Structure-dependent activity of plant natural products against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the major causes for nosocomial infections and has been classified as “high priority pathogen” by the World Health Organization. Its ability to develop resistances has been a challenge for the last decades and is still a threat to health care systems, as strains with resistances to the so-called drugs of last resort have been discovered. Therefore, new antibiotics are urgently needed. Natural products are an important source for the development of new drugs, thereby mostly serving as lead compounds for further modification. In this review, the data on plant natural products with reported anti-MRSA activity until the end of 2022 is discussed, highlighting the most effective drugs with respect to their inhibitory concentrations as well as with regard to eventual synergistic effects with existing antibiotics. In the latter sense, the class of alkaloids must be mentioned, exhibiting additive or synergistic effects by inhibiting bacterial efflux pumps. With regard to the antibiotic activity, phloroglucinol derivatives certainly belong to the most promising compounds, revealing several candidates with remarkable effects, e.g., lupulone, ivesinol, rhodomyrtone, aspidinol, or hyperforin. Also, the class of terpenoids yielded noteworthy compounds, such as the sesquiterpene lactones parthenolide and lactopicrin as well as acetophenone sesquiterpenes and sphaerodiene type diterpenoids, respectively. In addition, pronounced effects were observed for the macrolide neurymenolide A and three flavonol dicoumaroylrhamnosides.


Introduction
Due to the increasing age of the population, especially in industrialized nations, hospital or nursing home-acquired infections are becoming a major problem (Hasanpour et al., 2023).Antimicrobial resistance is prognosed to cause 10 million annual deaths by the year 2050, rendering it the second leading cause of death after cardiovascular diseases (Nandhini et al., 2022).Many of these infections are associated with methicillin-resistant Staphylococcus aureus (MRSA), which has a mortality rate of around 14% in patients with drug-resistant infections in the US (CDC, 2013).However, the prevalence of healthcare-associated MRSA varies for different countries, with, e.g., Portugal (58.4%),Pakistan (52%), India (46%), China (45%), and Norway (38.9%) showing high ratios, while other countries such as Mexico (19.1%),Australia (15.1%), and Germany (4.6%) are much less affected (Shoaib et al., 2022).Even though the first observation of resistance dates back to the 1960s (Jevons, 1961), MRSA is still a threat to Moreno Cardenas and Çiçek 10.3389/fmicb.2023.1234115Frontiers in Microbiology 02 frontiersin.orghealth-care systems, evolving more and more resistance mechanisms against a broad spectrum of antibiotics.Apart from communityassociated MRSA strains, livestock-associated strains also serve as a reservoir for resistance (Cuny et al., 2013).In 2017, MRSA was classified as a "high priority pathogen" by the world health organization (WHO, 2017), stating the urgent need to develop new treatments against this bacterium, i.e., new substances with antibiotic or antibiofilm activity.MRSA causes different kinds of infections including bacteremia, skin and soft-tissue infections, infective endocarditis, osteoarticular infections, pleuropulmonary infections, and many more (Tong et al., 2015).This often leads to complications or prolongation of clinical treatments.In the European Union, MRSA causes the second most infections of any drug-resistant pathogen, only surpassed by the cephalosporin-resistant Escherichia coli (ECDC, 2021).The treatment includes antibiotics of the so-called drugs of last resort (DoLR) like vancomycin, daptomycin, linezolid, and tigecycline (Choo and Chambers, 2016;Subramani et al., 2017).Alarmingly, strains with resistance to one of these antibiotics have already been detected (Layer et al., 2021).
An overriding cause for the development of microbial resistance is the misuse of antibiotics (Abreu et al., 2012;Peacock and Paterson, 2015;Rossiter et al., 2017).Most resistance mechanisms affect the uptake and efflux of the antibiotic, alter the target structure, modify the antibiotic, or even produce enzymes to inactivate the antibiotic (Peacock and Paterson, 2015;Rossiter et al., 2017).If a strain adopts a resistance the respective gene can be passed along through division or horizontal gene transfer (Peacock and Paterson, 2015;Álvarez-Martínez et al., 2020).Because most of the available antibiotics against MRSA target the cell wall biosynthesis, new lead compounds with other bacteriostatic or bactericidal mechanisms are urgently needed (Choo and Chambers, 2016).
In the past, several natural products have led to effective antibiotics, such as penicillin and vancomycin, with, however, a greater focus on fungal or bacteria-derived compounds (Katz and Baltz, 2016;Wright, 2017;Porras et al., 2021).Nevertheless, plants are a rich source of diverse lead compounds, such as alkaloids, terpenoids, quinones, or polyphenols.These substances protect organisms against harmful bacteria, fungi, and insects, thereby offering greater structural variety than standard small molecule libraries (Subramani et al., 2017;Zaynab et al., 2018;Atanasov et al., 2021).Considering the fact that no new antibiotic class has been approved by the FDA since the late 1980s, the broad diversity of specialized plant metabolites may contribute to the development of future antimicrobials (Durand et al., 2019).The traditional use of medicinal plant species against numerous kinds of infectious diseases is a good starting point for the exploration of new antibiotic lead structures (Anand et al., 2019;Porras et al., 2021).Some plant extracts are already used for the treatment of skin infections caused by MRSA, such as the essential oil from the tea tree Melaleuca alternifolia (Myrtaceae) (Nandhini et al., 2022).In addition, not only the pure antibiotic effect is of interest, but also potential additive or synergistic effects with existing antibiotics might be an approach against adapted resistances (Sadeer and Mahomoodally, 2021).
In the following, 223 natural products from various plant species are discussed with regard to their structure and antibacterial effects against 169 different MRSA strains (Supplementary Table S1) as well as for their synergistic effects with a range of different antibiotics.

