CTX-M-55-Type Extended-Spectrum β-lactamase-Producing Shigella sonnei Isolated from a Korean Patient Who Had Travelled to China
A 27-yr-old woman was admitted to the hospital with watery diarrhea, abdominal pain, and fever (39.3℃) on the day of her return from a 4-day trip to China. Laboratory investigation showed a peripheral blood leukocyte count of 8.2×109/L with 80% polymorphonuclear cells and a C-reactive protein level of 107 mg/L. Stool samples were inoculated on MacConkey and SS agar (BD Diagnostics Systems, Sparks, MD, USA) and incubated at 37℃ for 20 hr, and colorless medium-sized colonies developed. The bacterium was identified as S. sonnei using the Vitek2 GN ID card (bioMérieux, Marcy l'Etoile, France); a positive slide agglutination test with a Shigella serogroup D antiserum (Joong Kyeom Co., Anshan, Korea) confirmed the identification.\nThe pathogen's antimicrobial susceptibility was initially determined by the VITEK 2 AST N131 card (bioMérieux, Marcy l'Etoile, France). ESBL production was confirmed by the double disk synergy test using cefotaxime (30 µg), ceftazidime (30 µg) and amoxicillin-clavulanic acid (20/10 µg) disks (BD Diagnostics Systems) []. Minimum inhibitory concentrations (MICs) were determined by the CLSI agar dilution method or by the Etest (AB bioMérieux, Solna, Sweden) (). The isolate was resistant to cephalothin, aztreonam, and cefotaxime, but susceptible to cefoxitin and intermediate to ceftazidime. The patient was treated with oral ciprofloxacin, after which her symptoms began to improve. She was discharged 4 days after admission.\nTo determine the genetic characteristics of the ESBL, PCR and sequencing were carried out as described previously []. These investigations identified the ESBL as being encoded by blaCTX-M-55, which is identical to blaCTX-M-57 []. The blaCTX-M-55 gene and surrounding regions were investigated by PCR and sequencing [], and insertion sequence (IS)Ecp1 was found to be upstream of blaCTX-M-55. The blaCTX-M-55 gene was successfully transferred from the S. sonnei isolate to an Eecherichia coli J53 (azideR) recipient by plate mating. MacConkey agar containing cefotaxime (2 µg/mL) and sodium azide (100 µg/mL) was used as the selective media. The β-lactam resistance patterns of the S. sonnei isolate and the transconjugant were similar, but the MICs of ceftazidime and aztreonam for the transconjugant were increased 2-fold, from 8 µg/mL and 24 µg/mL to 16 µg/mL and 48 µg/mL, respectively (). To estimate the size of the plasmid harboring blaCTX-M-55, genomic DNA from S. sonnei isolate and its transconjugant were digested with S1 nuclease and separated by pulsed-field gel electrophoresis []. Southern blotting was performed on the agarose gels by in-gel hybridization with a blaCTX-M-55 probe labeled with digoxigenin (Roche Diagnostics, Indianapolis, IN, USA) []. This experiment showed that blaCTX-M-55 was located on a plasmid of approximately 130 kb in each of the strains (). Replicon typing of the plasmid by PCR was unsuccessful [].