MC21-A

Abstract: Methicillin-resistant Staphylococcus aureus (MRSA) is the most problematic gram-positive bacterium in public health not only because it is highly prevalent but also because it has become resistant to almost all available antibiotics except vancomycin and teicoplanin (44). Recently, its susceptibility to vancomycin has decreased, and vancomycin-intermediate and vancomycin-resistant S. aureus have increasingly been found (16, 21, 35) in several countries. Furthermore, a decrease in the susceptibility of MRSA to teicoplanin has also been reported in several hospitals around the world (28, 43). The evidence of MRSA resistance to vancomycin and teicoplanin, which are antibiotics of last resort, makes the need for alternative antibiotics and chemotherapeutics after vancomycin and teicoplanin treatments have failed particularly urgent. Although the chemical compounds of marine microorganisms are less well known than those of their terrestrial counterparts, in the last decade several bioactive substances have been isolated from marine bacteria and are new resources for the development of medically useful compounds. Antibiotics from marine microorganisms have been reported, including loloatins from Bacillus (14), agrochelin and sesbanimides from agrobacterium (1, 2), pelagiomicins from Pelagiobacter variabilis (23), δ-indomycinone from a Streptomyces sp. (6), and dihydrophencomycin methyl ester from Streptomyces (36). In particular, some species of the genus Pseudoalteromonas (formerly Alteromonas) (11) produce both antibiotics and several bioactive substances (12, 13, 18, 25, 29, 38, 39, 45). For example, Pseudoalteromonas rubra (12) and Pseudoalteromonas aurantia (13) have been reported to be antibiotic-producing bacteria. The several biologically active substances, antibacterial and algicidal toxins, as well as extracellular enzymes, produced by Pseudoalteromonas spp. have been reviewed by Holmstrom and Kjelleberg (22). With this background, we conducted a screening program for anti-MRSA substance-producing marine bacteria; one of the isolates, strain O-BC30T, showed high levels of anti-MRSA activity on a coculture plate with MRSA. Phenotypic characterization, 16S rRNA gene sequence analysis, and DNA-DNA hybridization suggested that strain O-BC30T is a new bacterial species in the genus of Pseudoalteromonas (24); Pseudoalteromonas phenolica sp. nov. is its proposed name. In this study we purified the anti-MRSA substance produced by strain O-BC30T, determined its chemical structure, and evaluated its antibacterial and bactericidal activities, especially against clinical isolates of MRSA, compared to those of vancomycin. This paper describes the purification, chemical structure elucidation, and in vitro antibacterial activity of the newly discovered anti-MRSA substance, MC21-A, as well as our investigation of its mechanism of action against MRSA.