| الملخص: | <i>Rhodococcus</i> members excrete secondary metabolites, especially compounds which act as biosurfactants. In this work, we demonstrated the ability of <i>Rhodococcus opacus</i> R7 to produce a novel bioactive compound belonging to the class of biosurfactants with antimicrobial properties during the growth on naphthalene. Chemical and biochemical analyses of the isolated compound demonstrated that the biosurfactant could be classified as a hydrophobic peptide. The ESI-full mass spectrometry revealed that the isolated biosurfactant showed a molecular weight of 1292 Da and NMR spectra evidenced the composition of the following amino acid residues: Ala, Thr, Asp, Gly, Ser. Surfactant activity of the <i>R</i>. <i>opacus</i> R7 compound was quantified by the critical micelle dilution (CMD) method and the critical micelle concentration (CMC) was estimated around 20 mg L<sup>−1</sup> with a corresponding surface tension of 48 mN m<sup>−1</sup>. Moreover, biological assays demonstrated that <i>R</i>. <i>opacus</i> R7 biosurfactant peptide exhibited antimicrobial activity against <i>Escherichia coli</i> ATCC 29522 and <i>Staphylococcus aureus</i> ATCC 6538 with the minimum inhibition growth concentration (MIC) values of 2.6 mg mL<sup>−1</sup> and 1.7 mg mL<sup>−1</sup>, respectively. In this study for the first time, a hydrophobic peptide with both biosurfactant and antimicrobial activity was isolated from a bacterium belonging to <i>Rhodococcus</i> genus.
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