Antimicrobial Agents from Marine Cyanobacteria and Actinomycetes

Continuous discovery of new antibiotics is essential due to the ever-growing resistance of microorganisms to existing drugs and the emergence of new pathogens. Many antibiotics in today’s clinical use are secondary metabolites of actinomycetes or their semisynthetic derivatives. Antibiotic from actinomycetes is an established yet interesting area for researchers, while secondary metabolite of cyanobacteria is still an emerging research field. Most antibiotics from actinomycetes were discovered with traditional whole-cell screening method by testing crude actinomycetes culture/extract against pathogens; examples include erythromycin, tetracycline, rifampicin, and daptomycin. Later many successful modifications were made to improve their activity – the basic idea was to retain the core scaffold of a chemical class and develop derivatives by changing chemical groups attached to the main scaffold. Often the modified antibiotics encounter the preexisting resistance mechanisms and questions arise regarding how many modified molecules can be developed using the known scaffolds. More recently, target-based discovery approach using bacterial genomics, combinatorial chemistry, and highthroughput screening was expected to change the scenario dramatically. Hundreds of microbial genes were sequenced and thousands of molecules were screened but that resulted in very few lead compounds, and none of them were approved for clinical use. These difficulties have led to a return to whole-cell screening as a meaningful way for searching new antimicrobial agents. Like other bioactive compounds, most of the antimicrobial agents have been derived from terrestrial organisms. Actinomycetes and fungi showed extreme productivity for novel antimicrobial agents, but the number of reports of novel antimicrobial scaffold from this group of microorganisms is gradually declining, which gives rise to the urge for widening the research area. As a result, bioprospecting has focused on various unexplored environments. Marine ecosystem is unique with diverse environmental parameters such as high salt concentration, extreme temperature and pH, high