Methods
A literature search was carried out using the Web of Science citation index, including all publications published until the end of 2022.The search terms "MRSA" and "natural products" were used yielding 833 results.These were reduced to the field of plant science resulting in 148 hits.Also, other sources were collected, which were not found by database search with the previous terms, yielding a total of 223 plant natural products with reported anti-MRSA activity.The compounds were sorted by compound class, thereby taking compound names and configurations "as is" from the original publications.Plants species names were checked using the "World Flora Online" and eventually changed into the accepted taxa.MIC/IC 50 values are given in μg/mL using the number of digits given in the original publications.Values found μmol/mL were converted to μg/mL.Values above 100 μg/mL were considered inactive and thus excluded from the review.Activities above 50 μg/mL were referred to as low, MIC and IC 50 values from 10 to 50 μg/mL as moderate and values below 10 μg/ mL as high.For synergistic effects factorial inhibitory concentration index (FICI) values above 2 indicate antagonism, values of 2 to 1 indifference, values of 0.5 to 1 additive effects, and values below 0.5 synergism.The values given in μg/mL represent the MIC of the combination.

Alkaloids
The first compound class to be discussed in this review is the class of alkaloids, which are a rich source for analgetic or cytostatic drugs, but are rather rare in the field of antibiotics.However, they acted as lead substances in the development of quinolones, thus remaining important scaffolds in drug development (Cushnie et al., 2014).
Our literature search revealed most reports for isoquinolines-type alkaloids with a total of eight compounds.Additionally, two lycorine derivatives, two pyrolin-2-on derivatives, two phenanthridine-type alkaloids, one quinolone, and one pyridine-derivative were reported (Figure 1).
Couroupita guianensis, which is used in traditional South American medicine, yielded 1, a compound with a low MIC value but also low toxicity (Costa et al., 2017).Liu et al. (2021) extracted two isoquinolines (2 and 3) from Doryphora aromatica leaves, with activities ranging from 9.9 to 39.6 μg/mL (2) and from 19.7 to 39.4 μg/ mL (3), respectively.Both compounds were first tested against methicillin-sensitive Staphylococcus aureus and showed higher activity against the resistant clinical isolates.Another two isoquinolines (4 and 5) were isolated from the roots of Zanthoxylum nitidum by Zeng et al. (2022).Compound 4 showed moderate and compound 5 showed low activity against MRSA, but both exhibited pronounced synergistic effects with ampicillin, which were attributed to the inhibition of the bacterial efflux pump.Compound 4 was, furthermore, found to successfully disturb the bacterial biofilm.Compound 6, a quinolone derivative was isolated from Zanthoxylum schreberi, formerly named Zanthoxylum monophylum, and displayed an IC 50 value of 1.5 μg/mL, which was rather low compared to the positive control ciprofloxacin (IC 50 of 0.06 μg/mL) (Rodríguez-Guzmán et al., 2010).Zuo et al. (2011) 2012) isolated 10, from Coptis chinensis, showing a slightly higher activity than 9.Moreover, 9 and 10 showed synergistic effects with different antibiotics.Also here, the synergistic effect was attributed to the inhibition of the bacterial efflux pump.Yu et al. (2019) tested 11 against an MRSA strain that overexpressed the NorA multidrug efflux pump, thus giving inferences for many wildtype-MRSA strains.While 11 was only slightly active when tested alone, a combination with norfloxacin or reserpine led to an eightfold increase in the antibiotic activity.Therefore, 11 was assumed an efflux pump inhibitor and suggested to be further investigated.The two lycorine derivatives 12 and 13 were isolated from Crinum ornatum, formerly referred to as Crinum distichum, by Koagne et al. (2018) and showed moderate activity.Two sesquiterpene alkaloids (14 and 15) were isolated from the semi-mangrove plant Myoporum bontioides (Dong et al., 2018).Both compounds showed high activity (6.5 μg/ mL) against the methicillin-resistant but vancomycin-sensitive S. aureus strain.Even more pronounced was the effect of 16, a sulfurcontaining pyridine derivative isolated from Allium stipitatum (Karunanidhi et al., 2019).The authors additionally performed a time-to-kill assay and observed bactericidal activity after 2 h.Pyridine or thiopyridine-based compounds were reported to have promising antibacterial activity before (Karunanidhi et al., 2019).In addition, compound 16 bears a carbonitrile group, which is also associated with strong antibacterial activity.Most of the discussed alkaloids exhibit only low to moderate activities against MRSA, with only a few exceptions, such as compounds 14-16.However, some compounds show auspicious potential as synergistic agents for existing antibiotics through their ability to inhibit the bacterial efflux pump.

