Molecular characterization of antimicrobial resistance in Shigella sonnei isolates in Korea Sung Yong Seol, Yong Tae Kim, Young Sook Jeong, Jae Young Oh, Hee Young Kang, Dong Chan Moon, Jungmin Kim, Yoo Chul Lee, Dong Taek Cho and Je Chul Lee Correspondence Je Chul Lee leejc@knu.ac.kr Department of Microbiology, Kyungpook National University School of Medicine, 101, Dongin-dong, Jung-gu, Daegu, 700-422, Korea Received 29 November 2005 Accepted 23 March 2006 The antimicrobial resistance of 122 Shigella sonnei isolates obtained in Korea during the period 1991–2000 was characterized. These isolates were highly resistant to traditional antibiotics such as trimethoprim (100 %), streptomycin (100 %), sulfamethoxazole (94 %), tetracycline (93 %) and nalidixic acid (90 %). All S. sonnei isolates carried Tn7 in their chromosomes. The 8?4 kb non-transferable resistance (R) plasmid carrying tetA, strA-strB and sul1 was found in 93 % of the S. sonnei isolates. Resistance to nalidixic acid first appeared in a S. sonnei isolate in 1997, and then in all S. sonnei isolates from 1998 and 1999. Resistance to commonly prescribed antibiotics such as ampicillin was increased in S. sonnei isolates during the outbreak period 1998–2000. Resistance to ampicillin was mediated by the conjugative R plasmids carrying bla TEM-1 . In conclusion, S. sonnei acquired antimicrobial resistance to commonly prescribed antibiotics through the horizontal transfer of conjugative R plasmids, while the genetic stability of transposon and non-transferable R plasmids was responsible for resistance to traditional antibiotics. INTRODUCTION Shigellosis remains an important cause of community- acquired gastrointestinal illness in both developing and industrialized countries. Since 1951, when Shigella sonnei was first reported to be sporadically isolated in Korea (Im & Choi, 1961), S. sonnei has become, in the 1990s and 2000s, the primary cause of shigellosis (Kim et al., 2002; Oh et al., 2003). Shigellosis caused by S. sonnei occurred sporadically in Korea until 1997, when the annual incidence was estimated to be 2–260 cases, but large outbreaks of shigellosis caused by S. sonnei have occurred since 1998, and the annual incidence of S. sonnei is now estimated to be 846–2400 cases (National Institute of Health in Korea, 2005). We previously reported that there are distinct phenotypic and genotypic differences in terms of biotypes, antimicrobial susceptibilities and PFGE profiles between the S. sonnei isolates from the 1980s and the 1990s (Jeong et al., 2003; Kim et al., 2002; Oh et al., 2003). S. sonnei isolates during the period 1977–1986 exhibited biotype a, while S. sonnei isolates during the period 1991–2000 exhib- ited biotype g. PFGE patterns of S. sonnei isolates during the period 1977–1986 were completely different from those of isolates during the period 1991–2000. This suggests that Korean endemic S. sonnei strains disappeared, and that new S. sonnei clones were introduced in the late 1980s or early 1990s. In Korea, S. sonnei isolated during the last decade were resistant to multiple antibiotics, and a high proportion of S. sonnei were commonly resistant to tetracycline, strepto- mycin, sulfonamide and trimethoprim (Jeong et al., 2003). Additionally, antimicrobial susceptibilities of S. sonnei isolates gradually changed over time. The most striking changes were an increase in resistance to ampicillin and the emergence of extended-spectrum b-lactamases (ESBLs) in S. sonnei isolates. ESBLs, including TEM-15, TEM-17, TEM-19, TEM-20, TEM-52 and CTX-M-14, have been detected among recently identified S. sonnei isolates (Kim et al., 2004; Pai et al., 2001). However, there have been no comprehensive studies involving the molecular character- ization of antimicrobial resistance in the S. sonnei isolates in Korea. The current study investigated the molecular mechanisms by which S. sonnei isolates during the period 1991–2000 were resistant to antimicrobial agents. METHODS Bacterial isolates. A total of 122 S. sonnei isolates were obtained from stool samples in different parts of Korea during the period 1991–2000 (Table 1). All S. sonnei isolates from sporadic shigellosis were collected. From the defined outbreaks, S. sonnei isolates that showed the same antibiogram and identical or similar PFGE pattern (similarity value of >95 %) were considered to be genetically Abbreviations: ESBL, extended-spectrum b-lactamase; R plasmid, resistance plasmid. 46441 G 2006 SGM Printed in Great Britain 871 Journal of Medical Microbiology (2006), 55, 871–877 DOI 10.1099/jmm.0.46441-0