Isolation and Identification of Pasteurella multocida From Bovine Respiratory Disease and the Effect of Baicalin on Its Drug Resistance.

This study aims to clarify the drug resistance status, carriage of drug-resistant genes, and the effect of baicalin on drug resistance in bovine respiratory Pasteurella multocida (Pm). Strains were isolated and cultured from bovine respiratory disease samples, and then PCR identification, drug sensitivity testing, and detection of drug-resistant genes were carried out. The minimum inhibitory concentration (MIC) of baicalin to Pm was determined by the microdilution method. The effects of baicalin at different concentrations on the drug-resistant phenotype of Pm were evaluated, and qRT-PCR was used to measure the impact of baicalin on the expression level of the aac(6')-Ib-cr gene. The fractional inhibitory concentration (FIC) of baicalin combined with enrofloxacin was determined by the microcheckerboard dilution method, and the quinolone resistance-determining region (QRDR) was compared. A total of 10 Pm strains were isolated, including 8 capsular Serotype A and 2 Serotype D; among them, 5 were multidrug-resistant. All 10 strains carried the strA, strB, and blaROB-1 drug-resistant genes. Baicalin at 1/2 MIC and 1/4 MIC concentrations reduced the drug resistance of Pm to amikacin, kanamycin, ciprofloxacin, and enrofloxacin with prolonged intervention time and significantly or extremely significantly decreased the expression level of the aac(6')-Ib-cr drug-resistant gene. The combination of baicalin and enrofloxacin showed a synergistic effect, and after combined culture, isoleucine (Ile) at Position 83 of the gyrA gene in Pm was replaced by arginine (Arg). The isolated Pm strains were predominantly Serotype A and mostly drug-resistant. Baicalin can effectively enhance the sensitivity of multidrug-resistant isolates to amikacin, kanamycin, ciprofloxacin, and enrofloxacin by inhibiting the expression of aac(6')-Ib-cr. The combination of baicalin and enrofloxacin exhibits a synergistic effect, mutually enhancing bacterial sensitivity to the drugs. Kaiwen Yin, Huiling Zhang, and Puguo Hao contributed equally to this work.