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Infections of clinical methicillin-resistant Staphylococcus aureus (MRSA) is a very tough public health problem and a challenge of new drug development. Nearly 90 Diels-Alder adducts (DAAs) have so far been isolated from Morus plants, but only a few of them have been evaluated for their anti-MRSA activities. To study the antibacterial compounds of DAAs from the root barks’ section of Morus alba L. and their synergism with antibacterial agents against clinical MRSA strains, bioassay-guided phytochemical methods were used to screen the active components. Minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) were assayed through broth serial microdilution. The synergism were evaluated by checker board microdilution and dynamic time-kill experiments. Three DAAs (multicaulisin (1), sanggenon G (2) and albanin G (3)) were isolated and identified from M. alba root barks. They were determined with potent effect against MRSA isolates with MICs/MBCs at 2–8/16–128 mg/L. They also showed synergy with conventional antibacterial agents, especially the aminoglycosides, with fractional inhibitory concentration indices (FICIs) ranged from 0.19 to 0.50 and the dose reduction indices (DRIs) ranged from 16 to 2. The MRSA resistance to the antibiotics could be reversed by compounds 1–3. The dose-dependent bactericidal synergism against MRSA was observed as well. The study released for the first time the anti-MRSA synergism of DAAs from M. alba root barks with antibacterial agents and the reversal of MRSA resistance to aminoglycosides. The results may be valuable for further development of new antibacterial drugs and synergists against MRSA infections.

The in vitro antibacterial activities of usnic acid (UA) in combination with six currently available antibiotics were evaluated through checkerboard microdilution and dynamic time-killing assays against Staphylococcus aureus and 10 clinical isolates of methicillin-resistant S. aureus (MRSA). The six antibiotics include three aminoglycosides (i.e., amikacin (AK), etimicin (EM), streptomycin (SM)), two glycopeptides (i.e., teicoplanin (TP), vancomycin (VA)) and a tetracycline (i.e., minocycline (MC)). UA alone showed MIC of 16 μg/mL against both S. aureus and MRSA strains. The checkerboard assay showed the range of fractional inhibitory indices (FICIs) as 0.156–1.500 against all the pathogens when UA was used in combination with the antibiotics. Significant bacteriostatic interactions of UA with TP and MC were observed. The enhancement of antibacterial activities against the tested pathogens were revealed by the bacteriostatic dose reduction indices (DRIs) ranges at 1–64 of UA and 1–32 of the antibiotics, especially the synergy of UA with TP by 90% and additive effects with VA by 50% isolates of MRSA strains, respectively. MC also showed 60% strains of synergy with UA. The time-killing curves further confirmed the bactericidal synergy among the combinations of UA with TP, AK, EM, and SM (1 × MIC, △LC24 = 3.406–4.344 log10CFU/mL) against one of the 10 MRSA strains, respectively. Other combinations showed additive effects or indifferences, while no antagonism occurred in all the tested combinations. The anti-MRSA potentiation is promising for further investigations in order to form a possible scenario of UA/antibiotics combinatory chemotherapy which would reduce their dosages and toxicological responses.

Objectives This study aims to investigate antimicrobial ingredients from Sappan Lignum and to evaluate their synergy on methicillin-resistant Staphylococcus aureus strains with antibiotics. Methods Bioactivity-guided phytochemical procedures were used to screen the active compounds. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were assayed by broth microdilution. The synergy was evaluated through checkerboard microdilution and loss of viability assays. Key findings Protosappanins A (PsA) and B (PsB) were identified from Sappan Lignum extracts. They showed active against both S. aureus and MRSA with MIC or MIC50 at 64 (PsA) and 128 (PsB) mg/L alone. When they were used in combination with antibiotics, they showed best synergy with amikacin and gentamicin with MIC50 (mg/L) of amikacin reduced more significantly from 32 to four (with PsA) and eight (with PsB), and the fractional inhibitory concentration index (FICI) ranged between 0.078 and 0.500 (FICI50 = 0.375). Moreover, the resistance of MRSA towards amikacin and gentamicin could be reversed by the Clinical and Laboratory Standards Institute criteria. The combined bactericidal mode could as well be synergy. PsA and PsB showed very low cytotoxicity in comparison with their promising activity against MRSA. Conclusions Protosappanins A and B showed both alone activities and resistance reversal effects of amikacin and gentamicin against MRSA, which warrant further investigations for potential combinatory therapy of MRSA infection.

