Intestinal bacteria isolated from seven freshwater cultured fish were examined for their antibacterial ability against 18 target strains, including members of the genus Aeromonas and human pathogenic bacteria, using a double-layer method. A total of 304 isolates, including aerobic and anaerobic bacteria, were classified into 18 taxonomic groups. All fish species harbored intestinal bacteria with antibacterial abilities. An average of 3.2% of tested strains exhibited antibacterial activity against 18 different species, including 12 Aeromonas species and pathogenic bacteria, but the activity varied with fish species, and taxonomic groups and strains of tested bacteria. All taxonomic groups tested, except for Acinetobacter spp., coryneforms and Staphylococcus spp., contained strains exhibiting antibacterial activity. In particular, 7–12 target strains were inhibited by strains of Aeromonas caviae, A. hydrophila, Pseudomonas spp. and Clostridium spp. These results suggest that the antibacterial ability of intestinal bacteria may protect host fish against pathogenic bacteria, to some extent.
Keywords
Antibacterial activity; Freshwater fish; Intestinal bacteria