Background：Increase in extended-spectrum β-lactamase(ESBL)-producing Escherichia coli and Klebsiella pneumoniae have been reported in Korea. The aim of this study was to determine the nationwide prevalence of ESBL-producing E. coli and K. pneumoniae, and to investigate the types of ESBLs.

Methods：A total of 2,221 E. coli and 1,128 K. pneumoniae consecutive isolates were yearly collected from 12 hospitals in 1999 and 2000. ESBL production was performed by National Committee for Clinical Laboratory Standards methods and double synergy tests. The type of ESBL was determined by polymerase chain reaction (PCR), isoelectric focusing, and nucleotide sequence analysis.

Results：ESBL-producing E. coli and K. pneumoniae isolates were detected from all 12 hospitals participated. The proportion of ESBL-producers was 9.1%(2.0-19.6%) of the E. coli and 29.2% (10.0-60.8%) of the K. pneumoniae isolates. Among the 22 isolates sequenced, SHV-12 was found in six isolates, SHV-2a in three isolates, TEM-52 in five isolates, TEM-106 in three isolates, and each of TEM-15, TEM-20, TEM-43, and TEM-107 in one isolate. CTX-M-14 was also found in one isolate.

Conclusion：ESBL-producing E. coli and K. pneumoniae are widespred to all levels of Korean hospitals. The most common types of ESBLs in Korea are SHV-12, SHV-2a, and TEM-52. In addition, we also identified new TEM-derived ESBLs. (Korean J Clin Microbiol 2003;6(2):149-155)