Background Clinical methicillin-resistant Staphylococcus aureus (MRSA) is a thorny problem in current anti-infective therapeutics and a challenge of new drug development. Plant prenylflavonoids possess anti-MRSA activity, but few of the prenylflavonoids have been reported the synergistic anti-MRSA effect when they are used in combination with conventional antibacterial agents. Purpose This study deals with anti-MRSA activity of four prenylflavonoids from the root bark of Morus alba and their synergism with 11 conventional antibacterial agents. Methods Chromatographic methods and spectral analysis were used to isolate and identify the prenylflavonoids. The antibacterial activity and synergism were assessed by the broth microdilution method, checkerboard dilution test, and time-kill curve assay, respectively. Results Four prenylflavonoids, i.e., cyclocommunol (Cy, 1), morusinol (Ml, 2), morusin (Mi, 3) and kuwanon E (Ku, 4), were isolated from Morus alba bark ethanol extract. Compounds 1, 3 and 4 showed high antimicrobial activity on both methicillin-susceptible S. aureus (MSSA) and MRSA strains with MICs/MBCs at 4–16/32–64 and 4–32/16–128 µg/ml, respectively. Ml (2) was not active. Compound 2 showed synergy with amikacin (AK) and streptomycin (SM) against all the ten MRSA isolates. Ml (2) and Ku (4) also showed synergy with ciprofloxacin (CI), etimicin (EM) and vancomycin (VA) against 7–9 isolates. The fractional inhibitory concentration indices (FICIs) ranged 0.09–1.00 and the dose reduction indices (DRIs) of these antibacterial agents ranged 2–128. Cy (1) and Mi (3) showed synergy with the tested antibacterial agents against only 1–3 MRSA isolates except VA. Furthermore, the MRSA resistance could be reversed in the combinations of AK with Cy, Ml, Mi and Ku; EM with Mi and Ku; and SM with Ml by the criteria of MIC interpretive standards for Staphylococcus spp. of CLSI. All the combinations showed only indifference in the 1 × MIC time-killing experiments. The prenylated substitutions play an important role in the activity of the compounds used alone and combined with the tested antibacterials. Conclusions The study revealed for the first time the anti-MRSA synergism of prenylflavonoids 1–4 with eleven antibacterial agents and the reversal of MRSA resistance to aminoglycosides, especially amikacin. The results might be valuable for the development of new antibacterial drugs and synergists against MRSA infection.

N-Methyl-dihydroberberine (M-Ber) was synthesized, and antibacterial activities of Berberine (Ber) and M-Ber alone and combined with antibiotics were studied against ten clinical MRSA isolates. MICs/MBCs (μg/ml, alone) ranges were 32–128/64–256 (Ber) and 64–128/256–1,024 (M-Ber) by a broth microdilution method. Significant synergies of Ber (M-Ber)/Azithromycin and Ber (M-Ber)/Levofloxacin combinations were observed by the chequerboard test. The Ber (M-Ber)/Ampicillin and Ber (M-Ber)/Cefazolin combinations showed indifference. These results demonstrated that Ber and M-Ber enhanced the in vitro inhibitory efficacy of Azithromycin and Levofloxacin, which had potential for combinatory therapy of patients infected with MRSA.

Two new compounds, khayseneganin I (1) and 2α,3α,16β-trihydroxy-20-acetoxy-20(R)-pregnane (2), along with six known compounds, 2α,3α,20-trihydroxy-16β-acetoxy-20(R)-pregnane (3), 2α,3β-dihydroxypregnan-16-one-2β,19-hemiketal (4), (+)-catechin (5), ivorenolide A (6), luteolin-7-O-α-l-rhamnoside (7), and ( - )-5'-methoxy-isolariciresinol-2a-O-β-d-xylopyranoside (8), were isolated from the leaves and twigs of Khaya senegalensis. The structures of new compounds were elucidated by 2D NMR spectroscopy and MS. Selected compounds (2-8) were evaluated for their antimicrobial activities and compounds 5 and 7 showed weak antimicrobial activities against MRSA 92(#) and MRSA 98(#).