Monoterpenoids and sesquiterpenoids
This section discusses one monoterpenoid and 17 sesquiterpenoids (Figure 2), with the latter compound class being known for some potent antimicrobials, such as artemisinin from Artemisia annua or santonin from Artemisia cina (Li et al., 2022).
The only monoterpene in this review, nodosol (17), was isolated from the marine angiosperm Cymodocea nodosa (Kontiza et al., 2008).It showed a MIC value of 16 μg/mL against strains with different resistance mechanisms.Interestingly, the compound showed higher activity than the reference antibiotics against three drug resistant strains.Four furyl bearing sesquiterpenes were isolated from Myoporum bontioides (18-21), of which two compounds display an additional dihydrofuran feature (18 and 19) (Dong et al., 2018).Interestingly, both compounds exhibited twice the activity of their ring-open counterparts (20 and 21).Compound 22 was isolated from Mentha pulegium and exhibited moderate activity with an IC 50 value of 8.5 μg/mL (Ibrahim, 2013).The dimeric sesquiterpenoid gossypol (23), which is also known from the cotton plant, was isolated from Thespesia garckeana by Masila et al. (2015) and showed an IC 50 of 4.66 μg/mL.Häkkinen et al. (2021) tested the germacranolide lactones parthenolide (24, from Tanacetum parthenium) and lactucopicrin (25, from Cichorium intybus) against a β-lactamase possessing strain, with pronounced effects (MIC value of 0.16 μg/mL).Ferula feruloides was the source for nine acetophenone sesquiterpenoids (26-34), which all exhibited activity against different MRSA strains (Sun et al., 2019).26 (MIC: 1 to 32 μg/mL) showed the highest effect against a strain that expresses a tetK efflux pump and against an epidemic strain.Its para-methyl ether 27, in contrast, was inactive against four out of five strains, indicating the importance of the para-hydroxy group for the anti-MRSA activity.This is even more clear when looking at the additional seven acetophenone sesquiterpenes, which (apart from 30) show pronounced growth inhibition.The number of different compounds in this class allows some further conclusions.Hydroxylation in the α-position of the side chain leads to an increase in activity against almost all strains (28, MIC: 1-4 μg/mL), whereas dihydroxylation in position ω-6 and ω-7 results in significantly lower effects (30, MIC: 64 μg/mL).Conversely, the formation of a dioxolane ring in the same position (31, MIC: 0.5-16 μg/mL) causes a significant increase in activity, especially for the tetK and the epidemic strain.This increase is much less pronounced when only one of the two hydroxy groups derivatized, as, e.g., for compound 34 (MIC: 16-64 μg/mL).Formation of a cyclic lactone in the α-position leads to MIC values of 2-8 μg/mL (32) and 2-16 μg/mL (33), respectively, whereas a rearrangement of the farnesyl unit toward a non-linear side chain causes even lower activities (29).Chemical structures of mono-and sesquiterpenoids with reported anti-MRSA activity.

Diterpenoids
This section comprises 34 diterpenoids of nine different scaffolds, of which most display meroditerpenoids (55-68) from the red algae Callophycus sp.Additionally, eight abietane type (47-54), six sphaerodiene-type 35-40, three labdane type (41, 42, and 43), two clerodane type (45 and 46), and one ent-kaurane-type (44) diterpenoid will be discussed (Figure 3).Smyrniotopoulos et al. (2008) isolated six brominated diterpenes (35-40) from the red alga Sphaerococcus coronopifolius.While compounds 35-37 and 39 were only low to moderately active, compounds 38 (MIC: 0.25 to 1 μg/mL) and 40 (MIC: 1 to 2 μg/mL) showed pronounced effects.The authors suggest that the presence of an α, β-unsaturated ketone at position C-12 may cause the activity, being able to act as a Michael-acceptor.Pfeifer Barbosa et al. ( 2019) isolated five diterpenoids of three different diterpenoid types (41-45) from the oleoresin of Copaifera reticulata and tested them for their anti-MRSA activity and their cytotoxicity.Kaurenoic acid (44, IC 50 of 3.4 μg/mL), kolavenic acid (45, clerodane-type) (IC 50 of 3.0 μg/mL), and 43 (labdane-type) (IC 50 of 2.5 μg/mL) showed the highest activity.Interestingly, the activity was not dependent on the diterpenoid type but increased with the compounds' lipophilicity.In a follow-up study by Çiçek et al. (2020) compounds 41 and 44 were subjected to semisynthetic derivatization targeting the exocyclic methylene group as well as the carboxylic acid functionality.Both features were found to be essential for the activity against MRSA.The second clerodane diterpene (46) was isolated by Dettweiler et al. (2020) from Callicarpa americana.It showed moderate activity (MIC: 16 μg/mL) against a β-lactam resistant strain.Zhao et al. (2021) isolated dihydrotanshinone I (47) from Salvia miltiorrhiza, which exhibited moderate effects that were proposed to result from disturbance of the cell wall and membrane.Starks et al. (2014) isolated one new and three known abietane-type diterpenoids (48-51) with pronounced activities from Taxodium ascendens, a species that was reclassified as Taxodium distichum.Out of the four compounds, 51 showed lower but broader activity with a MIC value of 4 μg/mL against five different strains.Desaturation and oxygenation in position 5 and 6, respectively, led to comparable activities (50, MIC: 1-4 μg/mL) while the exchange of a phenolic hydroxy group in position 11 toward an aliphatic alcohol (in position 3 or 29) led to a significant increase in activity against (at least) two strains (with MIC values of 1-2 μg/mL for compounds 48 and 49).Oluwatuyi et al. (2004) isolated the three abietane diterpenes, 12-methoxy-trans-carnosic acid (52), carnosic Chemical structures of diterpenoids with reported anti-MRSA activity.
Moreno Cardenas and Çiçek 10.3389/fmicb.2023.1234115Frontiers in Microbiology 06 frontiersin.orgacid (53), and carnosol (54), from Rosmarinus officinalis.All compounds showed low to moderate activity, with 54 being the most potent compound with a MIC value of 16 μg/mL.Thereby, the lactone bridge seems to be beneficial for the activity compared to a free carboxylic acid.Combinations of 53 and 54 with tetracycline revealed synergistic effects against the TetK-possessing strain XU212.
Compound 53, furthermore, exhibited potent synergistic effects in combination with erythromycin.Teasdale et al. (2012) isolated six brominated diterpenoids of the sphaerodiene type (55-60) from a member of the genus Callophycus.Thereby, compounds 59 and 57 were only moderately active, whereas compounds 58 and 55 showed a MIC value of 6.3 μg/mL and compounds 60 and 56 were active with a MIC value of 1.6 μg/mL.The genus Callophycus was also the source for eight halogenated meroditerpenoids (61-68).All compounds showed moderate activity, except compounds 63 (MIC: 1.4 μg/mL), 62 (MIC: 8 μg/mL), and 68 (MIC: 1.8 μg/mL), which exhibited pronounced effects (Lavoie et al., 2017).The structural complexity of this group of diterpenoids derives from the addition of a para-hydroxybenzoic acid to the diterpene scaffold either at the methyl group at position 8 (56) or at position 9 (55).Further condensation of 56 leads to the benzofuran moiety of compounds 57-60, and 61-62, respectively.Interestingly, the formation of a dihydropyran ring between the diterpene scaffold and the para-hydroxybenzoic acid moiety (as for compound 55) led to a four-fold lower activity, while the formation of a benzofuran-connected system (60) did not alter the antibacterial effect (compared to 56).Moreover, the formation of a methyl ester (61) as well as the oxygenation of the prenyl moiety (as for compounds 57-59) decreased the activity.This effect was less pronounced for the peroxide (58), though.

