title: In vitro synergism of magnolol and honokiol in combination with antibacterial agents against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA).
abstract: 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.
  
  
  CONCLUSIONS
  ML and HL showed synergistic potentiation of antibacterial agents against clinical isolates of MRSA and warrant further pharmacological investigation.
authors.affiliation: Research Center for Natural Medicines, Kunming General Hospital of Chengdu Military Command, Kunming, 650032, China. zuoguoying@263.net.
authors.forename: Guo-Ying
authors.initials: GY
authors.lastname: Zuo
authors.affiliation: Research Center for Natural Medicines, Kunming General Hospital of Chengdu Military Command, Kunming, 650032, China. 397148180@qq.com.|School of Pharmacy, Kunming Medical University, Kunming, 650032, China. 397148180@qq.com.
authors.forename: Xin-Juan
authors.initials: XJ
authors.lastname: Zhang
authors.affiliation: School of Basic Medical Sciences, Yunnan Traditional Chinese Medical College, Kunming, 650500, China. hanzjn@126.com.
authors.forename: Jun
authors.initials: J
authors.lastname: Han
authors.affiliation: School of Basic Medical Sciences, Yunnan Traditional Chinese Medical College, Kunming, 650500, China. zxylc2005@126.com.
authors.forename: Yu-Qing
authors.initials: YQ
authors.lastname: Li
authors.affiliation: Research Center for Natural Medicines, Kunming General Hospital of Chengdu Military Command, Kunming, 650032, China. kmwgc12@126.com.
authors.forename: Gen-Chun
authors.initials: GC
authors.lastname: Wang
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001713:Biphenyl Compounds / Q000494:pharmacology*; D004357:Drug Synergism; D017705:Lignans / Q000494:pharmacology*; D008712:Methicillin / Q000494:pharmacology; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D000498:Allyl Compounds; D010636:Phenols
chemical_list: D000900:Anti-Bacterial Agents; D001713:Biphenyl Compounds; D017705:Lignans; D008712:Methicillin; C005498:magnolol; C005499:honokiol; D000498:Allyl Compounds; D010636:Phenols