Protective effect of sodium alginates against bacterial infection in common carp, Cyprinus carpio L.

Abstract. A hot-water extract of Undaria pinnatifida was found to contain polysaccharide(s) which showed immunostimulating activity in fish. In order to identify the effective component(s), the hot-water extract was fractionated and acid-soluble (Fraction I) and acid-insoluble (Fraction II) polysaccharides were evaluated for their potential to enhance protection against Edwardsiella tarda infection in common carp. Intraperitoneal injection of Fraction II in carp 6 and 3 days prior to challenge with E. tarda resulted in a significantly greater survival rate than that of control fish at doses of 10–30mgkg^-1. On the other hand, Fraction I was not effective at any dose. Chemical and physicochemical analyses revealed that Fraction II was sodium alginate with a high degree of purity, with a molecular weight of 45000 and an M/G ratio of 0.8. Some commercial alginates were similarly evaluated for their efficacy. Alginates from Macrocystis pyrifera (MW, 146000-264000; M/G ratio, 0.9–1.0) significantly increased survival rate, but alginates from Lessonia nigrescens (MW, 177000–290000; M/G ratio, 1.3) had little effect. There are indications that the efficacy (protective effect) of alginates has some relationship to their M/G ratios.