Isolation and characterization of antibacterial substances produced by marine actinomycetes in the presence of seawater

Actinomycetes, which produced antibacterial substances against Gram-positive bacteria in the presence of seawater only, were isolated from marine environment and characterized. Of the 100 isolates from Otsuchi Bay in Iwate, 41 failed to produce any antibacterial substances in the absence of seawater. However, of the 41, two were found to produce antibacterial substances in the presence of seawater. Strain No. 18 exhibited higher activity than strain No. 28 and was, therefore, selected for further studies. Phylogenetic analysis and physiological characterization showed a high similarity between strain No. 18 and Micromonospora globosa. However, M. globosa JCM 3126 did not produce any antibiotics in the presence of seawater. Therefore, the effect of seawater on the growth of strain No. 18 and its production of antibacterial substances was investigated. Strain No. 18 grew in a medium prepared with artificial seawater at a concentration range of 0–140% (v/v). The optimal concentration of growth ranged from 10 to 30% (v/v). However, production of antibacterial substances was observed in the concentration range 60–110% (v/v) despite poor growth. Thus, the production of antibacterial substances is seawater-dependent. The purification and characterization of the substances is now in progress.