Novel R-plasmid conjugal transfer inhibitory and antibacterial activities of phenolic compounds from Mallotus philippensis (Lam.) Mull. Arg. Blessing O.M. Oyedemi a , Vaibhav Shinde b , Kamlesh Shinde b , Dionysia Kakalou a , Paul D. Stapleton a , Simon Gibbons a, * a Research Department of Pharmaceutical and Biological Chemistry, UCL School of Pharmacy, 29–39 Brunswick Square, London WC1N 1AX, UK b Department of Pharmacognosy, Poona College of Pharmacy, Pune 38, India 1. Introduction The problem of antimicrobial resistance continues to threaten the future of global health and healthcare systems [1]. Microbial infections caused by multidrug-resistant (MDR) Gram-positive bacteria such as Staphylococcus aureus as well as Gram-negative bacteria, including among others Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa, represent an increasing- ly growing problem. Bacterial plasmids have become broadly recognised as a major contributor to the emergence and burden of antibiotic resistance, especially in Gram-negative bacteria, owing to their ease of mobility across and within bacterial species using highly efficient type IV secretion systems (T4SSs) during conjuga- tion [2]. The T4SSs translocate DNA and protein substrates across the bacterial cell envelope and are widespread within Gram- negative bacteria [3]. Exemplary plasmids comprising different incompatibility (Inc) groups that are of clinical relevance include pUB307 [4], pKM101 [5], TP114 [6], R7K [7] and R6K [8]. Over the years, heterocyclic compounds, intercalators such as ethidium bromide and sodium dodecyl sulphate [9], acridine dyes, surface- active alkyl sulphates [10] and quinolones [11] have been reported as plasmid ‘curing’ agents. However, most of these compounds are Journal of Global Antimicrobial Resistance 5 (2016) 15–21 A R T I C L E I N F O Article history: Received 13 August 2015 Received in revised form 17 December 2015 Accepted 22 January 2016 Keywords: Multidrug resistance Bacterial plasmids Conjugation Natural products Antibacterial agent Anti-plasmid A B S T R A C T Antimicrobial resistance severely limits the therapeutic options for many clinically important bacteria. In Gram-negative bacteria, multidrug resistance is commonly facilitated by plasmids that have the ability to accumulate and transfer refractory genes amongst bacterial populations. The aim of this study was to isolate and identify bioactive compounds from the medicinal plant Mallotus philippensis (Lam.) Mull. Arg. with both direct antibacterial properties and the capacity to inhibit plasmid conjugal transfer. A chloroform-soluble extract of M. philippensis was subjected to bioassay-guided fractionation using chromatographic and spectrometric techniques that led to the isolation of the known compounds rottlerin [5,7-dihydroxy-2,2-dimethyl-6-(2,4,6-trihydroxy-3-methyl-5-acetylbenzyl)-8-cinnamoyl-1,2- chromene] and the red compound (8-cinnamoyl-5,7-dihydroxy-2,2,6-trimethylchromene). Both compounds were characterised and elucidated using one-dimensional and two-dimensional nuclear magnetic resonance (NMR). Rottlerin and the red compound showed potent activities against a panel of clinically relevant Gram-positive bacteria, including meticillin-resistant Staphylococcus aureus (MRSA). No significant direct activities were observed against Gram-negative bacteria. However, both rottlerin and the red compound strongly inhibited conjugal transfer of the plasmids pKM101, TP114, pUB307 and R6K amongst Escherichia coli at a subinhibitory concentration of 100 mg/L. Interestingly, despite the planar nature of the compounds, binding to plasmid DNA could not be demonstrated by a DNA electrophoretic mobility shift assay. These results show that rottlerin and the red compound are potential candidates for antibacterial drug lead development. Further studies are needed to elucidate the mode of inhibition of the conjugal transfer of plasmids. ß 2016 Published by Elsevier Ltd on behalf of International Society for Chemotherapy of Infection and Cancer. * Corresponding author. Tel.: +44 020 7753 5913; fax: +44 020 7753 5964. E-mail address: simon.gibbons@ucl.ac.uk (S. Gibbons). Contents lists available at ScienceDirect Journal of Global Antimicrobial Resistance jo u rn al h om ep age: w ww.els evier.c o m/lo c ate/jg ar http://dx.doi.org/10.1016/j.jgar.2016.01.011 2213-7165/ß 2016 Published by Elsevier Ltd on behalf of International Society for Chemotherapy of Infection and Cancer.