The Thai Journal of Veterinary Medicine


Twenty-eight nalidixic acid-resistant Salmonella isolates originated from chicken meat, pork and humans were examined for mutation in the quinolone resistance determining region (QRDR) of gyrA, gyrB, parC and parE genes. Four single-point mutations in gyrA (i.e. C248A, C248T, G259T and G259A) leading to amino acid substitutions Ser83Tyr, Ser83Phe, Asp87Tyr and Asp87Asn in GyrA, respectively, were identified in 11 quinolone-resistant strains. Amino acid change at position Ser83 was most frequently identified. Resistance to nalidixic acid was not always associated with resistance to ciprofloxacin. The presence of mutations in GyrA did not well correlate with ciprofloxacin resistance phenotype. No mutations were observed in gyrB and parE. A serovar Typhimurium resistant to nalidixic acid did not carry any mutations in QRDR of all genes tested. The results highlight the high frequency of mutation in gyrA in the quinolone-resistant isolates and the existence of alternative-quinolone resistance mechanisms.

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