Triterpenoids
In this section six cycloartane-type triterpenoids, two dammaranes, as well as one lupane, one ursane, one oleane, and one taraxastane will be discussed (Figure 4).

Phenolics
The group of phenolics is divided into seven sections, of which some summarize several smaller compound classes (in terms of the number of compounds with anti-MRSA activity).
Section 3.3.1,i.e., discusses 16 caffeic acid derivatives, of which six are chalcones, three are benzylchromanes, and two are lignans.

Caffeic acid derivatives
The root barks of Cordia gilletii yielded ferulaldehyd (81), which exhibited moderate effects in a study by Okusa et al. (2014).Rosmarinic acid (82) and its methyl ester (83) were isolated by Sabry et al. (2022) from the stem bark of Cordia africana.While 82 was only moderately active (with a MIC value of 31.25 μg/mL), the methylated form (83) showed pronounced effects (MIC: 7.81 μg/mL).Zuo et al. (2015) isolated two biphenyl compounds, 84 and 85, from Biancaea sappan, which was previously ascribed to the genus Caesalpinia, in a bioactivity-guided isolation procedure.Both compounds showed moderate to low activity but demonstrated synergistic effects in combination with amikacin and gentamycin (Figure 5).
Chan et al. ( 2012) isolated kuraridin (86), a prenylated chalcone, from the traditional Chinese medicial plant Sophora flavescens using high-speed counter-current chromatography.The compound showed high activity (MIC: 8 μg/mL) against the efflux pump expressing strains RN4220 (macrolides) and SA-1199B (fluoroquinolones) and against a representative healthcare-associated strain (SA-ST239).Additional checkboard studies found plenty of additive effects in combination with antibiotics, suggesting that the antibacterial action is not directly related to the efflux pump inhibition.Kim et al. (2017) FIGURE 5 Chemical structures of caffeic acid derivatives with reported anti-MRSA activity.
Moreno Cardenas and Çiçek 10.3389/fmicb.2023.1234115Frontiers in Microbiology 08 frontiersin.orgisolated licochalcone A ( 87) and E (88) from Glycyrrhiza inflata, also known as Chinese licorice.Both compounds showed moderate to high activity (MIC: 10 to 20 μg/mL) against six different strains, even partly surpassing the effect of the positive controls.Lee et al. (2009) tested the dihydrochalcone phloretin (89), which was obtained from a compound library.With low toxicity and a MIC value of 16 μg/mL, the authors suggest that the activity against MRSA correlates with the high affinity to the β-ketoacyl-acyl carrier potein synthase III (KASIII), a functional enzyme in the bacterial fatty acid biosynthesis.Bocquet et al. (2019) extracted eight different prenylated phenolic compounds from the female inflorescences of Humulus lupulus, which are used for beer brewing.Xanthohumol (90) was significantly active with a MIC value of 9.8 μg/mL, while desmethylxanthohumol (91) showed only moderate activity against the tested clinical isolates.Moreover, xanthohumol (90) displayed synergistic effects in combination with gentamycin, ciprofloxacin, oxacillin, and rifampicin.Honokiol (92) and magnolol ( 93), two lignans from the stem bark of a non-specified member of the genus Magnolia, which is used in traditional Chinese and Japanese medicine, were studied by Chiu et al. (2021).Both compounds demonstrated high activity against MRSA (MIC: 10 μg/mL).Further investigations showed that both compounds repressed the expression of mecA, a gene that is important for the β-lactam resistance of S. aureus.Additionally, both compounds decreased the biofilm formation in a sub-MIC concentration, which may result from repressing biofilm formation related genes.Zuo et al. (2014) isolated three 3-benzylchroman derivatives (94-96) from the heartwood of the Chinese drug Caesalpinia sappan (now Biancaea sappan).The compounds showed low to moderate activity with MIC values ranging from 16 to 64 μg/ mL.Further studies revealed high synergistic effects of brazilin (94) with gentamycin, etimicin, and streptomycin and of brazilein (95) with azithromycin, gentamycin, and ceftazidime, repectively.

