Despite the alarming rise in AMR over the last several years, resistance to antimicrobial drugs is nothing new. In 1945— two years after the mass-production of penicillin— over 20 % of S. aureus strains isolated from hospitalized patients were penicillin-resistant [22]. Subsequent antibiotics such as streptomycin, tetracycline, methicillin, cephalothin, gentamicin, cefotaxime, and linezolid all followed suit, resulting in the emergence of resistant bacterial strains one to four years after their introduction [22]. History indicates that for every new antimicrobial drug developed, a proportion of bacteria will become resistant to it and pose a greater threat to public health. Therefore, alternative strategies must be considered if antimicrobial resistance is to be successfully combated. One alternative is the use of medicinal plants, which for many cultures continue to play a crucial role in the primary care of patients [3]. In fact, the World Health Organization supports combining western with traditional medicinal practices in order to develop truly effective remedies for our modern health challenges [23].

It is widely accepted that the antimicrobial activities and physiological effects from plant therapies are due to the biosynthesis of secondary metabolites produced by plants as a defense mechanism [24]. Very limited chemical profiling work has been done on R. californica (as mentioned previously, earlier chemical studies reported the presence of anthraquinones [10, 11] in the genus Rhamnus), while terpenoids [9, 12], alkaloids [13], and flavonoids [14] are reported as major constituents of U. californica [15] with cyanogens, sugars and tannins as minor constituents [12]. Our study is the first to detect the presence of quinones, alkaloids, flavonoids, cardenolides, tannins and saponins in R. californica (leaves). In addition, alkaloids, flavonoids, cardenolides, saponins, tannins (leaves), and steroids (bark) were detected in U. californica. Individually, each of these classes of compounds varies considerably in antimicrobial capacity [25]. At the same time, some of the structural classes found in R. and U. californica have recently shown promising anti-MRSA potential. Specifically, a structural analog of the anthraquinone emodin, found in the genus Rhamnus, and a novel class of synthetic antibacterial cationic anthraquinones have shown potent antibacterial effects against MRSA [26, 27]. Likewise, structural analogs of flavonoids kaempferol, quercetin and eriodictyol [28], found in U. californica, and plant extracts containing terpenoids umbellulone, 1,8-cineole, α-terpineol and thymol among others [29–31] have shown anti-MRSA activity. Interestingly, even though 1,8-cineole exhibits little inherent antimicrobial activity, it has been shown to enhance the potency of other antimicrobials against MRSA [31]. Therefore, it is likely that the antimicrobial activity observed herein is due to the synergistic effects of all or most of the secondary metabolites present in each extract [31, 32]. In addition, the many compounds in the extracts may affect the bacteria via several different mechanisms, thus decreasing the likelihood of selecting for a resistant strain.

Our study is the first to report the antimicrobial potential of the methanolic extracts of leaves and bark of R. californica and U. californica, all of which significantly inhibited growth of Gram-positive bacteria such as B. cereus, S. pyogenes, and S. aureus, as well as the acid-fast organism M. smegmatis. Most intriguing was the discovery that these plant extracts were effective at controlling the growth of MRSA, one of the most ominous AMR strains, with MICs of 3.3-6.0 mg/ml. While we acknowledge that these MIC values are relatively high (in comparison to the generally accepted notion that plant extracts have significant antimicrobial activity if their MIC values are 100 μg/ml or lower), the fact that the two Gram-negative organisms E. coli and P. aeruginosa were virtually unaffected by these extracts is noteworthy and indicates that the antimicrobial effects on the Gram-positive organisms are specific and thus significant. This complete lack of activity on the Gram-negative organisms is most likely due to the protective nature of the outer membrane of their cell walls. Furthermore, MIC values similar to those obtained in this study have been reported recently by other researchers, such as: Farooqui et al., who tested the antibacterial activity of C. Sinensis and J. Regia against multidrug-resistant bacteria including MRSA [33]; Siwe Noundou et al., who demonstrated the antibacterial activity of A. floribunda against B. cereus and S. aureus [34]; and Zuo et al., who tested the antibacterial activity of 19 Chinese medicinal plants against MRSA [35]. Interestingly, the study by Zuo et al. showed that the MIC value of the isolated active component was 20-times greater than that of the crude extract [35].

Due to the limited amount of starting material available it was not possible to test the antimicrobial activity of our plant extracts on an extensive number of microorganisms. However, we successfully assessed the in vitro antimicrobial activity of these plant extracts against a MRSA strain (ATCC® BAA-1683, originally isolated from a patient at the Henry Ford Hospital in 2004) and a variety of Gram-positive and Gram-negative bacteria. Further investigation of the antimicrobial activity of these extracts against other clinical isolates of MRSA is warranted.