 The staffs in National Health Research Institute have established a long term surveillance of antimicrobial resistance from 22 to 44 hospitals distributed in all parts of Taiwan-Taiwan Surveillance of Antimicrobial Resistance TSAR since 1998. It has been conducted five times in year 1998, 2000, 2002, 2004, and 2006 and were named as TSAR-I, TSAR-II, TSAR-III, TSAR-IV and TSAR-V. In this study, isolates of Enterobacteriaceas from TSAR-IV will be recruited. Each 50 isolates of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter cloacae, Serratia marcescens, and Morganella morganii will be enrolled about 10 of the TSAR IV collected isolates for the following microbiological studies. 