| Objective and methodThe three following experiments were conducted to investigate the effect of verapamil on enrofloxacin resistance in enrofloxacin-resistant E.coli.1 .The isolation of Enrofloxacin-resistant E.coli strains and sensitivity test: Serotype O of avian E.coli, isolated from broiler farms in HeBei province, were identified.The resistance of 20 strains taken randomly were tested by using Kirby-Bauer disk diffusion test. Eight strains of enrofloxacin-resistant E.coli were selected for the experiment in vitro.2. The experiment in vitro: The minimal inhibitory concentrations(MICs) of enrofloxacin and verapamil on 10 strains E.coli (including standard O2.O78) were tested with the method of broth two-fold dilution. Then the checkboard system was used to determine the fractional inhibitory concentration (FIC) index of enrofloxacin combined with verapamil. The one strain was selected for experiment in vivo.3. The experiment in vivo: 240 commercial broiler chickens were randomly divided into 4 groups on 30 days of age. The positive control and EN, EN+VP groups were inoculated with 1.5ml E.coli suspension by an intraperitoneal injection to induce Avian colibacillosis. The fouth group was negative control. When clinical signs emerged 4 hours postinoculation, the EN group was administrated with EN at a dose of 2.5 mg/kg.b.w twice each day , while the EN+VP group with EN (2.5 mg/kg.b w) and VP (6.0 mg/kg.b.w) twice each day. The mortality, curative ratio, and weight gain were observed every day. At the day 2,4,6,8,10 after inoculation, 6 birds were randomly taken from each group for detecting the mean femoral artery pressure (FAPm), macroscopic leisions and positive rate of E.coli.Result and conclusion1. 183 isolates of E.coli were got from 206 samples, with a mean isolation rate of 83.9%. 160 of 180 isolates were identified as 32 serotypes. The main serotypes were O78.O2.O1.O88.O76 in this district. The sensitivity test showed that 20 strains of E.coli taken randomly from the 160 were all resistant to two or more antibiotics. They could be distinguished to belong to 16 resistance spectrum . All of them showed multiple-resistant. The enrofloxacin resistance rate was 60%.2. The combined sensitivity test in vitro indicated that synergic effect were 87.5 % (7/8) of risistant-EN E.coli, indifferent effect was showed in two standard strains (O78. O2) and no antagonistic effect was found. That means VP can reduce the resistance of E.coli to EN in vitro. The MICs of verapamil against 10 strains revealed efflux pump inhibitor (verapamil ) has little effect itself on the E.coli.3. The experiments in vivo indicated that VP can promote the susceptibility of E.coli to enrofloxacin. The results were as follow: (1?The mortality of EN+VP group was lower than that of EN group, while the protective rate and curative ratio were a bit higher than that of EN group. (2) The weight gain of EN+VP group was significantly higher than that of EN group. (3) The rate of typical pathologic changes (pericarditis, perihepatitis) caused by colibacillosis of EN+VP group lower than that of EN at the same day, the positive rate of E.coli was the same. (4) At 2,4 days postinoculation, the FAPm of EN+VP group was higher than that of EN group. These conclution proved that enrofloxacin combined with verapamil by mascular injection no effect on cardiovascular system of chickens obviously.
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