Enhanced Production of Antimicrobial Compounds by Three Salt-Tolerant Actinobacterial Strains Isolated from the Sundarbans in a Niche-Mimic Bioreactor

A novel reactor system, the rotating disk bioreactor (RDBR), was used to mimic the niche environmental conditions of three salt-tolerant estuarine actinobacteria isolated from the Sundarbans region off the Bay of Bengal, designated MS310 (99% similar in its 16S rRNA gene sequence to Streptomyces parvallus), MS3/20 and MS1/7. The RDBR, operated at a rotational speed of one revolution per day, 50% submergence of discs, aeration rate of 1.0 vvm, and with a sucrose-containing medium, faithfully mimicked the intertidal estuarine habitat of these marine isolates, and supported biofilm formation and production of antimicrobial metabolites—in particular, actinomycin D by MS310. Onset of antibiotic production by MS310 occurs at 20 h in the RDBR compared to 55 h in a conventional stirred-tank bioreactor (STBR). Furthermore, peak antimicrobial activity is attained much earlier in the RDBR with MS310 (at 45 h) than that reported with a terrestrial strain of S. parvallus grown in a STBR (at 144 h). Peak antimicrobial activity of metabolites produced by MS1/7 and MS3/20 were also attained earlier in the RDBR (at 25 and 12 h, respectively) than in a STBR (at 80 and 28 h, respectively). Antibiotic synthesis in the three isolates, in general, appears to be associated with their growth. Overall, the RDBR may be considered the preferred alternative to the STBR for production of antimicrobials by biofilm-forming estuarine bacteria for its much higher surface/volume ratio, lower costs, and easy operability

Studies on the Production and Purification of an Antimicrobial Compound and Taxonomy of the Producer Isolated from the Marine Environment of the Sundarbans

A microorganism isolated from the Sundarbans region of the Bay of Bengal, India, showed potent antimicrobial activity against gram-positive and gramnegative bacteria, molds, yeast and several multiple-drugresistant (MDR) bacteria, including methicillin-resistant Staphylococcus aureus (MRSA). The isolate grew in the presence of 20% (w/v) NaCl, antibiotic production being maximum with 5% (w/v) NaCl in the production medium. Natural seawater stimulated antibiotic biosynthesis. The absence of catabolite repression during the synthesis of the antimicrobial substance was demonstrated by the utilization of glucose by this isolate. The 16S rRNA gene of this aerobic, gram-positive, mycelium- and spore-forming microorganism was amplified, and molecular phylogenetic analysis of the DNA sequence showed less than 93% similarity with its closest relative, indicating differentiation at the genus level. The highly stable, active principle was purified by butyl acetate extraction and silica-gel chromatography and a single compound was found to posses the broad-spectrum activity. Molecular characterization showed that the active compound is a lipid. Bioreactor studies demonstrated that antibiotic production is strongly dependent on the scale of operation and there is a definite relation between the dissolved oxygen concentration, medium pH, glucose utilization, cell differentiation and antibiotic production. Maximum production in 30 h could be obtained by regulation of the medium pH in the alkaline range by a combination of controlled addition of NaOH, regulation of the air supply and changes in the reactor configuration. Considering all of the above evidences and based on comparison with the current literature, a novel antimicrobial appears to have been isolated