Background Methicillin-resistant Staphylococcus aureus (MRSA) poses a serious therapeutic challenge in current clinic and new drug development. Natural coumarins have diverse bioactivities and the potential of resistance modifying effects. Purpose This study is to present in-depth evaluations of in vitro antimicrobial activities of four natural coumarins 5-geranyloxy-7-methoxycoumarin (Gm, 1 ), (5,7-dimethoxy-8-prenyloxycoumarin (artanin, Ar , 2 )), isopimpinellin (Is, 3 ) and phellopterin (Ph, 4 ) from Zanthoxylum nitidum (Roxb.) DC. (Rutaceae) extracts, focusing on their potential restoration the activity of conventional antibacterial agents against clinical MRSA strains. Methods Bioactivity-guided fractionation and spectral analyses were used to isolate the coumarins and identify the structures, respectively. The double broth microdilution method was used to assay the coumarins’ alone activity. The classic checkerboard microdilution and dynamic time-killing methods were used to evaluate combinatory effects. Results The four plant coumarins Gm ( 1 ), Ar ( 2 ), Is ( 3 ) and Ph ( 4 ) were isolated and identified from Z. nitidum extracts. Coumarins 1 – 4 displayed promising inhibition against both MSSA and MRSA with minimal inhibitory concentrations (MICs) of 8–64 µg/ml, but very weak against Gram-negative pathogen and yeast with MICs of 256 to ≥ 1024 µg/ml. The geranyloxy and prenyloxy substitutions showed to be more active than the methoxy substitution on the coumarin skeletons. 1 – 4 also showing different extent of synergism with a total of eight conventional antibacterial agents, i.e. chloramphenicol (CL), gentamicin (CN), fosfomycin (FF), levofloxacin (LE), minocycline (MI), piperacillin/tazobactam (P/T), teicoplanin (TE) and vancomycin (VA) against ten clinical MRSA strains. Four to ten of the tested MRSA strains showed bacteriostatic synergy in the eleven combinations. The anti-MRSA modifying effects were related to different arrangement in the combinations with fractional inhibitory concentration indices (FICIs) from 0.187 to 1.125 and the three combinations CN (Is), CL (Ph) and MI (Gm) were the best ones. The enhancement of activity was also shown by 2–64 of dose reduction indices (DRIs) of the combined MICs, with VA (Ph) combination resulted the biggest DRI. The resistance of MRSA to antibacterial agents could be reversed in the combinations of CL (Gm or Ph), LE (Ph) and MI (Is) following the Clinical and Laboratory Standards Institute (CLSI) criteria. Six combinations P/T (Gm), TE (Ar), CN (Is), VA (Ph) and CL (Gm or Ph) also showed bactericidal synergy with Δlog 10 CFU/ml >2 at 24 h incubation. Conclusions The coumarins showed high potentiating effects of the antibacterial agents against multi-drug resistant SA. The resistance reversal effect of CL, LE and MI warrants further pharmacological investigation on combinatory therapy for the sake of fighting against MRSA infections.

Antibacterial activity of berberine (Ber) and 8-acetonyl-dihydroberberine (A-Ber) alone and combined uses with antibacterial agents ampicillin (AMP), azithromycin (AZM), cefazolin (CFZ) and levofloxacin (LEV) was studied on 10 clinical isolates of SCCmec III type methicillin-resistant Staphylococcus aureus (MRSA). Susceptibility to each agent alone was tested using a broth microdilution method and the chequerboard and time-kill tests for the combined evaluations, respectively. The alone MICs/MBCs (mg/mL) ranges were 32–128/64–256 (Ber) and 32-128/128-512 (A-Ber). Significant synergies were observed for the Ber (A-Ber)/AZM and Ber (A-Ber)/LEV combinations against 90% of the tested MRSA strains, with fractional inhibitory concentration indices (FICIs) values ranged from 0.188 to 0.500. An additivity result was also observed for the Ber/AZM combination by time-kill curves. These results demonstrated for the first time that Ber and A-Ber enhanced the in vitro inhibitory efficacy of AZM and LEV to a same extent, which had potential for further investigation in combinatory therapeutic applications of patients infected with MRSA.