Flavonoids
The present section discusses eight flavonols, four flavones, three flavanones, two flavans, and one flavanone (Figure 6).Prenylated flavonoids will be dealt with in the subsequent section (3.3.3).Koagne et al. (2018) isolated 97 and 98 together with two alkaloids from Crinum distichum, which was reclassified as Crinum ornatum.104) as a potent compound against MRSA (with a MIC value of 16 μg/mL).Commiphora pedunculata, a traditionally used medicinal plant species common in Africa, Arabia, and the Indian subcontinent, is the source of dihydrokaempferol ( 105), which displayed moderate activity against a clinical isolate (Tajuddeen et al., 2014).Oluwatuyi et al. (2004) investigated 106 from Rosmarinus officinalis against three different drug-resistant strains.Thereby, the highest activity (16 μg/ mL) was found against the tetracycline efflux pump possessing strain.
Nzogong et al. ( 2018) isolated 107 and 110, two low-to moderately active glycosylated flavonoids from Dissotis senegambiensis, which was regrouped into the genus Antherotoma, along with four triterpenoids and five tannins.Two flavanonol rhamnosides (108 and 109) were isolated by An et al. (2011) from Hypericum japonicum and tested against 10 methicillin-resistant and one methicillin-sensitive strain.While 108 was affecting all tested strains, 109 was only inhibiting three strains.However, MIC values of both compounds were only at a moderate to low level (ranging from 32 to 64 μg/mL).The glycosylated flavonoids with the highest activity were isolated from the leaves of Platanus occidentalis, the American sycamore (111-114) (Ibrahim et al., 2009).All four compounds display kaempferol 3-O-dicoumaroylrhamnosides, with different geometric isomerism for the coumaroyl moieties.Thereby, the derivative bearing two trans-coumaroyl moieties (111) exhibited the lowest activity (MIC value of 10 μg/mL), while two cis-coumaroyl moieties (114) led to a MIC value of 0.6 μg/mL and thus to a very  Chemical structures of isoflavonoids with reported anti-MRSA activity.
pronounced effect.Also, compounds bearing one cis-and one transmoiety each were still inhibiting at a very high level, with MIC values of 1.7 μg/mL (112) and 1.3 mg/mL (113), respectively.The isolated compounds were additionally tested against different microorganisms, however, with no observed effects.The position and number of the coumaroyl moiety also seem to have an influence on the activity, considering the low activity of compound 110.The authors, moreover, suggest that the hydroxy groups at positions 5, 7, and 4′ were important for the anti-MRSA activity (Ibrahim et al., 2009).
The class of (non-prenylated) flavonoids displays mostly low to moderately active constituents.
However, for three dicoumaroylrhamnosides pronounced effects have been demonstrated.

Prenylated flavonoids
The 14 compounds discussed in this section comprise seven flavones, six flavanones, and one flavanonol-type flavonoid (Figure 7).In addition, there is a broad variety of prenylation pattern, such as mono-, or di-prenylated flavonoids as well as ring or chain prenylated compounds.
Pang et al. ( 2022) isolated five prenylated flavonoids (115-119), with activities ranging from low MIC values of 100 μg/mL (115) and 74.95 μg/mL (117) over 25.02 μg/mL (119) to pronounced effects of 12.53 μg/mL (116) and 3.08 μg/mL (118), respectively.Compound 116 was also tested by Aelenei et al. (2020), who determined a MIC value of 6.25 μg/mL and noteworthy synergistic effects (FICI values of 0.13-0.16)with ciprofloxacin, gentamicin, oxacillin, and tetracycline, respectively.The number of similar structures allow several conclusions.First, the hydration of the C-3 prenyl chain almost completely abolishes the antimicrobial effect and second, ring closure between the B-and C-ring also leads to a decrease in activity.Furthermore, chain prenylation in the α-position (C-8) results in a significantly higher activity than ring prenylation. of 2 to 4 μg/mL, but was also found cytotoxic at the same concentration.Kuwanon G (127) was isolated from Morus sp. by Aelenei et al. (2020) and showed a MIC value of 12.5 μg/mL as well as synergistic effects with four different antibiotics.Even more active was cudraflavone C (128), which was isolated from Artocarpus hirsutus and tested against 12 different MRSA strains (Meenu et al., 2021).The compound affected all strains with a MIC value of 4 μg/mL, induced a reduction of biofilm formation by 30.4% and showed synergistic effects in combination with gentamycin.

