Abstract: 【Objective】 The purpose of the experiment was to screen plant extracts that had inhibitory effects on canine Escherichia coli (E.coli),provide a reference for the application of plant extracts in the prevention and control of bacterial diseases in dogs. 【Method】 Fifteen plant extracts including shikimic acid,rubusoside,chlorogenic acid,etc.,were selected.The antibacterial activity of these extracts against E.coli was determined using the hole-punching method to identify those with significant antibacterial effects.The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of plant extracts to canine E.coli were measured using the double dilution method.The MIC dose of each plant extract was co-cultured with E.coli in a constant temperature shaker.Samples were taken periodically,centrifuged,and supernatants were collected.The absorbance at D_{600 nm},D_{260 nm} and D_{280 nm} values of the supernatant was used to assess the impact of different plant extracts on the growth curve and extracellular nucleoprotein of E.coli.Meanwhile,the activity of alkaline phosphatase (AKP) in the supernatant and lactate dehydrogenase (LDH) in the bacteria was measured using kits. 【Result】 Shikimic acid,rubusoside,chlorogenic acid,phloretin and salidroside showed good antibacterial effects against E.coli.Based on the size of the inhibition zone diameter,their antibacterial capacities ranked as follows:Shikimic acid＞chlorogenic acid＞rubusoside＞phloretin＞salidroside.The MICs of phloretin,shikimic acid,rubusoside,chlorogenic acid and salidroside to canine E.coli were 0.48,15.625,62.5,31.25 and 125 mg/mL respectively,while their MBCs were 0.48,31.25,125,62.5 and 250 mg/mL,respectively.Growth curve measurements indicated that all plant extracts at MIC doses inhibited the growth of E.coli.Analysis of the absorbance at D_{260 nm} and D_{280 nm} values revealed that chlorogenic acid,salidroside,rubusoside and phloretin significantly damaged the cell membrane of E.coli.AKP activity in the supernatant under the influence of rubusoside and salidroside was significantly higher than that of control group (P<0.05),indicating that they could destroy the cell wall of E.coli and exert antibacterial effects.LDH activity in bacteria treated with shikimic acid,rubusoside,chlorogenic acid,phloretin and salidroside was significantly lower than that of control group (P<0.05),suggesting that all five plant extracts could inhibit intracellular LDH activity and affect bacterial metabolism. 【Conclusion】 Shikimic acid,rubusoside,chlorogenic acid,phloretin and salidroside had significant inhibitory effects on canine E.coli.

Key words: plant extract; canine Escherichia coli; in vitro antibacterial activity