Thesis (Ph.D.)--Chulalongkorn University, 2011

Three hundred and forty-four Escherichia coli isolates from clinically healthy fattening pigs were evaluated for susceptibilities to 8 antimicrobials, class1 integrons, antimicrobial resistance genes, mutation in Quinolone Resistance-Determining Regions (QRDRs), virulence genes and phylogenetic groups. E. coli isolates were resistant to tetracycline (96.2%) and followed by ampicillin (91.6%). Seventy-three percent contained intI1 gene, of which 22.3% carried inserted gene cassettes, i.e., incomple sat, aadA22, aadA1, dfrA12-aadA2, and sat-psp-aadA2. Two most commonly observed gene cassettes were aadA2 (42.9%) and aadA1 (26.8%). Horizontal transfer of class 1 integrons was detected in 8 E. coli isolates with class1 integrons carrying dfrA12-aadA2 gene cassettes. Sixteen resistance genes were detected in E. coli isolates and their presence was correlated to resistant phenotype. The amino acid substitutions Ser-83-Leu, Asp-87-Asn and Gln-94-Pro were observed in GyrA and Ser-58-Ile in ParC. Ten virulence genes including elt, estA, estB, astA, faeG, fasA, fedA, eseA, paa and sepA were detected, of which fasA (98.3%) was most commonly observed. Eighty-two percent of the E. coli isolates were assigned to group B1 followed by group A (8%), B2 (7%) and D (3%). Statistically-significant associations were found among the specific virulence genes, the specific resistance phenotypes and genotypes (P<0.05). The results supported the significant role of commensal E. coli as reservoirs for antimicrobial resistance determinants and virulence factors.