Evaluation of antibacterial potential of mangrove sediment-derived actinomycetes

Actinomycetes are well-known as the source of bioactive metabolites. In this work, 16 out of 118 (13.6%) isolates of mangrove sediment-derived actinomycetes showed potential antibacterial activity against at least one bacterial strain. Five extracts from isolates AMA11, AMA12 and AMA21 exhibited a broad spectrum antibacterial activity against Staphylococcus aureus ATCC25923, Staphylococcus epidermidis ATCC35984, methicillin-resistant S. aureus (MRSA) SK1, Acinetobacter baumannii NPRC004 and Escherichia coli ATCC25922. Ethyl acetate extract from the cells of AMA11 (AMA11CE) showed high activity against S. aureus and MRSA with the lowest minimum inhibitory concentration (MIC) of 0.5 μg ml^-1. At concentration of four times its MIC, AMA11CE destroyed MRSA cells as analysed by the scanning electron microscopy. In addition, AMA11CE, ethyl acetate extract from the culture broth of AMA12 (AMA12BE), AMA12CE and AMA21CE reduced violacein production in Chromobacterium violaceum. Furthermore, at concentrations lower than 10 μg ml^-1, all five extracts inhibited biofilm formation by S. epidermidis ATCC35984. The chemical analysis of the most active fraction from AMA11CE by GC-MS revealed the presence of 3-nitro-1,2-benzenedicarboxylic acid, hexadecanoic acid, quinoxaline-2-carboxamide and pentadecanoic acid. The 16S rDNA sequencing analysis revealed that these three potential isolates belonged to the genus Streptomyces. The results revealed that the actinomycetes from mangrove environment would be a good source of bioactive metabolites against pathogenic bacteria.