Abstract
Objective: Our study evaluated the inhibitory effects of 13 plant materials on bacterial growth, so as to explain their traditional uses in bacterial infections. In addition, the correlation between antibacterial effects of plants and the polyphenol contents or antioxidant activities was also verified to investigate if these properties were responsible for the plant antibacterial effects.
 Methods: We tested extracts with 20 bacterial strains purchased from Japan Veterinary Products Association (JVPA, Tokyo, Japan) and also the Escherichia coli (E. coli) and Streptococcus suis (S. suis) strains that we had previously isolated from livestock. After polyphenol contents and antioxidant activities were measured, we examined their correlations with antibacterial effects.
 Results: The results showed that all extracts exerted inhibitory effects on tested bacteria and at different levels. Among all of samples, betel and mother rhizome of turmeric showed the strongest effects and also exerted inhibition on antibiotic-resistant E. coli and S. suis bacteria. In addition, direct and significant correlations between antibacterial effects’ and polyphenol content’s values were established and thus demonstrating that antibacterial effects of plants were attributable, at least in part, to the presence of polyphenol compounds.
 Conclusions: Our study demonstrated the antibacterial effects of 13 plant materials and, therefore, partly gives pharmacological basis to explain their traditional uses in diseases associated with bacterial infections. In addition, as betel and mother rhizome of turmeric showed the highest effects, we suggested that they might serve as the most promissory candidates for bacterial treatment. However, follow-up researches are still required to assess their potentials.
Highlights
The appearance of antibiotic resistance has become a global concern, and there is a continuous and urgent need to discover new antimicrobial compounds with diverse chemical structures and novel mechanisms of action to replace antibiotics in the treatment of bacterial infections [1]
When considering the effects with both Gramnegative and Gram-positive bacteria together, the strongest effects belonged to betel and turmeric mother rhizome, because regardless of bacterial grams, these two materials always exerted the lowest minimum inhibitory concentration (MIC) and demonstrating the highest inhibitory effects
The inhibitory effects of betel and turmeric mother rhizome have been demonstrated on E. coli and S. suis strains that had already been resistant to antibiotics and indicating their potentials to alternate antibiotics in bacterial infections, especially when resistance to antibiotics had occurred and reduced antibiotic effects
Summary
The appearance of antibiotic resistance has become a global concern, and there is a continuous and urgent need to discover new antimicrobial compounds with diverse chemical structures and novel mechanisms of action to replace antibiotics in the treatment of bacterial infections [1]. We selected to test these plant materials because traditional herbalism has suggested their potent antimicrobial property and/or they have been traditionally employed to treat diseases that associated with bacterial infections [4a-g, 5a-k, 6]. Plant extracts were first tested with 12 bacterial strains including six Gram-positive and six Gram-negative bacteria purchased from Japan Veterinary Products Association (JVPA, Tokyo, Japan), to evaluate their antibacterial effects through the determination of the minimum inhibitory concentration (MIC) values. Along with MIC determination, our study measured polyphenol contents and antioxidant activities of extracts and investigated their relationships with antibacterial effects because these values had been reported to associate with antimicrobial properties of plants [7,8]. Because antibacterial effects and many other pharmacological functions and health improvement of plants are attributed to polyphenol contents and/or antioxidant activities [9]; determination on these values is certainly useful to assess their biological properties
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