High antimicrobial effects
The antimicrobial effects of Tropaeolum majus L. (TR) and Armoracia rusticana P.Gaertn., B.Mey. & Scherb (AR) against oral pathogenic bacteria have not been studied sufficiently, so far. Hence, the aim of our study was to determine the susceptibilities of clinically important oral pathogens and to show that TR and AR are feasible for the usage in antimicrobial therapy in patients.
In addition to the frequently used standard MIC-Methods a modified gas test set up was used to evaluate the antimicrobial effects of the dried plant powder [19, 20].
Prior to testing the antimicrobial activity, the active substances of TR and AR were analyzed in detail using headspace gas chromatography mass spectrometry (GC-MS). We found chemically different ITCs in the two plants, which favors the use of a combination of ITC-containing plants. For our susceptibility tests, we therefore used a mixture of TR and AR at a proportion of 2.5:1 and a combination of synthetically produced ITCs with matches the proportions of ITCs in the plants.
With the exception of Veillonella parvula, all tested species were highly susceptible to herbal TR/AR in the gas test and synthetic ITCs in the agar dilution, with MICs ranging between 50/20 mg and 200/80 mg TR/AT, and 0.01 and 0.08 mg ITC/mL, respectively. Tannerella forsythia, Porphyromonas gingivalis, Fusobacterium nucleatum and Prevotella baroniae were found to be the most sensitive of the tested species, as 50/20 mg TR/AR, which equates to only 1/4 tablet of the commercially available drug (Angocin®), was enough to stop growth. MBC values were about 4 to 10 times higher than MIC values. Although the order of herbal MIC values does not exactly correlate with the sequence of synthetic ITC MIC and MBC values, all determined concentrations could easily be reached in the oral cavity by topical application as mouthwash, gel, chip, or, in combination with fluoride, as toothpaste.
Our GC-MS experiments demonstrated, that a mixture of herbal TR/AR powder can release a spectrum of ITCs in high amounts. Previous studies tested TR alone for its bactericidal effect on certain oral pathogens. However, the ITC solution was sucked in a paper disk and placed on the agar. The MBC values of the ITCs extracted from TR and the synthetic allyl- ITC were higher than in our experiments . This could mean that the combination of TR and AR is more effective than TR alone.
Furthermore, Salvadora persica, sticks of which have been used as natural toothbrushes for centuries, were found to contain high amounts of benzyl-ITC and show considerable antimicrobial effects on Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis . This is consistent with the results presented in this study for ITCs.
Why phytotherapeutic drugs?
Although TR and AR are cultivated and have been used for hundreds of years, relevant bacterial resistances to ITCs have not as yet been reported. The safety of systemic TR/AR administration up to 1200/480 mg daily was demonstrated in a clinical trial . Adverse side effects were significantly lower in a TR/AR group than an antibiotic group . Negative effects on the gut microbiota were not observed. Moreover, the treatment costs with TR/AR are substantially lower than with antibiotic prescriptions. Additionally, chlorhexidine, which has been considered the gold standard in dental plaque control , is also cytotoxic, as reported for human gingival fibroblasts, osteosarcoma cells and osteoblasts [24, 25]. Moreover, human saliva can to some extent inactivate the antibacterial effects of chlorhexidine against some oral bacteria, inducing selective processes in the bacterial populations of human saliva . Furthermore, a correlation of resistance towards chlorhexidine and different medically relevant antibiotics cannot be excluded due to the similar mechanisms of resistance which include multidrug efflux pumps and cell membrane changes as reported in an own review of the literature . Another frequently used oral health product is Listerine® . Although there is accumulating evidence that Listerine® is effective in improving oral health, the absence of systematic toxicological studies means that an accurate safety assessment cannot be made . Hence, new natural antibacterial compounds such as ITCs from plants could be promising components for dental oral care. However, the direct comparison of ITCs effects on oral pathogens with standard antibiotics or chlorhexidine is still pending, which must be acknowledged as a limitation of our study approach.
Out of the tested species, Aggregatibacter actinomycetemcomitans, Campylobacter rectus, Eikenella corrodens, Fusobacterium nucleatum, Porphyromonas gingivalis, Prevotella intermedia, Tanerella forsythia, and Veillonella parvula are highly associated with periodontitis [30, 31]. With the exception of Veillonella parvula, all these pathogens were found to be highly susceptible to ITCs. The topical use of herbal TR/AR, e.g. as antiseptic mouthwash, gel or chip, should be considered, but also systemic administration, since the compliance to phytotherapy is usually good, and spread of antibiotic resistance could be avoided. Activity exhibited by ITCs against biofilms was demonstrated by the example of Pseudomonas aeruginosa . The effects against the diverse array of oral bacteria tested in the present study suggest an anti-biofilm effect of ITCs. Such potential should be examined in future studies to clarify inhibition of formation or degradation of already formed oral biofilm.
Endocarditis prophylaxis for dental procedures should predominantly cover Staphylococci, Streptococci, Enterococci, and Candida spp., but also incidental pathogens such as HACEK organisms . Our in vitro-study demonstrated that HACEK organisms are highly susceptible to TR/AR. These results support and expand our previous findings of the antibacterial effect of mustard oil-containing plants against the predominant endocarditis relevant oral bacteria .