**Antimicrobial Constituents from** *Machaerium* **Pers.: Inhibitory Activities and Synergism of Machaeriols and Machaeridiols against Methicillin-Resistant** *Staphylococcus aureus***, Vancomycin-Resistant** *Enterococcus faecium***, and Permeabilized Gram-Negative Pathogens**

**Ilias Muhammad 1,\*, Melissa R. Jacob 1, Mohamed A. Ibrahim 1, Vijayasankar Raman 1, Mallika Kumarihamy 1, Mei Wang 1,2, Taha Al-Adhami 3, Charlotte Hind 4, Melanie Cli**ff**ord 4, Bethany Martin 4, Jianping Zhao 1, J. Mark Sutton <sup>4</sup> and Khondaker Miraz Rahman 3,\***


Academic Editor: Daniela Rigano

Received: 18 November 2020; Accepted: 16 December 2020; Published: 18 December 2020

**Abstract:** Two new epimeric bibenzylated monoterpenes machaerifurogerol (**1a**) and 5-*epi*machaerifurogerol (**1b**), and four known isoflavonoids (+)-vestitol (**2**), 7-*O*-methylvestitol (**3**), (+)-medicarpin (**4**), and 3,8-dihydroxy-9-methoxypterocarpan (**5**) were isolated from *Machaerium* Pers. This plant was previously assigned as *Machaerium multiflorum* Spruce, from which machaeriols A-D (**6**–**9**) and machaeridiols A-C (**10**–**12**) were reported, and all were then re-isolated, except the minor compound **9**, for a comprehensive antimicrobial activity evaluation. Structures of the isolated compounds were determined by full NMR and mass spectroscopic data. Among the isolated compounds, the mixture **10** + **11** was the most active with an MIC value of 1.25 μg/mL against methicillin-resistant *Staphylococcus aureus* (MRSA) strains BAA 1696, −1708, −1717, −33591, and vancomycin-resistant *Enterococcus faecium* (VRE 700221) and *E*. *faecalis* (VRE 51299) and vancomycin-sensitive *E. faecalis* (VSE 29212). Compounds **6**–**8** and **10**–**12** were found to be more potent against MRSA 1708, and **6**, **11**, and **12** against VRE 700221, than the drug control ciprofloxacin and vancomycin. A combination study using an in vitro Checkerboard method was carried out for machaeriols (**7** or **8**) and machaeridiols (**11** or **12**), which exhibited a strong synergistic activity of **12** + **8** (MIC 0.156 and 0.625 μg/mL), with >32- and >8-fold reduction of MIC's, compared to **12**, against MRSA 1708 and −1717, respectively. In the presence of sub-inhibitory concentrations on polymyxin B nonapeptide (PMBN), compounds **10** + **11**, **11**, **12**, and **8** showed activity in the range of 0.5–8 μg/mL for two strains of *Acinetobacter baumannii*, 2–16 μg/mL against *Pseudomonas aeruginosa* PAO1, and 2 μg/mL against *Escherichia coli* NCTC 12923, but were inactive (MIC > 64 μg/mL) against the two isolates of *Klebsiella pneumoniae*.

**Keywords:** *Machaerium* Rimachi 12161; machaerifurogerol; 5-*epi*-machaerifurogerol; machaeriol A–C; machaeridiol A–C; isoflavonoid; MRSA; VRE; gram negative bacteria

#### **1. Introduction**

The genus *Machaerium* Pers. (Fabaceae) consists of approximately 130 species, which are primarily distributed in the tropical Americas [1]. It is a genus of shrubs or lianas and small to medium-sized trees occurring throughout Southern Mexico to Brazil and Northern Argentina and Peru. These species are indigenous to all climatic regions ranging from equatorial rainforests to the verges of dry and cold deserts [2–4]. Several species of this genus are used in traditional medicines are considered to have multiple medicinal properties. Generally, various plant parts of *Machaerium* are used as an antitussive, and the sap is used to cure aphthous ulcers of the mouth [4]. *M. floribundum* is used to treat diarrhea and menstrual cramps [4]. The presence of a wide array of secondary metabolites from *Machaerium*, including flavonoids, terpenoids, and oxygenated phenolic compounds, together with their bioactivities, was recently reviewed by Amen et al. (2015) [2].

