we analyzed all 140 mrsa isolates we obtained during a previous study ( 2 ) and 50 other isolates selected from our collection of nosocomial mrsa isolates obtained in 2003 .
characteristics of these 190 isolates are shown in table 1 . * ni , nosocomial infection acquired at national taiwan university hospital ( ntuh ) ; nc , nosocomial colonization at ntuh ; ieoh , mrsa infection detected at ntuh within 48 h after transfer from another hospital . 
bl , blood ; ur , urine ; sp , sputum ; pu , pus ; wo , wound ; ns , nostril ; ct , catheter tip ; st , stool . 
pulsed - field gel electrophoresis patterns were interpreted according to procedures previously reported ( 7,8 ) .
thirty - four isolates belonged to pulsotype a , 49 to pulsotype b , 69 to pulsotype c , 6 to pulsotype d , 2 to pulsotype e , 3 to pulsotype f , 11 to pulsotype k , 5 to pulsotype l , 8 to pulsotype m , and 3 ( all isolated in 2003 ) to 3 minor pulsotypes .
all isolates were tested by sccmec element typing and multilocus sequence typing ( mlst ) ( 9 ) and were analyzed for the panton - valentine leukocidin ( pvl ) gene ( 10 ) and drug susceptibility to erythromycin , clindamycin , gentamicin , amikacin , ciprofloxacin , levofloxacin , tetracycline , trimethoprim - sulfamethoxazole , rifampin , and vancomycin by using the disk diffusion method ( 11 ) .
isolates with pulsotype a are sequence type 254 ( st254 ) ; those with pulsotype b are st241 ; those with pulsotypes c , k , and l are st239 ; and those with pulsotypes d , e , f , and m are st59 , st 254 , st30 , and st5 , respectively .
all mrsa isolates with pulsotypes a , d , e , and f have the type iv sccmec element .
however , only isolates with pulsotypes d and f , as well as 2 isolates from 2003 with 2 minor pulsotypes , have the pvl gene .
isolates with pulsotypes b and c have the type iii sccmec element , and isolates with pulsotype m have the type ii sccmec element . *
p , pulsotype ; scc , staphylococcal cassette chromosome ; mlst , multilocus sequence typing ; pvl , panton - valentine leukocidin ; n , no ; y , yes .  ox , oxacillin ; em , erythromycin ; cl , clinidamycin ; gm , gentamicin ; am , amikacin ; cp , ciprofloxacin ; lv , levofloxacin ; tc , tetracycline ; ts , trimethoprim - sulfamethoxazole ; rf , rifampin ; va , vancomycin . 
results of mlst and typing of sccmec elements of the 3 isolates with 3 minor pulsotypes obtained in 2003 are shown in table 2 .
the correlation between sccmec element typing and mlst in this study corresponds to findings of previous reports ( 6,1214 ) .
enright et al . identified st254-iv mrsa isolates in germany and the united kingdom ( 12 ) , and chongtrakool et al . identified st239-iii and st241-iii mrsa isolates in several asian countries ( 14 ) .
we demonstrate that the predominant mrsa clone at ntuh in early 1990s had the type iv sccmec element . however , the predominant mrsa clones at ntuh from 1994 to 2003 had the type iii sccmec element .
these findings differ from those of wisplinghoff et al . , who reported that that the sccmec element in predominant mrsa clones at their institute changed from type iii in 1984 to 1988 to type i in 1989 to 1998 ( 6 ) .
mrsa isolates of pulsotypes b and c are more resistant than isolates of pulsotype a to certain antimicrobial drugs , especially fluoroquinolones and trimethoprim - sulfamethoxazole ; mrsa isolates with pulsotype c are more resistant to clindamycin but less resistant to rifampin than those with pulsotype b ( table 2 ) . from 1993 through 2000 , annual use of fluoroquinolones increased 3 at ntuh ; however , use of trimethoprim - sulfamethoxazole , clindamycin , and rifampin did not change ( 15 ) .
therefore , the shift of predominant mrsa clones , which also led to the shift in types of sccmec elements at ntuh , might be caused by selective pressure from antimicrobial drugs , especially fluoroquinolones .
the mrsa clone ( pulsotype a ) that predominated in 1992 and 1993 at ntuh has the type iv sccmec element .
although the first study of mrsa with the type iv sccmec element reported that this element was found in community - acquired mrsa ( ca - mrsa ) ( 5 ) , some studies have reported mrsa isolates with this element in a hospital environment ( 12 ) . however , to our knowledge , these reports did not demonstrate that mrsa isolates with the type iv sccmec element became predominant among all mrsa isolates in an institution , especially before the mid-1990s .
furthermore , 4 st59 mrsa isolates obtained in 1994 and 1996 and 3 st30 mrsa isolates obtained in 1992 and 1993 have the type iv sccmec element and pvl gene .
recently , st59 mrsa isolates were found to cause ca - mrsa infection in taiwan ( 13 ) . among these st59
ca - mrsa isolates , some had the type iv sccmec element , and others had the type v sccmec element ( 13 ) . although the type iv sccmec element could be transferred to ca - mrsa clones with other genetic backgrounds , our finding supports the possibility that st59 mrsa isolates with the sccmec element type iv in taiwan may originate from hospital strains but transfer into ca - mrsa strains .
recently reported the results of sccmec typing of 615 mrsa isolates obtained in 1998 and 1999 from 11 asian countries ( 14 ) .
the st239-iii , st241-iii , st254-ii , and st5-ii mrsa isolates were prevalent in many asian countries .
however , the st254-iv , st30-iv , and st59-iv mrsa isolates from our study were not found in other asian countries .
in addition , st254-iv mrsa isolates have been found in germany and the united kingdom ( 12 ) . whether st254-iv mrsa isolates in our study
were transmitted from those in germany or the united kingdom by international travel requires further study .
the first mrsa isolate with the type iv sccmec element in our hospital appeared as early as 1992 . the sccmec element carried by predominant mrsa clones changed from type iv to type iii sccmec element during the period 19922003 at ntuh .
because the major mrsa clones isolated in 19942003 are more resistant to antimicrobial drugs , especially fluoroquinolones and trimethoprim - sulfamethoxazole , than those obtained in 1992 and 1993 , this shift may be caused by selective pressure from indiscriminate use of antimicrobial drugs .
 to determine the predominant staphylococcal cassette chromosome ( scc ) mec element in methicillin - resistant staphylococcus aureus , we typed 190 isolates from a hospital in taiwan . 
 we found a shift from type iv to type iii sccmec element during 19922003 , perhaps caused by selective pressure from indiscriminate use of antimicrobial drugs . 
The Study
Conclusions