Trigoflavidols A (1) and B (2), tetranorditerpenoid dimers possessing a rearrangement skeleton with a spiroketal core moiety, and trigoflavidol C (3), a hexanorditerpenoid, have been isolated from Trigonostemon flavidus along with two known compounds. Compounds 1 and 2 showed moderate antimicrobial activities (MIC values: 3.12-6.25 μg/mL) against Staphylococcus aureus, 8(#)MRSA, and 82(#)MRSA, and 1, 2, and 5 showed weak activities (IC(50) values: 3.75-28.99 μM) against various human tumor cell lines.

The antibacterial activity of two bisbenzylisoquinoline alkaloids, tetrandrine (Tet) and demethyltetrandrine (d-Tet), alone and in combination with the antibiotics ampicillin (AMP), azithromycin (AZM), cefazolin (CFZ) and levofloxacin (LEV) against 10 clinical isolates of staphylococcal chromosomal cassette mec (SCCmec) III type methicillin-resistant Staphylococcus aureus (MRSA) was studied. Susceptibility to each agent alone was tested using a broth microdilution method. The chequerboard and time-kill tests were used for the combined evaluations. The minimal inhibitory concentrations/minimal bactericidal concentrations (MICs/MBCs, μg/mL) ranges alone were 64–128/256–1,024 for both Tet and d-Tet. Significant synergies against 90% of the isolates were observed for the Tet/CFZ combination, with their MICs being reduced by 75–94% [fractional inhibitory concentration indices (FICIs) ranged from 0.188 to 0.625], respectively. An additive bactericidal result was also observed for the Tet (d-Tet)/CFZ combination in the time-kill experiments. These results demonstrated that Tet and d-Tet enhanced the in vitro inhibitory efficacy of CFZ. Their potential for combinatory therapy of patients infected with MRSA warrants further pharmacological investigation.

One new flavonoid, 5,6,7-trimethoxyflavone-8-O-β-d-glucopyranoside (1), along with six known compounds 2–7, was isolated from Oroxylum indicum. Their structures were determined on the basis of spectral data. The antibacterial activities of compounds 1–4 were studied. Compounds 1 and 3 showed medium antibacterial activity against Staphylococcus aureus with MIC/MBC at 32–128 μg/ml.

A new limonoid, 3-de(2-methylbutanoyl)-3-propanoylcipadesin (1), along with 10 known limonoids and 1 known triterpenoid, was isolated from the fruits of Cipadessa cinerascens. Their structures were elucidated on the basis of spectroscopic analysis. All compounds were evaluated for their antimicrobial activities, and compounds 6 and 12 showed weak antimicrobial activities against MRSA 82(#) and MRSA 92(#).

Two new degraded diterpenoids, trigohowilols A (1) and B (2), four new heterodimers, trigohowilols C-F (3-6), one new homodimer, trigohowilol G (7), and three known degraded diterpenoids (8-10) were isolated from the methanol extract of the stems of Trigonostemon howii. Compounds 1-7 were evaluated for their cytotoxic activity against five human tumor cell lines by an MTT assay, and trigohowilols E (5) and F (6) exhibited inhibitory activity with IC50 values ranging from 2.33 to 12.57 mu M Moreover, compounds 1-6 showed weak antimicrobial activities (MIC values: 6.25-25 mu g/mL) against Staphylococcus aureus, Pseudomonas aeruginosa, MRSA 92(#), and MRSA 98(#) using a 2-fold dilution method.

The in vitro antimicrobial activities of 30 Chinese medicinal plants were evaluated with reference to the treatment record of infectious diseases in the Traditional Chinese Medicine (TCM) literature. The plant materials were extracted with 80% ethanol and the extracts were primarily screened against conventional clinical pathogens like Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Candida albicans by the agar diffusion method. Their inhibition zone diameters (IZDs, mm, 50 mg/mL) ranged from 2,048 by the standard broth microdilution method. The seven extracts from M. yunnanensis, S. sinensis, G. morella, E. daneillii, M. squamulata, S. arborescens and B. hancei were determined as the most active extracts, with MICs of 8–64 μg/mL. The results were in good agreement with their traditional applications in skin and other infections.

