasing the amount of SFW as feed. However, the fish showed a digestive capacity to use residual nutrients up to 66 %, increasing their initial weight by up to 25 %. The increase of shrimp farming waste as feed negatively affected the whole-body proximal composition. The digestibility results showed that the fish could digest up to 41 % of the shrimp farming waste protein. Results suggest that juvenile grey mullets are capable of utilizing residual nutrients from shrimp farming waste. However, it is necessary to use an alternative feed source to induce an optimal growth performance for the juvenile grey mullets. It is also recommended that mullets be fed with formulated feed to meet their nutritional requirements to maintain the protein and lipid content of the whole-body under culture conditions.

Intestinal Microbial Ecology and Fillet Metal Chemistry of Wild Grey Mullets Reflect the Variability of the Aquatic Environment in a Western Mediterranean Coastal Lagoon (Santa Giusta, Sardinia, Italy)

Fish populations play an active role in the maintenance of aquatic ecosystems biodiversity. Their intestinal microbiota and fillet chemistry depend on abiotic and biotic factors of the water environments that they inhabit. The present study investigated the grey mullets’ gut microbiota from a transitional aquatic ecosystem (Santa Giusta Lagoon, Sardinia, Italy) by a multidisciplinary approach which refers the results of (1) gut cultivable microbiota analyses (MA), (2) the trace metal assessment of fish muscle (TM), (3) the physico-chemical water monitoring (PC). MA detected the greatest number of total aerobic heterotrophic bacteria, Enterobacteriaceae and coliforms in Autumn (mean values 1.3 × 105, 2.4 × 104, 1.1 × 104 cfu g−1, respectively) when the accumulated rain and mean values of nutrients (reactive phosphorous and silica) were the highest. Marine bacteria were more numerous in Summer (mean value 7.4 × 105 cfu g−1) when the highest mean values of water temperature and salinity were registered. The gut bacteria were identified as Pseudomonas spp. (64%), Aeromonas spp. (17%), Ochrobactrum pseudogrignonense (10%), Providencia spp. (5%), Enterobacter ludwigii (2%) and Kocuria tytonicola (2%). TM showed that Ca, Na, B and Ni increased their concentrations in Winter while maxima of P, Zn, Cu and Fe were found in muscles of fish sampled in Summer. This study highlighted that the fish intestinal microbiota and metal composition of the fillet reflected the seasonal aquatic environmental variability.

Nutritional quality of the fresh and processed grey mullet (Mugilidae) products: a short review including data concerning fish from freshwater

This short review compiles the data concerning the quality of flesh, processed roe and smoked fillets of grey mullets (Mugilidae), including when available the data concerning the fish from freshwater and in particular those from Tunisian reservoirs. The biochemical and mineral composition of the fish, the nutritional benefits and the potential health risks related to the consumption of fish products are discussed with regard to human health, taking into consideration the nutritional recommendations and normative toxicological limits defined by leading health authorities. Flesh proximal compositions of fish from freshwater are relatively scarce, but the few available lipid data are within the very large proximal range (from less than 1% to more than 11%) reported for grey mullets in general. Most of the studies reveal the predominance of unsaturated fatty acids with a substantial proportion ω3 type not only for fish from marine environment but also for those from freshwater. Flesh constitutes a source of essential amino acids and mineral nutrients too. The mullets are generally reported to be safe for human consumption except the fish from heavily contaminated zones. Globally, it should be regarded as an interesting contribution to a healthy diet. The mullets are also greatly appreciated for the production of salted and dried roe as well as smoked fillets, which allow increasing their economic value while preserving products health beneficial attributes. Data concerning processing mullet's roe from freshwater are missing, but it may be supposed that these roes have acceptable texture, taste and flavour as they are highly sought by connoisseurs. It is recommended to perform studies on the qualities of freshwater mullet's roe and to look for labelling the origin for both the roes and the smoked fillets of mullet from Tunisian reservoirs dedicated to the production of drinking water as it would guarantee that the fish come from unpolluted environments.