Antibacterial activity and synergy of antibiotics with sanguisorbigenin isolated from Sanguisorba officinalis L. against methicillin-resistant Staphylococcus aureus

In Vitro Synergistic Inhibitory Effects of Plant Extract Combinations on Bacterial Growth of Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most common pathogens of healthcare-associated infections. Medicinal plants have long been used in the traditional treatment of diseases or syndromes worldwide. Combined use of plant extracts could improve the effectiveness of pharmacological action by obtaining synergism, acting on multiple targets simultaneously, reducing the doses of individual components, and minimizing side effects. We aimed to investigate the synergistic inhibitory effects of selected medicinal plants (Caesalpinia sappan L. (CS), Glycyrrhiza uralensis Fisch. (GU), Sanguisorba officinalis L. (SO), and Uncaria gambir Roxb. (UG)) on the bacterial growth of MRSA and its clinical isolates. SO and UG extracts generated the best synergistic interaction as adjudged by checkerboard synergy assays. MICs of the individual extracts decreased 4-fold from 250 to 62.5 μg/mL, respectively. The SO + UG combination was further evaluated for its effects on bacterial growth inhibition, minimum bactericidal/inhibitory concentration (MBC/MIC) ratio, and time-kill kinetics. The results indicate that the SO + UG combination synergistically inhibited the bacterial growth of MRSA strains with bactericidal effects. SO + UG combination also exhibited more potent effects against clinical isolates. In multistep resistance selection experiments, both standard and isolates of MRSA showed no resistance to the SO + UG combination even after repeated exposure over fourteen passages. Our data suggest that using plant extract combinations could be a potential strategy to treat MRSA infections.

Inhibitory Effect of Andrographis paniculata Lactone on Staphylococcus aureus α-Hemolysin

We investigated the effect of andrographolide (AP) on the hemolytic capacity of Staphylococcus aureus (S. aureus) isolated from our region. AP is a labdane diterpenoid isolated from the stem and leaves of Andrographis paniculata. The hla gene from 234 S. aureus strains and the quality control standard strain ATCC29213 in dairy cows in some areas of Ningxia was analyzed. Evolutionary analysis, homology modeling, and functional enrichment annotation of α-hemolysin Hla detected from our region were performed through bioinformatics. The hemolytic ability of S. aureus isolates from the region was examined using the hemolysis test, and the effect of AP on S. aureus was quantified. Moreover, the effect of AP on the transcript levels of hla and genes highly related to hla (i.e., clfA and fnbA) was examined through fluorescence quantitative PCR. The mode of action of AP on the detected Hla was analyzed through molecular docking and dynamic simulation. The results showed that S. aureus in our region has a high rate of hla carriage. The hemolytic activity of strains NM98 and XF10 was significant, and ATCC29213 also exhibited some hemolytic activity. AP could inhibit the expression of Hla and its related proteins by downregulating hla, clfA, and fnbA transcript levels, which in turn attenuated the S. aureus hemolytic activity. Meanwhile, the AP molecule can form three hydrogen bonds with residues ASN105, SER106, and THR155 of Hla protein; bind with PRO103 through alkyl intermolecular forces; and form carbon hydrogen bonds with LYS154, reflecting that the AP molecule has a comparatively ideal theoretical binding activity with Hla protein. Among them, PRO103 and LYS154 are highly conserved in Hla protein molecules and play pivotal roles in the biological functions of Hla, and their binding may affect these functions. Their binding may also prevent the conformational transition of Hla from a monomer to an oligomer, thus inhibiting Hla hemolytic activity. This study offers a molecular basis for use of AP as an antivirulence drug and new ideas for developing novel drugs against S. aureus infection.

Combination of Sanguisorbigenin and Conventional Antibiotic Therapy for Methicillin-Resistant Staphylococcus aureus: Inhibition of Biofilm Formation and Alteration of Cell Membrane Permeability

Methicillin-resistant Staphylococcus aureus (MRSA) infection is challenging to eradicate because of antibiotic resistance and biofilm formation. Novel antimicrobial agents and alternative therapies are urgently needed. This study aimed to evaluate the synergy of sanguisorbigenin (SGB) isolated from Sanguisorba officinalis L. with six conventional antibiotics to achieve broad-spectrum antibacterial action and prevent the development of resistance. A checkerboard dilution test and time-to-kill curve assay were used to determine the synergistic effect of SGB combined with antibiotics against MRSA. SGB showed significant synergy with antibiotics and reduced the minimum inhibitory concentration of antibiotics by 2-16-fold. Biofilm inhibition assay, quantitative RT-PCR, crystal violet absorption, and transmission electron microscopy were performed to evaluate the synergy mechanism. The results indicated that SGB could inhibit biofilm formation and alter cell membrane permeability in MRSA. In addition, SGB was found to exhibit quite low cytotoxicity and hemolysis. The discovery of the superiority of SGB suggests that SGB may be an antibiotic adjuvant for use in combination therapy and as a plant-derived antibacterial agent targeting biofilms.

Bisdemethoxycurcumin Reduces Methicillin-Resistant Staphylococcus aureus Expression of Virulence-Related Exoproteins and Inhibits the Biofilm Formation

Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen of nosocomial infection, which is resistant to most antibiotics. Presently, anti-virulence therapy and anti-biofilm therapy are considered to be promising alternatives. In the current work, we investigated the influence of bisdemethoxycurcumin (BDMC) on the virulence-related exoproteins and the biofilm formation using a reference strain and clinic isolated strains. Western blotting, quantitative RT-PCR, and tumor necrosis factor (TNF) release assay were performed to assess the efficacy of BDMC in reducing the expression of Staphylococcus enterotoxin-related exoproteins (enterotoxin A, enterotoxin B) and α-toxin in MRSA. The anti-biofilm activity of BDMC was evaluated through a biofilm inhibition assay. The study suggests that sub-inhibitory concentrations of BDMC significantly inhibited the expression of sea, seb, and hla at the mRNA level in MRSA. Moreover, the expression of virulence-related exoproteins was significantly decreased by down-regulating accessory gene regulator agr, and the inhibition of biofilms formation was demonstrated by BDMC at sub-inhibitory concentrations. Consequently, the study suggests that BDMC may be a potential natural antibacterial agent to release the pressure brought by antibiotic resistance.

Trans-Cinnamaldehyde Exhibits Synergy with Conventional Antibiotic against Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major nosocomial pathogen worldwide and has acquired multiple resistance to a wide range of antibiotics. Hence, there is a pressing need to explore novel strategies to overcome the increase in antimicrobial resistance. The present study aims to investigate the efficacy and mechanism of plant-derived antimicrobials, trans-cinnamaldehyde (TCA) in decreasing MRSA’s resistance to eight conventional antibiotics. A checkerboard dilution test and time–kill curve assay are used to determine the synergistic effects of TCA combined with the antibiotics. The results indicated that TCA increased the antibacterial activity of the antibiotics by 2-16-fold. To study the mechanism of the synergism, we analyzed the mecA transcription gene and the penicillin-binding protein 2a level of MRSA treated with TCA by quantitative RT-PCR or Western blot assay. The gene transcription and the protein level were significantly inhibited. Additionally, it was verified that TCA can significantly inhibit the biofilm, which is highly resistant to antibiotics. The expression of the biofilm regulatory gene hld of MRSA after TCA treatment was also significantly downregulated. These findings suggest that TCA maybe is an exceptionally potent modulator of antibiotics.