ISOLATION AND CHARACTERIZATION OF CYANOBACTERIA POSSESSING ANTIMICROBIAL ACTIVITY FROM THE SUNDARBANS, THE WORLD'S LARGEST TIDAL MANGROVE FOREST

Eight obligately halophilic, euryhaline cyanobacteria from intertidal soil were isolated in artificial sea-water nutrients III (ASN-III) medium. Antimicrobial activity, 16S rRNA gene sequences, phenotypic characters as well as growth and antibiosis in response to variable salinity, temperature, phosphate concentration, and pH were studied. Minimum inhibitory concentrations (MIC) of the extracts against Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa, and multiple drug-resistant clinical isolates ranged between 0.25 and 0.5 mg . mL(-1). Cytotoxicity tests showed 73%-84% human colon adenocarcinoma (HT-29/C1) cell survival at MIC values, indicating that the extracts were nontoxic. Morphologically, six cyanobacteria were assigned to the Lyngbya-Phormidium-Plectonema (LPP) group B, and one each was assigned to Oscillatoria and Synechocystis genera. Glycerol, mannitol, and starch supported better photoheterotrophic growth than simpler mono-and disaccharides. No heterocyst formation was observed when grown under nitrogen-starved conditions. All isolates survived 7 parts per thousand salinity, grew at minimum 32 parts per thousand salinity, and showed sustained growth throughout 32 parts per thousand-82 parts per thousand salinity but matured poorly in freshwater medium supplemented with 30.0 g . L-1 NaCl. Antimicrobial production occurred only at 32 parts per thousand salinity. While four of the eight isolates demonstrated sustained growth at 37 degrees C, maximum antimicrobial activity was obtained at 25 degrees C. All strains showed maximum growth and antimicrobial elaboration at 0.04 . g L-1 phosphate. All isolates thrived at pH 9.5; six grew at pH 4.5, though antimicrobial production occurred only at pH 7.5. Molecular phylogenetic analysis based on 16S rRNA gene sequences of the filamentous isolates validated the previous taxonomic affiliations established on morphological characteristics. This is the first study of antimicrobial-producing halophilic cyanobacteria from the mangroves.