JAC 49/2 pp. 261-267

Extended-spectrum -lactamases (ESBLs), such as the plasmid-mediated class A TEMand SHV-type enzymes, have developed by stepwise mutations in their structural genes, resulting in either single or multiple amino acid changes in the encoded enzymes, and these changes sufficiently remodel the active site to allow attack on aminothiazolyl compounds. Since they were first identified at the beginning of the 1980s, ESBL-producing microorganisms, belonging mostly to the Enterobacteriaceae, have spread by nosocomial routes throughout the world. The incidence of ESBL producers in Korean isolates of Escherichia coli and Klebsiella pneumoniae were in the range 4.8–22.5% and 13.2–22.4%, respectively. In contrast to the USA and Europe, where SHV-2, SHV-4 and SHV-5 are the most prevalent extendedspectrum SHV enzymes, SHV-12 and SHV-2a are the most frequently identified extended-spectrum SHV enzymes among K. pneumoniae strains in Korea. They are also found in E. coli and even in Enterobacter cloacae, in which chromosomal AmpC cephalosporinases predominate. SHV-2a first appeared in Germany in 1991. It has since been seen around the world and is particularly common in countries that border the Mediterranean, where it often occurs in Salmonella isolates. Recently, SHV-2a has been reported to be produced by Pseudomonas aeruginosa, indicating that the ESBL gene is no longer limited to Enterobacteriaceae. SHV-12 was first described in a survey of SHV -lactamases in Switzerland in 1997. Although the prevalence of SHV-12 in Enterobacteriaceae in Europe and the USA is not known, it is widespread among K. pneumoniae and E. coli strains in Korea. In addition, no other SHV-ESBLs besides SHV-2a and SHV-12 have been isolated in Korea. The reason for the prevalence of SHV-2a and SHV-12 in Korea is not clear, but our study provides some clues.