molecules Communication Potent and Non-Cytotoxic Antibacterial Compounds against Methicillin-Resistant Staphylococcus aureus Isolated from Psiloxylon mauritianum, A Medicinal Plant from Reunion Island Jonathan Sorres 1,2,*, Amandine André 2,3 , Elsa Van Elslande 2 , Didier Stien 2,4 and Véronique Eparvier 2,* 1 Association DESIBER, 98 rue Roger Payet, Rivière des Pluies, La Réunion, 97438 Sainte Marie, France 2 CNRS, Institute of Chemistry of Natural Substances UPR2301, University of Paris-Saclay, 91198 Gif-sur-Yvette, France; [email protected] (A.A.); [email protected] (E.V.E.) 3 Laboratoire Shigeta, 62 boulevard Davout, 75020 Paris, France 4 Laboratory of Biodiversity and Microbial Biotechnologies (LBBM), Sorbonne University, CNRS, 75006 Paris, France; UPMC Univ Paris 06, Banyuls-sur-Mer Oceanological Observatory, 66650 Banyuls-sur-Mer, France; [email protected] * Correspondence: [email protected] (J.S.); [email protected] (V.E.); Tel.: +33-169-823-679 (V.E.) Received: 22 July 2020; Accepted: 3 August 2020; Published: 5 August 2020 Abstract: With the occurrence of antibiotic-resistant Staphylococcus aureus strains, identification of new anti-staphylococcal drugs has become a necessity. It has long been demonstrated that plants are a large and diverse source of antibacterial compounds. Psiloxylon mauritianum, an endemic medicinal plant from Reunion Island, was chemically investigated for its reported biological activity against S. aureus. Aspidin VB, a phloroglucinol derivative never before described, together with Aspidin BB, were first isolated from the ethyl acetate extract of P. mauritianum leaves. Their structures were elucidated from spectroscopic data. Aspidin VB exhibited strong antibacterial activity against standard and methicillin-resistant S. aureus strains, with a minimal inhibition concentration (MIC) of 5 0.25 µg/mL, and no cytotoxicity was observed at 10− M in MRC5 cells. Due to its biological activities, Aspidin VB appears to be a good natural lead in the fight against S. aureus. Keywords: Aspidin derivatives; acylphloroglucinol; Psiloxylon mauritianum; Staphylococcus aureus; antibacterial compounds 1. Introduction Staphylococcus aureus is a Gram-positive bacterium and the major cause of hospital-acquired infections, often resulting in longer stays and increases in patient mortality [1]. Such S. aureus infections, promoted by the use of ventilators or venous catheters, affect the bloodstream, lower respiratory tract, and the skin and soft tissues [2]. The microbial world is ruled by adaptation to environmental pressure, and S. aureus has developed very effective tools to resist antibiotics since the introduction of penicillin in the 1940s to cure infections. The selective pressure of antibiotics continually promotes the emergence of drug-resistant strains of S. aureus, which have dramatically increased and spread around the world [3]. Methicillin-resistant Staphylococcus aureus (MRSA) emerged quickly after introduction of the first semi-synthetic β-lactam in 1961 and has become a major worldwide health care problem [2]. Due to the rapidity and extent of its spread, as well as the high diversity among clones and strain virulence, the WHO has classified MRSA as a high priority target for new antibiotic development [4]. Molecules 2020, 25, 3565; doi:10.3390/molecules25163565 www.mdpi.com/journal/molecules Molecules 2020, 25, 3565 2 of 8 Molecules 2020, 25, x FOR PEER REVIEW 2 of 8 Even ifif pharmaceuticalpharmaceutical companies companies prefer prefer combinatorial combinatorial chemistry chemistry library library strategies, strategies, the large the diversity large ofdiversity natural of products natural oproductsffers a wide offers range a ofwide antimicrobials range of antimicrobials [5]. Plant sources [5]. Plant of anti-staphylococcal sources of anti- compoundsstaphylococcal should compounds be highlighted should be due highlighted to reports due in to the reports literature in the of remarkableliterature of activitiesremarkable of acylphloroglucinolsactivities of acylphloroglucinols or terthiophenes, or terthiophene which haves, which minimum have minimum inhibitory inhibitory concentrations concentrations (MIC) of less(MIC) than of less 1 µg than/mL [15 ].μg/mL [5]. Psiloxylon mauritianum Baill. is is a a dioecious glabrous flowering flowering plant classified classified as a member of the Myrtaceae family family and and is is a a unique unique species species of of the the genus genus PsiloxylonPsiloxylon [6].