## 4 Discussion  
Several species of interest to aquaculture are constantly threatened by microbial infections and diseases, which cause several damages in a field of great importance at industrial level. The use of commercial antibiotics for the treatment of diseases in farmed fishes has been underway for a long time, with consequent onset of bacterial resistance to drugs in both humans and fishes, and environmental impact by the non-absorbed fractions of antibiotics [3]. Moreover, the use of antibiotic reduces the confidence level of potential clients in the high quality of aquaculture products, thus increasing the numerous difficulties for a field that is still supporting competition with the wild resource and facing the reluctance of customers. [PERSON] et al. [36] suggested that bacteria which developed resistance to antibiotics have to be fronted with novel and safer antibiotics and recognized in the natural products of marine origin a possible candidate [37]. The marine environment is the last frontier in the search for natural antibiotics, by virtue of the high biodiversity which offer a good probability of success. Marine plant and benthic organisms, with particular regard to seaweeds [38], have been studied as a source of several metabolites, such as polysaccharides, polyunsaturated fatty acids, phenolic compounds, and carotenoids, with a wide \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Origin** & **Isolate** & \multicolumn{2}{c}{_Photobacterium damselae_ subsp. _piscicida_} \\ \hline  & & & **TSB+Glu** & **TSB+Suc** & **TSB+Gly** \\ \hline  & Water & _Pseudomonas_ sp. C1S1 & - & - & - \\  & Water & _Salinibacterium_ sp. C2W9 & - & \(10.0\pm 0.0\) & - \\  & Water & _Salinibacterium_ sp. C3W3 & - & \(10.0\pm 0.0\) & - \\ \hline  & _M. claparedei_ & _Cellulophag_ sp. Mc108 & - & - & - \\  & _M. claparedei_ & _Jostella_ sp. A8 & - & - & - \\  & _M. claparedei_ & _Idiomarina_ sp. A19 & \(10\pm 0.0\) & \(10\pm 0.0\) & - \\  & _H. panica_ & _Bacillus_ sp. SpN3 & - & - & - \\  & _P. spinosum_ & _Bacillus_ sp. PS62 & \(12.3\pm 0.6\) & \(12.3\pm 0.6\) & - \\  & _P. spinosum_ & _Brevibacterium_ sp. & - & - & - \\  & Lake Faro water & _Tenacibaculum_ sp. W14 & \(8.3\pm 0.6\) & \(8.3\pm 0.6\) & - \\  & Control & Tetracycline & & \(37\pm 0.0\) & \\ \hline  & & & **Glu**, glucose; **Suc**, sucrose; **Gly**, glycerol. & & \\ \end{tabular}
\end{table} 
Table 3: Inhibitory activity exhibited by concentrated supernatants (CFSc) of selected isolates. spectrum of activities, i.e., antiviral, antiprotozoan, antifungal, and antibacterial properties [39]. On the contrary, literature on marine bacterial metabolites active against pathogens involved in aquaculture diseases is really scant. Moreover, it is known that many bioactive compounds isolated from marine plants and animals actually derive from bacteria associated with them in symbiotic relationships. The main aim of the present study consisted in the investigation of marine bacterial strains of different origins against the most dangerous pathogens in aquaculture. Strains of Mediterranean origin, some of which are associated with benthic organisms, have been used because of their simplicity of cultivation in vitro, the speed of growth, and their potentiality reported in biopolymer production [19; 20; 23]. On the other hand, strains of Arctic origin were used as strong specialised microorganisms, able to produce specific cellular constituents [40] or high concentration of inhibitory enzymes [41] with peculiar chemical properties and effectiveness. One of the most used assays to select bacterial candidates able to produce bioactive molecules with antibacterial activity is the agar diffusion method, through which the pathogen agents are exposed to extracellular products released by bacterial producers. Despite the great reproducibility of the test and the low cost, it could be not used to investigate possible in vivo effects [42].