Monoterpene Hydroxy Lactones Isolated from Thalassiosira sp. Microalga and Their Antibacterial and Antioxidant Activities

Two monoterpenoid lactones, loliolide (<b>1</b>) and <i>epi</i>-loliolide (<b>2</b>), were isolated from the crude dichloromethane extract of a microalga, <i>Thalassiosira</i> sp.). The structures of loliolide (<b>1</b>) and <i>epi</i>-loliolide (<b>2</b>) were elucidated by 1D and 2D NMR analysis, as well as a comparison of their ¹H or/and ¹³C NMR data with those reported in the literature. In the case of loliolide (<b>1</b>), the absolute configurations of its stereogenic carbons were confirmed by X-ray analysis, whereas those of <i>epi</i>-loliolide (<b>2</b>) were determined by NOESY correlations. Loliolide (<b>1</b>) and <i>epi</i>-loliolide (<b>2</b>) were tested for their growth inhibitory activity against two Gram-positive (<i>Staphylococcus aureus</i> ATCC 29213, <i>Enterococcus faecalis</i> ATCC 29212) and two Gram-negative (<i>Escherichia coli</i> ATCC 25922, <i>Pseudomonas aeruginosa</i> ATCC 27853) bacteria, as well as one clinical isolate (<i>E. coli</i> SA/2, an extended-spectrum β-lactamase producer-ESBL) and two environmental isolates, <i>S. aureus</i> 74/24, a methicillin-resistant (MRSA), and <i>E. faecalis</i> B3/101, a vancomycin-resistant (VRE) isolates. The results showed that none of the tested compounds exhibited antibacterial activity at the highest concentrations tested (325 μM), and both revealed low antioxidant activity, with ORAC values of 2.786 ± 0.070 and 2.520 ± 0.319 µmol TE/100 mg for loliolide (<b>1</b>) and <i>epi</i>-loliolide (<b>2</b>), respectively.