Molecular characterization of strains of fluoroquinolone-resistant Salmonella enterica serovar Schwarzengrund carrying multidrug resistance isolated from imported foods.
OBJECTIVE
To determine the fluoroquinolone resistance determinants in Salmonella enterica serovar Schwarzengrund from imported foods.


METHODS
Antibiotic susceptibility of Salmonella Schwarzengrund to 16 antibiotics was examined using disc agar diffusion and Etest. Quinolone resistance determinants were examined by sequence analysis of gyrA, gyrB, parC and parE, PCR amplification of qnrA, qnrB and qnrS, and expression of acrB, ramA, marA, soxS and rob using quantitative RT-PCR. The contribution of efflux pump activities to antibiotic resistance was determined by the addition of carbonyl cyanide m-chlorophenylhydrazone (CCCP). The effect of ramR deletion on ciprofloxacin resistance was determined by complementing with wild-type ramR.


RESULTS
Salmonella strains 30 and 487 were susceptible to ciprofloxacin and had a single mutation in gyrA as compared with strain 75, which was highly resistant to ciprofloxacin and had a double mutation in gyrA. Increased expression of ramA was associated with high resistance to ciprofloxacin. Strain 75 had a deletion of 315 bp in the ramR gene, which regulates ramA expression. Overexpression of ramA was possibly related to a loss of ramR. Introduction of ramR decreased the MIC of ciprofloxacin from 48 to 24 mg/L. The addition of CCCP did not reduce antibiotic resistance. To our knowledge, this study reports for the first time the natural deletion of ramR in Salmonella Schwarzengrund.


CONCLUSIONS
This study indicates that fluoroquinolone-resistant Salmonella are prevalent in imported food. Double mutations in gyrA and a loss of ramR were associated with high-level quinolone resistance in multidrug-resistant Salmonella Schwarzengrund strain 75.
Microbiology Division,  US Food and Drug Administration, National Center for Toxicological Research, Jefferson, AR 72079,  USA.
Tatsuya
T
Akiyama
Ashraf A
AA
Khan
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D024901:Drug Resistance, Multiple, Bacterial; D024841:Fluoroquinolones / Q000494:pharmacology*; D005516:Food Microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D019779:Salmonella enterica / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification; D017422:Sequence Analysis, DNA
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004269:DNA, Bacterial; D024841:Fluoroquinolones