Isoflavonoids
In addition to the prenylated flavonoids there is a total of 13 isoflavonoids discussed in this review, with seven of them being isoflavanones and six being isoflavones (Figure 8).
Gallic acid (142) was isolated by Lakornwong et al. ( 2018) from the stems of Rhodamnia dumetorum, which is traditionally used for its astringent effect.The compound only showed moderate activity, Chemical structures of gallic acids, stilbenoids, phenanthrenes, biphenyls, and diarylheptanoids with reported anti-MRSA activity.Chemical structures of xanthones and anthranoids with reported anti-MRSA activity.
Four phenanthrene derivatives (153-156) were isolated by Tóth et al. ( 2016) from Juncus inflexus.Antibacterial screenings resulted in juncuenin D (155) as the most potent anti-MRSA agent (MIC value of 12.5 μg/mL).Additionally, juncusol (154) and dihydrojuncuenin B (156) showed a MIC value of 25 μg/mL, whereas jinflexin B (153) was almost inactive.Chen et al. (2018) isolated blestriacin (157), a dimeric phenanthrene derivative, from the fibrous roots of Bletilla striata.The compound showed high activity against the standard strain and two clinical isolates (MIC: 2 μg/mL).To further investigate the mechanism of action of 157, a growth kinetic assay was performed comparing the compound to antibiotics with known mechanisms.Interestingly, blestriacin (157) exhibited a growth kinetic similar to rapidly lytic membrane-active agents like polymyxin and nisin.Therefore, disrupting membrane potential and integrity could be a possible mechanism of action.In addition, two moderately active prenylated biphenyls (158 and 159) were isolated from Garcinia esculenta by Zheng et al. (2021).
Lavoie et al. ( 2019) isolated seven oligomeric biphenyls (160-166) from the green alga Cladophora socialis.The highest activity was determined for cladophorol C (160) with a MIC value of 1.4 μg/ mL.Thereby, those oligomers which were not condensed with protocatechuic acid exhibited higher effects.Moreover, the number of monomers seem to have an impact on the activity, with the most potent compounds containing nine to ten monomers.
Interestingly, the common feature of the highly active compounds (173, 174, 179, 180, and 181) is the missing ring prenlyation attached to the B-ring of the xanthone scaffold.Additionally, multiple chain prenylation and/or methylation of one of the hydroxy groups contribute to the activity, indicating the lipophilicity of the compounds as an important feature for the anti-MRSA effect.
Phytochemical investigations of Stereospermum fimbriatum, a plant that is traditionally used for itchy skin, earache, stomachache, and postpartum treatment yielded the angucycline derivative 183, which displayed a MIC value of 6.25 μg/mL (Awang et al., 2020).Five anthraquinones (184-189) were studied in a in silico-based screening for new anti-microbial agents by Alhadrami et al. (2022).Quinizarin (184) and alizarin (185) showed weak activity, while rhein (186) was  Chemical structures of additional phenolic and non-phenolic compounds with reported anti-MRSA activity.

Phloroglucinol derivatives
In this section, a total of 15 phloroglucinol derivatives will be discussed, some of which belong to the most active compounds in this review (Figure 11).
Bocquet et al. ( 2019) isolated three acyl phloroglucinols (192-194) from hops (Humulus lupulus).While humulone (192) and colupulone (193) showed only weak to moderate activity, lupulone (194) exhibited remarkable effects with a MIC value of 0.6-1.2μg/ mL.Wisetsai et al. (2022) extracted two prenylated phloroglucinol derivatives (195 and 196) from the fruit of Acronychia pedunculata.Also here, one of the two isolates (195) was only moderately active, while the dimeric compound 196 showed pronounced effects (MIC value of 8 μg/mL).Another highly active dimer, namely rottlerin (197) was isolated from Mallotus philippensis (Oyedemi et al., 2016).While the monomer 198 showed only moderate activity, the dimer rottlerin (197) demonstrated high activity (MIC: 2-8 μg/mL) against six out of nine different strains.Wang et al. (2018) isolated garcinol (199) from Garcinia mangostana and determined a MIC value of 15.6 μg/mL against two different strains.With a MIC value of 16 μg/mL, the acyl phloroglucinol 200 isolated from Hypericum foliosum, showed comparable activity (Gibbons et al., 2005).Ahmed et al. (2014) studied the antibacterial effect of two monomeric (201 and 202) and one dimeric (203) phloroglucinol derivative from Ivesia gordonii.Thereby, 201 was found moderately active, while 2,4-diacetylphloroglucinol (202) exhibited pronounced effects.Even more interesting was the activity of ivesinol ( 203), which with a MIC value of 0.31 μg/mL is one the most promising plant natural products with anti-MRSA activity.Rhodomyrtone (204) was purified by Sianglum and Sirmanote (2012) from Rhodomyrtus tomentosa and likewise showed promising activity with a MIC value of 0.5 μg/mL.Aspindol (205), which was isolated from the fern Dryopteris fragrans, was studied for its anti-MRSA and antibiofilm activity and for its mechanism of action (Hua et al., 2018).The compound demonstrated high activity against 19 clinical isolates with MIC values ranging from 0.5 to 2 μg/mL.Additional RNA-sequencing of the MRSA strain revealed the downregulation of genes involved in the ribosome and amino acid synthesis, iron transport, and β-lactam resistance.The last phloroglucinol derivative in this review, hyperforin (206), was isolated from Hypericum perforatum and was as well tested for its anti-MRSA and anti-biofilm activity (Schiavone et al., 2013).Also, for hyperforin (206) significant antibacterial effects were determined with MIC values of 0.5-1 μg/mL, thus confirming the potential of phloroglucinol derivatives as anti-MRSA lead compounds.

