| Summary: | 博士 === 國立臺灣大學 === 獸醫學研究所 === 92 === Antimicrobial usage in pig industry in Taiwan for prevention and treatment of bacterial diseases has been applied for decades. The pressure of antimicrobial selection contributes to the emergence of antimicrobial-resistant porcine bacterial pathogens. Three common bacterial pathogens, Actinobacillus pleuropneumoniae, Escherichia coli, and Salmonella enterica serovar Choleraesuis, were investigated for antimicrobial susceptibility and mechanisms of antimicrobial resistance in this study. The quinolone resistance-determining regions (QRDRs) of the gyrA gene of nalidixic acid-resistant S. enterica serovar Choleraesuis isolated from swine were sequenced. Four types of point mutation were found in this regions: Ser-83-to-Phe (TCC→TTC), Ser-83-to-Tyr (TCC→TAC), Asp-87-to-Gly (GAC→GGC), and Asp-87-to-Asn (GAC→AAC). PCR-RFLP and MAS-touch down PCR were developed for detected of point mutations in QRDRs of the gyrA gene. Fifty swine isolates of S. enterica serovar Choleraesuis collected from 1997 to 2002 were examined by PCR-RFLP and MAS-touch down PCR. The result indicated 7 isolates with point mutations in codon 83, 13 with point mutations in codon 87, and 30 with double mutations in both codons 83 and 87. The minimum inhibition concentrations (MICs) of enrofloxacin of the isolates with a single mutation in codon 83 or 87 were < 2 mg/ml, while the MICs of the isolates with double mutations in both codon 83 and 87 ranged from 2 to > 128 mg/ml. A class I integron comprised of dhfr, orfF and aad2 was also identified in both human and swine S. enterica serovar Choleraesuis isolates. In addition, the susceptibility of 16 common antimicrobials among 60 isolates of each of A. pleuropneumoniae, E. coli, and S. enterica serovar Choleraesuis isolated from diseased pigs from 1997 to 2001 were tested. The MIC values indicated that ceftiofur and amikacin were highly active against isolates of A. pleuropneumoniae, E. coli, and S. enterica serovar Choleraesuis; cephalothin, doxycycline, florfenicol, and gentamicin were highly to A. pleuropneumoniae but moderately active to E. coli, and S. enterica serovar Choleraesuis; trimethoprim was only highly active against A. pleuropneumoniae; flumequine was only moderately active against S. enterica serovar Choleraesuis; ampicillin, chloramphenicol, kanamycin, and tetracycline were moderately active against A. pleuropneumoniae only. The other antimicrobials tested were inactive. E. coli and S. enterica serovar Choleraesuis isolates were analyzed for antimicrobial resistance genes, integrase gene and virulence factor by multiplex PCR. The prevalence of integrase gene in E. coli and S. enterica serovar Choleraesuis were 97% and 95%, respectively. The prevalence of extended- spectrum β-lactamase (tem) and florfenicol resistance (floR) in E. coli were 38% and 55%, respectively. The prevalence of tem, floR and spvC (Salmonella plasmid virulence C gene) in S. enterica serovar Choleraesuis were 60%, 25%, and 98%, respectively. This study indicated the emergence of multidrug resistance in A. pleuropneumoniae, E. coli and S. enterica serovar Choleraesuis is a serious problem in swine industry. A surveillance system is necessary on the swine industry to monitor the emergence of fluoroquinolone and/or multidrug resistant bacterial pathogens in Taiwan.
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