Earlier studies on one of the *Machaerium* species (Manuel Rimachi, Y-12161), named *M. multiflorum* Spruce, yielded four unique (+)-*trans*-hexahydrodibenzopyrans (HHDBP), machaeriols A-D, and three 5,6-*seco*-HHDBPs, machaeridiols A-C [5,6]. An unprecedented structural similarity for the HHDBP nucleus was observed in machaeriol and hexahydrocannabinol, and the 5,6-*seco*-HHDBP nucleus in machaeridiol and dihydrocannabidiol. Since these are the first reports of novel phytocannabinoids from a higher plant other than *Cannabis*, a recollection of the plant material was necessary from the original source. Unfortunately, there is an absence of documentary evidence for the existence of the species *M. multiflorum*. This species name was not included in the regional Floras as well as in major online databases (i.e., the International Plant Names Index (http://www.ipni.org/index.html) and The Plant List (http://www.theplantlist.org). Therefore, it was assumed that the plant sample was misidentified and was given the name combination *M. multiflorum* Spruce in error. An investigation was carried out on the identity of the plant, and a re-examination of the voucher specimen (Rimachi # 12161) at the Missouri Botanical Garden (MBG) concluded that this species should be treated only as an unidentified species of *Machaerium* Pers., as determined by the collection information (Manuel Rimachi, Y. 12161) [7].

The significance of the chemistry and biological activity of these aralkyl class of phytocannabinoid-type compounds led to the re-examination of the *n*-hexane and DCM fractions of the stem bark EtOH extract of the original plant material [5,6], as well as previously unexamined root and leaf extracts, which showed significant enhancement of antimicrobial activity against the various strains of methicillin-resistant *Staphylococcus aureus* (MRSA) and vancomycin resistant *Enterococci* (VRE). MRSA and VRE represent two potential threats to human health. According to the Centers for Disease Control and Prevention (CDC), MRSA can cause serious health problems, such as bloodstream infections and pneumonia. CA-MRSA occurs with a higher incidence rate in the United States and in particular amongst people who are in close physical contact, such as football athletes and childcare workers [8]. A recent national estimate for invasive MRSA incidence rates showed one in three people carry *S. aureus* in their nose and two in 100 people carry MRSA. *Enterococci* bacteria have the ability to survive for months in humans and animals. Similar to MRSA, VRE infections are commonly acquired by hospitalized patients. Enterococcal infections can be lethal, particularly those caused by VRE. According to the CDC, the number of nosocomial VRE isolates increased in the United States 20-fold, between 1989 and 1993. VRE is now the second to third most common cause of nosocomial infections in the USA [9].

In order to acquire substantial quantities of machaeriol A-D (**6**–**9**) and machaeridiol A-C (**10**–**12**) for comprehensive antimicrobial evaluations against MRSA and VRE, a reinvestigation was conducted on stem bark, leaves, and roots of the original plant material. During the course of this work, the novel epimeric mixture of bibenzylated furanoid monoterpenes, machaerifurogerol (**1a**), and 5-*epi*-machaerifurogerol (**1b**), together with the known isoflavons (+)-vestitol (**2**) and 7-*O*-methylvestitol (**3**), and pterocarpans (+)-medicarpin (**4**) and 3,8-dihydroxy-9-methoxypterocarpan (**5**), as well as previously isolated [5,6] machaeriol A-C (**6**–**8**) and machaeridiol A-C (**10**–**12**), were isolated. In this study, we report the correction of the previously reported botanical identity of the plant *M. multiflorum*, the structure elucidation of compounds **1**–**5**, and comprehensive antimicrobial activities of compounds **1**–**8** and **10**–**12**.
