Abstract: The objective of this study was to determine the mutation frequencies of three Salmonella strains with different serotypes (Indiana,Enteritidis and Typhimurium) when selected under increasing ciprofloxacin pressures,as well the mutations in the quinolone resistance determining regions (QRDRs) in four target genes,mutations in ramA,ramR and ramA-ramR intergenic region,and the AcrAB-TolC efflux activity of the respective mutants of different serotypes during resistance development.PCR and DNA sequencing were applied to determine mutations within the above mentioned genes,and RT-PCR amplification of 16S rRNA,tolC,acrB,acrA and ramA were used to detect the activities of AcrAB-TolC of each mutants. Accumulation of ciprofloxacin in each Salmonella parent strains in the presence and absence of carbonyl cyanide 3-chlorophenylhydrazone (CCCP) was detected using HPLC to determine the presence of active efflux pump.The results showed that the Indiana serotype Salmonella strain presented a much higher mutation frequency and was easily ready to develop resistance to ciprofloxacin comparing with Enteritidis and Typhimurium serotype strains.The development of resistance in Indiana strains were mainly due to the sin-gle mutation in gyrA,and further high level resistance were obtained with additional activated AcrAB-TolC pump.While the resistance to ciprofloxacin in Enteritidis strains was mainly resulted from the double mutations in amino 83 and 87 in gyrA,and additional multiple mutations in gyrB and parC were contributed to the high level resistance in Enteritidis strains.However,Typhimurium strains were hard to development resistance under ciprofloxacin pressures.The development of resistance in Typhimurium strains in vitro was firstly derived from a single mutation in gyrB,and subsequently increasing resistance was obtained with additional mutation in parC and a weak AcrAB-TolC role.

Key words: serotype; Salmonella; drug accumulation; QRDRs; mutation frequency; fluorescence quantitative