Antimicrobial mechanism of semi-bionic extracts of three traditional medicinal plants—Rheum palmatum L., Scutellaria baicalensis Georgi, and Houttuynia cordata Thunb—That can be used as antibiotic alternatives

Abstract

The Chinese traditional medicinal plants <i>Rheum palmatum L., Scutellaria baicalensis Georgi</i>, and <i>Houttuynia cordata Thunb</i> in a ratio of 108:65:27 form a compound named Dahuang Qinyu San (DQS), which inhibits and kills <i>Escherichia coli</i> and <i>Salmonella</i> to a certain extent in fish and shrimp aquaculture environments. The active ingredients quercetin, emodin, baicalin, and aloe-emodin are obtained from the semi-biomimetic extract of DQS (SEDQS). However, the antibacterial mechanism of SEDQS against <i>Salmonella</i> is still unclear. This study used the microwell-plate method to determine the Minimum Inhibitory Concentration (MIC) of SEDQS against <i>Salmonella enteritidis</i> (<i>S. enteritidis</i>) isolated from geese. In addition, the effect of SEDQS on the growth curve, respiratory metabolic system, cell wall, soluble protein, and nucleic acid in bacterial liquid of <i>S. enteritidis</i> was detected by spectrophotometer and reagent kit. The effects of SEDQS on <i>S. enteritidis</i> DNA, binding gel blocking, virulence gene expression, and pathogenicity-related proteins were determined by gel electrophoresis, SDS-PAGE, and fluorescence quantitative PCR. The study found that a concentration of 1/4 MIC-2 MIC (2.27-18.2 mg/ml) SEDQS can significantly inhibit the normal growth of <i>S. enteritidis</i>, destroy the cell membrane structure of bacteria resulting in the leak of nucleic acid, protein, and other contents (<i>P</i> < 0.01). It also significantly inhibited the activities of succinate dehydrogenase (SDH) and malate dehydrogenase (MDH; <i>P</i> < 0.01) in a concentration-dependent manner. When the concentration of SEDQS was 1/2 MIC to 2 MIC (4.55-18.2 mg/ml), the expression levels of <i>gyrB, fimA, filC, spvR, Hcp</i>, and <i>vgrG</i> virulence genes (<i>P</i> < 0.01) all decreased by more than 31, 11, 18, 30, 34, and 21% respectively compared with the control group. SEDQS could significantly inhibit the expression of six virulence genes and play an important role in the pathogenicity of <i>the S. enteritidis</i> infected host. The SEDQS could exert antibacterial pharmacological effects by inhibiting the growth and metabolism of <i>S. enteritidis</i> and inhibiting the expression of major virulence factors. It has potential application value as an antibiotic alternative.