Additional phenolic and non-phenolic compounds
The last section of this review summarizes natural products, which could not be attributed to the before discussed compound classes and were not enough in number to create respective sections.These compounds comprise one acylresorcinol, one mono-and one polyacetylene, one benzofuran, one prenylated benzopyran, one pyran derivative, one sulfur containing cyclopentapyran, two macrolides, and seven chrysophaentins (Figure 12).Luís et al. (2013) isolated an acylresorcinol derivative (207) from the fruits of Hakea sericea, which only showed moderate effects (MIC value of 20 μg/mL) against two strains.
A natural polyacetylene, namely falcarindiol (208), was tested by Lechner et al. (2004) after extraction from Angelica dahurica.The compound showed high to moderate activity, with MIC values ranging from 8 to 32 μg/mL.The monoacetylene 209 was isolated from the essential oil of Carlina acaulis by Herrmann et al. (2011).The compound showed moderate activity, with a MIC value of 15 μg/mL.Tanaka et al. (2015) isolated a benzofuran derivative (210) from Erythrina variegata, which, however, only exhibited moderate effects.
Ruiz Mostacero et al. (2021) extracted the prenylated benzopyran 211 from Peperomia obtusifolia in a bioassay-guided isolation approach.The compound showed high activity (MIC: 4 μg/mL) against the community-associated strains.Moreover, fluorescence microscopy revealed the ability of 211 to disturb and interact with the bacterial cell wall, which could be due to the amphipathic characteristics of the compound.Also, the pyran derivative 212, isolated from Rhodamnia dumetorum, exhibited pronounced effects with a MIC value of 8 μg/mL (Lakornwong et al., 2018).

Current status and future perspectives
In the last 40 years, more than 160 antibacterial drugs were approved by the U.S. Food and Drug Administration, half of them being of natural origin or derived from natural leads (Newman and Cragg, 2020).Still, none of these compounds were isolated from plants though several plant natural products exhibited noteworthy effects at a preclinical stage.In the case of MRSA, the many kinds of infections, e.g., skin and soft tissue infections, infective endocarditis, or pleuropulmonary infections, are certainly hindering when it comes to clinical trials (Tong et al., 2015).Especially, with regard to topical applications the amount of drug material that is needed stands against the limited amounts obtained by isolation procedures.There have been significant technical approaches toward the discovery of potential drug candidates, e.g., molecular networking and machine learning approaches (Fox Ramos et al., 2019;Vamathevan et al., 2019), their fast and efficient isolation, e.g., counter-current chromatography or centrifugal partition chromatography (Ito and Bowman, 1970;Murayama et al., 1982), and their final structure elucidation at even sub-milligram amounts, e.g., anisotropic NMR or microcrystal electron diffraction (Kunde and Schmidt, 2019;Liu et al., 2019), which facilitate the gathering of (new) natural products including eventual first bioactivity assays.However, the obtained quantities resulting from bioassay-guided isolation studies are, in general, far below the amounts necessary for clinical applications.Therefore, it is not surprising that the few reported trials applied essential oil (Caelli et al., 2000;Dryden et al., 2004;Edmondson et al., 2011;Blackwood et al., 2013) or extracts (Umashankar et al., 2018) rather than pure compounds (Rahim et al., 2021).
Though the so far conducted clinical trials were not convincing due to small sample sizes, missing randomization, or statistically insignificant results, they show a possible solution to overcome the problem of insufficient amounts of substance, namely the application of enriched (and standardized) extracts.As observed in this review, not seldom several constituents of a compound class and plant species exhibit positive effects, such as acetophenone sesquiterpenes, sphaerodiene diterpenes, and meroterpenes, which were isolated together with additional (less active) components of the same compound class.This complexity and the close structural similarity of the constituents hamper the isolation of the most active compounds and usually result in loss of substance.With the use of enriched extracts, preferably standardized to a certain (or minimum) concentration of active compound(s) or compound class, sufficient amounts could be provided for, e.g., topical formulations.Of course, when it comes to eventual further semi-synthetic drug optimization, pure substances will be needed.These optimizations not solely aim at    potentiating the effect of a compound but are also needed to increase the compounds' bioavailability and physical properties.For example, phenolic compounds, such as flavonoids, are rapidly eliminated, whereas many terpenoids show low solubility in aqueous solutions.
Here, biotechnological approaches could guarantee provision of highpurity compounds that are produced in bioreactors.However, this requires understanding of the biosynthetic pathways leading to the natural product of interest, which currently is not the case for many plant species (Porras et al., 2021).Here, computational techniques such as genome mining and the use of genomic and transcriptomic databases show possible approaches to enable the synthesis of natural products in microorganisms.Alternatively, plant species such as Nicotiana benthamiana could serve as producing organisms, being more closely related and thus more suitable to create, e.g., glycosylated natural products (Molina-Hidalgo et al., 2021).

