Abstract: This study was conducted to screen out Chinese herbal formula with antimicrobial activity against antibiotic-resistant E.coli, and provide basis for prevention and clinical cure of colibacillosis.According to the principles of compatibility,5 different kinds of Chinese herbal formulae were made by 12 kinds of Chinese herbal medicines with strong antimicrobial activity,screened out from our previous study,and then their in vitro antimicrobial activities against 10 isolated strains of antibiotic-resistant E.coli and 3 type strains of E.coli were evaluated by determining the diameter of inhibition zone (IZD) and minimal inhibitory concentration (MIC).Results showed that different formulae had different IZD on the same tested bacteria,in other words,the IZD were Formula 5 > Formula 4 > Formula 1 (except for SEC616,SEC911,SEC1284,and C197)> Formula 2 > Formula 3.The Formula 5 had the maximum IZD and minimum MIC against the 13 tested strains of E.coli, respectively,which suggested that this formula presented the strongest in vitro antibacterial activity;the Formula 3 had the minimum IZD and maximum MIC against 13 tested strains of E.coli, which suggested that Formula 3 presented the weakest in vitro antibacterial activity.In addition,the IZD and MIC values of Formula 1,Formula 2,and Formula 4 against different tested strains of E.coli were also different.These findings indicated that Formula 5 had the strongest in vitro antimicrobial activity against antibiotic-resistant E.coli, which could be an alternative prescription for prevention and cure of colibacillosis in swine production.

Key words: Chinese herbal formula, E.coli, antimicrobial activity, swine