Establishment Of Epidemiological Cut-off Values For Three Animal Specific Drugs Against Escherichia Coli From Chicken's Respiratory Tract

Avian colibacillosisi is an infectious disease caused by Escherichia coli.It is an important infectious disease that endangers animal husbandry and often causes huge economic losses.At present,the problem of resistance caused by irrational or improper use of antibiotics is becoming more and more serious in the aquaculture industry in China.Florfenicol,apramycin and danofloxacin,as animal-specific drugs,play an important role in the prevention and treatment of E.coli infection in animals.However,there is no breakpoint about florfenicol,apramycin and danofloxacin on CLSI standard.And the lack of interpretive criteria seriously affects the development of drug resistance monitoring for these three drugs.Therefore,in order to better monitor the emergence of drug resistance,analyze the current situation and development of drug resistance,and more scientifically guide clinical drug use and new drug development,in this study,we established epidemiological cut-off values of florfenicol,apramycin and danofloxacin according to CLSI procedure.A number of 363 E.coli isolates were isolated from chicken's tracheitis that were collected from Guangdong Province.The antimicrobial susceptibilities of 363 isolates were determined by a agar dilution method recommended by CLSI.By plotting the histogram of MIC distribution for florfenicol,apramycin and danofloxacin and using the statistical analysis and NRI analysis as well as the ECOFFinder.The epidemiological cut-off values for florfenicol,apramycin and danofloxacin was ?16 ?g/ml??16 ?g/ml and ?0.125 ?g/ml respectively.The result of antimicrobial sensitivity test showed that these isolated strains were resistant to florfenicol 66.7%(242/363),apramycin 22.3%(81/363)and danofloxacin92.3%(335/363).PCR was used to detect the related antimicrobial resistance genes(floR,aac(3)-IV,PMQR and QRDR genes)in these strains.For florfenicol,the strains within the epidemiological cut-off values(?16 ?g/ml)do not carry floR;for apramycin,the strains within the epidemiological cut-off values(?16 ?g/ml)do not carry aac(3)-IV;for dalfloxacin,the strains within the epidemiological cut-off values(?0.125 ?g/ml),there is no mutations of QRDR and PMQR(qnrA,qnrB,qnrC,qnrD aac-(6')-Ib-cr,qepA,and oqxB)are not detected.However,qnrS is still detected in a few of the strains at MIC=0.06 ?g/ml.In the situation of lack of susceptibility criteria,the establishment of epidemiological cut-off values can identify the presence of non-wild-type strains and is beneficial to the development of drug resistance surveillance.