Purpose. This study describes the antibacterial effect of extracts and compounds isolated from the aerial part of Chelidonium majus Linn. (Papaveraceae) acting against clinical strains of methicillin-resistant Staphylococcus aureus (MRSA). Methods. The activities were evaluated by using the macrobroth dilution method and reported as the MICs/MBCs. Results. Bioassay-guided fractionation of the most active extract from the aerial parts (EtOAc) led to the isolation of benzo[c]phenanthridine-type alkaloids 8-hydroxydihydrosanguinarine (hhS), 8-hydroxydihydrochelerythrine (hhC), which were potently active against MRSA strains. Conclusions. The selective antibacterial activity reported in this paper for 8-hydroxylated benzo[c]phenanthridine-type alkaloids isolated from C.majus opens the possibility that they could be helpful for the developing of new antibacterial agents for treating the infection of MRSA which has created nosocomial problem worldwide.

Salvianolate (SAL) is a prescribed medicine from the Chinese herb Danshen (Salvia miltiorrhiza Bunge). It has been widely used in treatment of coronary and other diseases with significant effects. The in vitro antimicrobial activities of SAL against infectious pathogens were assayed and its combined effects on 10 clinical isolates of SCCmec III type methicillin-resistant Staphylococcus aureus (MRSA) with ten antibiotics were evaluated. Susceptibility to each agent alone was tested using a broth microdilution method, and the chequerboard and time-kill experiments were used for the combined activities. The results showed MIC was 128–256 mg/L for SAL used alone against MRSA. Significant synergies were observed for SAL/Ampicillin (Fosfomycin, Erythromycin, Piperacillin-tazobactam or Clindamycin) combination against over half of the isolates, with their MICs reduced by times of dilution (TOD) to 4–32 (FICIs 0.375–0.5), respectively. SAL/AMP combination showed the best combined effect of synergy on bacteriostatic and bactericidal activities, while SAL/AMK combination reversed the resistance of MRSA to AMK. The results demonstrated that SAL enhanced widely the in vitro anti-MRSA efficacy of the ten antibacterial agents, which had potential for combinatory therapy of patients infected with MRSA and warrants further investigations.

Background Methicillin-resistant Staphylococcus aureus (MRSA) is a problematic pathogen posing a serious therapeutic challenge in the clinic. It is often multidrug-resistant (MDR) to conventional classes of antibacterial agents and there is an urgent need to develop new agents or strategies for treatment. Magnolol (ML) and honokiol (HL) are two naturally occurring diallylbiphenols which have been reported to show inhibition of MRSA. In this study their synergistic effects with antibacterial agents were further evaluated via checkerboard and time-kill assays. Methods The susceptibility spectrum of clinical MRSA strains was tested by the disk diffusion method. The minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of ML and HL were assayed by broth microdilution. The synergy was evaluated through checkerboard microdilution and time-killing experiments. Results ML and HL showed similar activity against both MSSA and MRSA with MIC/MBC at 16 ~ 64 mg/L, with potency similar to amikacin (AMK) and gentamicin (GEN). When they were used in combination with conventional antibacterial agents, they showed bacteriostatic synergy with FICIs between 0.25 ~ 0.5, leading to the combined MICs decreasing to as low as 1 ~ 2 and 1 ~ 16 mg/L for ML (HL) and the agents, respectively. MIC50 of the combinations decreased from 16 mg/L to 1 ~ 4 mg/L for ML (HL) and 8 ~ 128 mg/L to 2 ~ 64 mg/L for the antibacterial agents, which exhibited a broad spectrum of synergistic action with aminoglycosides (AMK, etilmicin (ETM) and GEN), floroquinolones (levofloxacin (LEV), ciprofloxacin and norfloxacin), fosfomycin (FOS) and piperacillin. The times of dilution (TOD, the extent of decreasing in MIC value) were determined up to 16 for the combined MIC. A more significant synergy after combining was determined as ML (HL) with AMK, ETM, GEN and FOS. ML (HL) combined with antibacterial agents did not show antagonistic effects on any of the ten MRSA strains. Reversal effects of MRSA resistance to AMK and GEN by ML and HL were also observed, respectively. All the combinations also showed better dynamic bactericidal activity against MRSA than any of single ML (HL) or the agents at 24 h incubation. The more significant synergy of combinations were determined as HL (ML) + ETM, HL + LEV and HL + AMK (GEN or FOS), with △LC24 of 2.02 ~ 2.25. Conclusion ML and HL showed synergistic potentiation of antibacterial agents against clinical isolates of MRSA and warrant further pharmacological investigation.