Macroalgal activity against multiple drug resistant Aeromonas hydrophila: A novel treatment study towards enhancement of fish growth performance

Objective: The aim of this study was to evaluate the efficiency of macroalgal extracts as antibacterial agent against multidrug-resistant (MDR) bacteria isolated from Nile tilapia (Oreochromis niloticus) as well as to enhance the fish growth performance by macroalgae diet application. Methods: A total of 50 swabs were collected from the diseased organs of tilapia fish including gills, skin, spleen, intestine, liver, kidney and muscle. The isolated bacteria were identified and then confirmed by using VITEK 2. Eight macroalgal species were collected from Abu-Qir, Alexandria coast, Egypt. After determination of their biomass, three solvents were used to prepare algal extracts. The antibacterial activities of different macroalgal extracts were measured against MDR Aeromonas hydrophila 6 (MDRAH6) using well-diffusion method. The mechanism by which macroalgal extract affects MDR bacteria was conducted by using transmission electron microscope (TEM). To evaluate the safety of the promising algal extract, GC-MS was performed to detect the composition of S. vulgare extract. In addition, growth performance was measured as an application of algal extracts into fish feed. Results: Between eight collected macroalgal species, Sargassum vulgare showed the highest biomass production (53.4 g m(-2)). In addition, its ethanolic extract showed the highest significant antibacterial activity with MIC value of 250 mu g ml(-1). TEM examination showed distinctive changes in the treated MDRAH6 cells including rupture of the cell wall, leakage of cytoplasmic contents, alterations in the cytoplasm density in addition to totally cell deformation. In addition, GC-MS analysis revealed eleven identified components in S. vulgare ethanolic extract, in which 9,12-octadecadienoyl chloride and hexadecanoic acid methyl ester were dominant (46.6 and 19.7 %, respectively). Furthermore, dietary replacement of fish meal with S. vulgare ethanolic extract significantly enhanced the growth performance and survival of Nile tilapia with a significant reduction in the total bacterial count. Conclusion: Ethanol extract of the brown macroalga S. vulgare could be a promising antibacterial and a new active agent against MDR A. hydrophila, which could be a major causative agent of Nile tilapia fish diseases. In addition, this study recommended S. vulgare as a natural and effective source to enhance the growth performance of Nile tilapia. In fact, isolation and examination of the individual antibacterial active compounds of the S. vulgar ethanolic extract are under investigation. (C) 2016 Elsevier Ltd. All rights reserved.