| Plants are the richest source of bioactive chemicals and many of them are extensively used as naturally occurring antimicrobial agents.In relation to green consumption and human health,much current concern has been focused on the exploitation and use of natural plants.Meanwhile,the misuse of antibiotics contributes to many problems such as the development of reservoir of resistant bacteria.There is therefore an urgent need to search for alternatives to antibiotics in industry.In this work we carried out some studies aimed at using of plant extracts to control bacterial contamination in ethanol fermentation so as to totally eliminate the use of antibiotics during ethanol production.In addition,a series of experiments were conducted to examine the effects of plant extracts on the growth of intestinal bacteria with the aim of discovering and developing potential prebiotics.The effects of aqueous extracts from 120 kinds of plants on the growth of Saccharomyces cerevisiae(S.cerevisiae) and Lactobacillus fermentum(L.fermentum), which is the major bacterial contaminants in ethanol fermentation,in pure culture were first examined by impregnated paper disk method.Results indicated that 13 kinds of plant extracts such as Prunus mume(P.mume) extract and Coptis extract inhibited the growth of yeast and lactobacillus to different extents,while 4 kinds of plant extracts such as Black cohosh extract revealed growth-inhibiting activity towards L.fermentum to a certain extent,with no adverse effects on the growth of yeast.Thereafter,Coptis extract and P.mume extract were chosen as the potential antibacterial agents.The effects of them on the growth of yeast and L.fermentum in pure culture were further examined by spectrophotometric method.The viabilities of L.fermentum were significantly decreased by Coptis extract and P.mume extract at concentrations of 0.01 g/L and 0.05 g/L,respectively.However,P.mume extract stimulated the growth of yeast to a certain extent at the same concentrations,while Coptis extract did not change the viability of S.cerevisiae.Furthermore,P.mume extract was used to control bacterial contamination in ethanol fermentation.The presence of P.mume extract at a concentration of 0.05 g/L resulted in a 1.45 mg/mL improvement in ethanol produced after 12 h fermentation.It also reduced the numbers of L.fermentum from 6×10~7 CFU/mL to 1.0×10~4 CFU/mL.In the same manner,the effects of aqueous extracts from 120 kinds of plants on the growth of Lactobacillus casei(L.casei),Lactobacillus acidophilus(L,acidophilus), Escherichia coli(E.coli) and Staphylococcus aureus(Staph.aureus) in pure culture were first examined by impregnated paper disk method.The astringent plant extracts which abound with phenolics,especially the plants belonging to Rosacea,own the best antibacterial activity,.Acrid plant(herb) extracts which are rich in alkaloids, especially the plants belonging to Rutaceae and Ranunculaceae,had the better antibacterial activity.The antibacterial activity of plant extracts which contain glycosides,especially the plants being sweet in taste,was the worst.Then,the effects of Coptis extract and P.mume extract on the growth of L.casei,L. acidophilus,E.coli,Staph.aureus and Clostridium perfringens(CI.perfringens) in pure culture were further examined by spectrophotometric method.Both Coptis extract and P.mume extract exhibited potent inhibition against CI.perfringens and Staph.aureus at concentrations of 0.01 g/L and 0.05 g/L,respectively.But no inhibition against E.coli was observed from treatment with Coptis extract even at 0.01 g/mL.On the contrary,P.mume extract showed slight promoting effects on the growth of E.coli at 0.001 g/mL,but significant growth inhibition at 0.005 g/mL.Moreover,P mume extract also exhibited selective growth-inhibiting activity towards E.coli in mixed culture of lactic aicd bacteria and E.coli.Therefore,P.mume may play an important role for preventing intestinal disturbance.
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