Community of soft coral Alcyonium digitatum associated bacteria and their antimicrobial activities

Abstract

Discovery of novel natural products from soft corals were reported with many achievement so far. Soft coral associated bacteria have gained interest for several reasons such as for marine environmental protection and bio-mining of natural products from microorganism as compared to the animals. This work focused on communities of soft coral associated bacteria and on the search for new antimicrobial substances and new natural products as potential candidates for new drugs. Thus, soft coral associated bacteria were quite abundant and good sources of antimicrobial producers. The results from cultivation-based studies and sequence analysis of 16S rRNA genes of bacterial communities associated with the soft coral A. digitatum are reported in this study. A total of 251 isolates were identified as belonging to 4 phyla Firmicutes, Actinobacteria, Gammaproteobacteria, and Alphaproteobacteria. Four of these isolates may be new species, 2 isolates of Firmicutes, 1 Gammaproteobacterium and 1 Alphaproteobacterium. The high diversity of the bacterial communities and their antimicrobial activities suggest the contribution of the associated bacteria to the coral health. The results from antimicrobial tests demonstrated that A. digitatum was the source of various bacteria producing antimicrobial substances. Antimicrobial activity of the isolates were found only in BM medium (3%), only in GYM medium (2%) or in both of these media (44%). Quite significant, all isolates belonging to the two species B. amyloliquefaciens and B. methylotrophicus showed antimicrobial activities. Some strains of the two species inhibited all four test microorganisms. Chemical analyis revealed that the two Bacillus sp. strains produced various known cyclic peptides as well as presumably new metabolites. They represented highly potent producers of antimicrobials. All isolates of Actinobacteria produced antimicrobial substances, that inhibited particulary Gram-positive bacteria. In addition to a marine Micrococcus isolate, three isolates of Firmicutes were thoroughly studied in regard to analysis of production of new compounds. In addition a number of known compounds were identified, including several peptides. The metabolites of the four strains were examined and purified by using several HPLC systems. The biological activities of pure compounds were studied using antibacterial, antifungal, cytotoxic, and enzyme assays. Eleven compounds were purified and applied to bioassays. One of the proposed known cyclic peptides showed antimicrobial activity against both B. subtilis and E. coli, whereas the others showed inhibition of methicillin-resistant Staphylococcus aureus (MRSA) and phosphodiesterase 4β2. One of compounds, which were considered as new compounds, inhibited fungus Trichophyton rubrum and enzyme phosphodiesterase 4β2. The structure elucidations of all pure compounds are in progress.