[6 ].P. P.mauritianum mauritianum is endemicis endemic to toReunion Reunion Island Island and and Mauritius Mauritius and and used used there there as as a amedicinal medicinal plant plant for for the the treatment of common infectious andand inflammatory inflammatory diseases, diseases, hypercholesterolemia, hypercholesterolemia, gout, dysenterygout, dysentery and to alleviate and to symptoms alleviate ofsymptoms amenorrhea of amenorrhea [7,8]. In 2013, [7,8]. the leaves In 2013, of P. the mauritianum leaves ofwere P. mauritianum listed in the Frenchwere listed pharmacopoeia in the French and constitutepharmacopoeia one of and the best-selling constitute one medicinal of the plantsbest-selli on Reunionng medicinal Island. plants Aqueous on Reunion extracts Island. of P.mauritianum Aqueous haveextracts also of demonstrated P. mauritianum antiviral have activityalso demonstrated against strains antiviral of Zika activity and Dengue against viruses strainsin vitro of Zika, without and exhibitingDengue viruses genotoxic in vitro, effects, without in several exhibiting mammalian genotoxic cell effects, types in [9several]. The mammalian crude acetone cell extracttypes [9]. of P.The mauritianum crude acetonewas foundextract to of harbor P. mauritianum antioxidant was activity found and to showed harbor antimicrobialantioxidant activity activity, and with showed an MIC ofantimicrobial 51 µg/mL recorded activity, against with anS. MIC aureus of. 51 Through μg/mL bioassayrecorded guided against fractionation, S. aureus. Through this anti-staphylococcal bioassay guided activityfractionation, was linked this anti-staphylococcal to the presence of corosolicactivity was and linked asiatic to acids the presence [10]. Despite of corosolic its promising and asiatic biological acids activities[10]. Despite and aits large promising consumption biological of the activities leaf infusions and a by large Reunionese consumption people, of very the few leaf phytochemical infusions by studiesReunionese were people, found in very the literature,few phytochemical and to date, studies only thewere two found pentacyclic in the triterpenesliterature, and mentioned to date, above only havethe two been pentacyclic isolated from triterpenesP. mauritianum mentioned[8,10 above]. have been isolated from P. mauritianum [8,10]. In anan e ffeffortort to to identify identify new new natural natural antimicrobial antimicrobial compounds compounds and to and explore to explore the chemical the chemical diversity ofdiversity plants fromof plants Reunion from Island, Reunion we Island, found thatwe fo theund ethyl that acetate the ethyl extract acetate (EtOAc) extract from (EtOAc)P. mauritianum from P. demonstratedmauritianum demonstrated strong antimicrobial strong anti activitymicrobial against activityS. aureus against(MIC S. ofaureus 8 µg /(MICmL). of 8 μg/mL). 2. Results and Discussion Bioassay guided fractionation fractionation of of the the antibacterial antibacterial EtOAc EtOAc extract extract of ofPsiloxylonPsiloxylon mauritianum mauritianum ledled to toisolation isolation of ofthe the known known molecules molecules Aspidin Aspidin BB BB (1 ()1 [11,12],)[11,12 ],ursolic ursolic acid acid (3 (3) )and and oleanic oleanic acid acid ( (44))[ [13],13], along with compoundcompound 22 thatthat hadhad not not previously previously been been isolated isolated or or described described in in the the literature literature (Figure (Figure1). 1 13 The1). The known known compounds compounds were were identified identified by by comparison comparison of ofH 1H and and 13CC datadata withwith valuesvalues reported in the literature, together with crystallography data for 1. The The common triterpenic triterpenic acids acids 3 and 4 were 1 13 isolated as a 6:46:4 ratioratio mixture,mixture, andand thethe completecomplete 1HH andand 13C-NMR assignments were deduced from NMR 1D and 2D experiments conducted on a 700 MHz NMRNMR spectrometer.spectrometer. Figure 1. StructureStructure of the compounds isolated from Psiloxylon mauritianummauritianum.. Compound 2 was initially obtained as a white amorphous solid. HRESIMS analysis of 2 revealedrevealed + a molecular formula ofof CC2727H36OO88 (m/z(m/z 489.2490489.2490 for for [M [M + H]H]+),), implying implying 2 2 C C and and 4 4 H H more more than in 1 Aspidin BB 1.. The 1H-NMRH-NMR spectrum displayed remarkably downfield-shifteddownfield-shifted singuletsingulet signalssignals atat δδHH 15.86, 11.41 and, 10.05, which are characteristic of the hydroxyl groups found in acylphloroglucinols 1 Aspidin derivativesderivatives [ 14[14].]. The TheH-NMR 1H-NMR data data of 2ofwere 2 were very very similar similar to those to forthose1 except for 1for except the presence for the ofpresence a supplementary of a supplementary signal at signalδH 1.39 at (m) δH 1.39 integrating (m) integrating four protons four protons (H11 and (H11 H12), and and H12), a two and methyl a two methyl triplet at δH 0.98 (J = 7.4 Hz) and δH 0.93 (J = 7.1 Hz), which are the common signals for methyl terminal groups