Antibacterial low-molecular-weight compounds produced by the marine bacterium Rheinheimera japonica KMM 9513.

Strain KMM 9513 was isolated from a sediment sample collected from the Sea of Japan seashore and selected due to its ability to inhibit indicator bacterial growth. The strain KMM 9513 has been recently described as a novel species Rheinheimera japonica. This study was undertaken to determine which substances produced by strain KMM 9513 could be responsible for its antimicrobial activity. Eight compounds were obtained from an ethyl acetate extract of R. japonica KMM 9513. The structures of five diketopiperazines (4-8) and diisobutyl-, dibutyl- and bis(2-ethylhexyl) phthalates (1-3) were established on the basis of detailed interpretation of NMR data, by Marfey method and optical rotation data. The structures of diketopiperazines were determined as cyclo-(L-valyl-L-proline), cyclo-(L-valyl-D-proline), cyclo-(L-phenylalanyl-L-proline), cyclo-(L-leucyl-L-proline), and cyclo-(L-phenylalanyl-D-proline). Compounds 1-3, 5 and 8 revealed antimicrobial activities against Bacillus subtilis and/or Enterococcus faecium and Staphylococcus aureus. In this paper, we describe the isolation and structural elucidation of the isolated compounds 1-8. This is the first report of the characterisation of low molecular weight antibacterial metabolites produced by a member of the genus Rheinheimera.