Summary and conclusion
In this review a total of 223 plant natural products from more than 20 different compound classes are presented and discussed.Though most of the reported compounds display moderate effects, with activities ranging from 10 to 50 μg/mL, a few exceptions act at low or even submicromolar concentrations.Two such compounds are the sesquiterpene lactones parthenolide (24) and lactupicrin (25), both showing a MIC value of 0.16 μg/mL.In addition, the subclass of acetophenone sesquiterpenes present a set of interesting candidates, with four compounds displaying pronounced effects (26-28, 31).Similarly, the subclass of meroditerpenes, which also originate from two different biosynthetic pathways, yield several highly active constituents (56,60,63,68), with MIC values between 1.4 and 1.8 μg/ mL.Further potent diterpenes were found in the class of abietanes (48, 49) and sphaerodienes (38, 40), respectively.Of note, for compound 38 MIC values of 0.25-1 μg/mL against five different strains have been determined.The class of triterpenoids only revealed one interesting compound (80), as did the class of alkaloids (6).However, many of the reported alkaloids displayed synergistic effects with existing antibiotics, which result from the inhibition of the bacterial efflux pump.(Table 1) Strong synergistic effects (with ciprofloxacin, gentamicin, oxacillin, and tetracycline) were also reported for the prenylated flavones morusin (116) and kuwanon G (127).Likewise, taxifolin 7-O-α-L-rhamnopyranoside (108), displayed synergism with ceftazidime and levofloxacin, but was only slightly active alone.Three flavonol dicoumaroylrhamnosides (112-114), in contrast, exhibited pronounced effects with MIC values of 1.7-0.6 μg/mL.Other interesting phenolic compounds were found in the classes of isoflavonoids (132), xanthones (170, 171), anthranoids (187, 191), and above all, in the class of phloroglucinols.Here, several compounds exhibited remarkable effects, such as lupulone (194, MIC values of 0.6-1.2μg/mL), ivesinol (203, MIC: 0.31 μg/mL), rhodomyrtone (204, MIC: 0.5 μg/mL), aspidinol (205, MIC: 0.5-2 μg/mL), and hyperforin (206, MIC: 0.5-2 μg/mL), thus rendering phloroglucinol derivatives the most promising compound class for further drug development.Finally, two rather unusual plant natural products have been reported, which were the cyclopentapyran 213 (with a MIC value of 1 μg/mL) and the macrolide neurymenolide A (214, with an IC 50 value of 0.77 μg/mL).
In summary, several compounds with pronounced and/or synergistic effects against Methicillin-resistant Staphylococcus aureus have been isolated from plant species.Some of them even showed MIC values comparable to the established antibiotics vancomycin, daptomycin, tigecycline, and linezolid, respectively (Niveditha and Sujatha, 2015).Still, none of the mentioned compounds managed to proceed toward a commercial antibiotic drug.Whether the restricted activity to only a few out of several strains or their limited availability were the reason to not further develop the respective compounds could not be determined.In the latter case, the future will show if emerging biotechnological methods will help to provide higher amounts of natural products for subsequent clinical studies or semi-synthetic and technological modifications.In addition, more comprehensive approaches, e.g., studies with enriched extracts or defined fractions, may present an alternative for some stages in drug development.And last but not least, ethnopharmacological research will have to adapt to international conventions, e.g., the Nagoya protocol, taking them as what they are, namely measures to protect biodiversity and natural heritage and not obstacles in drug discovery.
bisbenzylisoquinoline alkaloids (7 and 8) from Stephania tetrandra, a traditional drug in Chinese medicine, with both compounds showing low activity.Yu et al. (2005) and Zuo et al. (2012) both studied compound 9, an isoquinoline alkaloid from Coptis chinensis against different clinical isolates resulting in low to moderate antibacterial effects.However, the compound was found to prevent the bacterium from adhesion and invasion into human gingival fibroblasts, thus also affecting the virulence of MRSA.Additionally,Zuo et al. (

FIGURE 6
FIGURE 6Chemical structures of flavonoids with reported anti-MRSA activity.

FIGURE 7
FIGURE 7Chemical structures of prenylated flavonoids with reported anti-MRSA activity.

FIGURE 8
FIGURE 8 Navrátilová et al. (2016) studied six C-6-geranylated flavonoids (120-125) isolated from Paulownia tomentosa against six MRSA strains with additional resistance against tetracycline.All compounds showed high to moderate effects, with mimulone (124, MIC values ranging from 2 to 16 μg/mL) and 3-O-methyldiplacol (122, MIC values of 4-8 μg/mL) exhibiting the most pronounced activity.The latter two compounds were further investigated for their synergistic effect resulting in additive effects with oxacillin against most MRSA strains.The authors suggest that the synergistic effect is caused by the inhibition of PBP2a and bacterial efflux pumps or by interactions with membrane peptidoglycans.Chan et al. (2012) isolated sophoraflavanone G (126) from Sophora flavescens.The compound showed high activity in the range

FIGURE 11
FIGURE 11Chemical structures of phloroglucinol derivatives with reported anti-MRSA activity.

FIGURE 12
FIGURE 12 Salam et al. (2021) both showing complete growth inhibition at 16 μg/mL.Unfortunately, not enough compound could be isolated for the determination of MIC values.Salam et al. (2021)isolated 75 from the leaves of Castanea sativa, showing only moderate anti-MRSA activity.However, further studies showed that the virulence was attenuated by 75.
from Oncoba manii, FIGURE 4Chemical structures of triterpenoids with reported anti-MRSA activity.

TABLE 1
Overview on additive and synergistic effects with existing antibiotics indicated by FICI values and combined MIC values, respectively.