en Anti-microbial agents, resistance and treatment protocols Anti-microbial agents, resistance and treatment protocols Original article Original article <strong>Introduction:</strong> Red seaweeds are the source of polyanionic polymers that play a critical role in ionic, mechanical, and osmotic functions of the cells. The <em>Gracilaria </em>polysaccharides have numerous biological activities. This research aimed to compare the <em>in vivo</em> and <em>in vitro</em> effects of the various carbohydrate fractions of the seaweed <em>Gracilariopsis persica</em>.<strong> Methods: </strong>The crude polysaccharide of the <em>G. persica</em> seaweed was extracted using three methods, including soaking in water, hot water extraction, and acid extraction. On the optimal conditions, the seaweed polysaccharides were extracted using HCl 0.1 M 10% (w/v), and the crude carbohydrates were precipitated by ethanol. The extract was fractionated on diethylaminoethyl cellulose (DEAE-C) column using a NaCl gradient. The antimicrobial activity of each fraction was assessed by microdilution broth method against 6 bacteria species, including<em> Staphylococcus aureus</em>, <em>Escherichia coli</em>, <em>Methicillin</em>-<em>resistant Staphylococcus aureus</em> (<em>MRSA</em>), <em>Salmonella typhimurium, Pseudomonas aeruginosa, </em>and<em> Aeromonas hydrophila. </em>Moreover, the obtained fractions were orally administered (100 &micro;g/day) for 7 days to 10 groups of 4 adult NMRY mice. The effects of various fractions were evaluated based on the bactericidal effect of the sera and some immune response indicators, including complement activity and humoral immune response against sheep red blood cells (SRBC)<em>. </em><strong>Results: </strong>Most of the fractions had direct antibacterial effects; however, oral administration of the fractions neither increased the antibacterial effect of sera nor triggered the complement activity<em>.</em> However, the fractions 1, 2, 5, and 6 significantly induced the humoral immune response against SRBC<em>.</em><strong> Conclusion: </strong>The <em>G. persica</em> seaweed has direct antibacterial effects. However, unlike the humoral immune response induction, the carbohydrate fractions have no effects on innate immune responses. Gracilaria, Mice, Anti-bacterial Agents, Immunity 57 61 http://jommid.pasteur.ac.ir/browse.php?a_code=A-10-89-3&slc_lang=en&sid=1 Mohammad Khosravi m.khosravi@scu.ac.ir 0000-0001-9510-3464 Yes Department of Pathobiology, Faculty of Veterinary Medicine, Shahid Chamran University of Ahvaz, Iran Darioush Gharibi d.gharibi@scu.ac.ir 0000-0002-3614-0254 No Department of Pathobiology, Faculty of Veterinary Medicine, Shahid Chamran University of Ahvaz, Iran Farnoosh Kaviani f.kaviani89dvm@yahoo.com 0000-0002-9669-4056 No Student of clinical pathology, Faculty of Veterinary Medicine, Shahid Chamran University of Ahvaz, Iran Mehrdad Mohammadidust mmohammadidoust@yahoo.com 0000-0002-5530-420x No Aquaculture Research Center, Ahvaz, Iran.