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we conducted a retrospective review of electronic records for all cultures positive for s. aureus that originated from hospital residents during 19972006 .
data collection was approved by the committee on human research , office of research administration , at the university of california , san francisco .
isolates were tested for oxacillin resistance by the salt agar method , and the presence of the meca gene was confirmed by pcr .
susceptibility to other antimicrobial drugs was determined by using microbroth dilution with the microscan walkaway 96 instrument ( dade behring , deerfield , il , usa ) .
inducible clindamycin - resistance testing ( d - zone test ) was performed by using the agar disk - diffusion method for isolates obtained during 20052006 .
results were interpreted in accordance with guidelines ( m7-a5 ) of the clinical and laboratory standards institute ( wayne , pa , usa ; www.clsi.org ) .
nonduplicated mrsa isolates were genotyped by pulsed - field gel electrophoresis ( pfge ) after digestion of chromosomal dna with smai ( 6 ) , spa typing ( 7 ) , and multilocus sequence typing ( mlst ) ( 8) .
usa300 was defined by the presence of panton - valentine leukocidin ( pvl ) genes ( lukf - pv and luks - pv ) and the arginine catabolic mobile element ( acme ) , detected by pcr .
staphylococcal cassette chromosome mec ( sccmec ) type was identified by using a pcr - based protocol ( 9 ) .
chi - square tests were used for bivariate analysis , and  tests for trend were used to evaluate secular trends .
all statistical analysis was conducted by using stata version 9.1 ( stata corp . , college station , tx , usa ) .
the proportion of mrsa among s. aureus isolates increased from 38.1% ( 56/147 ) in 1997 to 72.3% ( 99/137 ) in 2006 ( p<0.0001 ; figure 1 ) .
median age ( interquartile range [ iqr ] ) was 67 years ( 5478 years ) for patients with mrsa and 64 years ( 4978 years ) for those with methicillin - susceptible s. aureus ( mssa ) ( p = 0.22 ) .
we observed a decrease in the proportion of urinary or respiratory specimens from 69.7% ( 131/188 ) during 19972000 to 49.5% ( 236/477 ) during 20012006 , and a concomitant increase in the proportion of skin or skin structure specimens from 22.3% ( 42/188 ) to 42.8% ( 204/477 ) ( p<0.0001 ; figure 2 ) .
longitudinal dynamics of methicillin resistance and methicillin - resistant staphylococcus aureus ( mrsa ) clones at a long - term care facility , san francisco , california , usa , 19972006 . evolution of methicillin - resistant staphylococcus aureus isolate specimen sources at a long - term care facility , san francisco , california , usa , 19972006 . three pfge clonal groups ( usa100 [ st5 , sccmec type ii ] , usa300 [ st8 , sccmec type iv ] , and usa500 [ st8 , sccmec type iv ] ) accounted for 85.2% ( 563/661 ) of genotyped mrsa isolates .
usa300 was first isolated in 2001 and accounted for 11.3% of all mrsa ( 7/62 ) in 2002 , 30.1% ( 25/83 ) in 2003 , 47.9% ( 45/94 ) in 2004 , 49.5% ( 49/99 ) in 2005 , and 64.0% ( 55/86 ) in 2006 ( p<0.0001 ; figure 1 ) .
compared with non - usa300 mrsa , usa300 was most frequently isolated from skin or skin structures ( 64.3% vs. 27.0% , risk ratio 2.38 , 95% confidence interval 1.982.87 , p<0.0001 ) and was recovered from younger patients with a median age ( iqr ) of 62 years ( 5175 years ) vs. 68 years ( 5479 years ; p = 0.001 ) . among all mrsa
isolates , gentamicin resistance decreased from 46.9% in 1997 to 4.7% in 2006 , and clindamycin resistance decreased from 71.4% to 47.6% for the same years ( p<0.0001 ) .
overall , 285/744 ( 38.3% ) mrsa isolates were multidrug resistant ( resistant to > 3 non-lactam antimicrobial drugs ) , including 177/299 ( 59.2% ) usa100 , 39/182 ( 21.4% ) usa300 , and 32/82 ( 39.0% ) usa500 .
the rate of multidrug resistance for usa300 was lower than for non - usa300 ( p<0.0001 ) but increased from 0% in 2001 to 14.3% ( 1/7 ) in 2002 , 12.0% ( 3/25 ) in 2003 , 13.3% ( 6/45 ) in 2004 , 24.5% ( 12/49 ) in 2005 , and 30.9% ( 17/55 ) in 2006 ( p = 0.015 ) .
usa300 was less frequently resistant than non - usa300 to clindamycin ( p<0.0001 ) , gentamicin ( p = 0.0004 ) , and trimethoprim / sulfamethoxazole ( p = 0.0001 ) and more frequently resistant to tetracycline ( p = 0.0002 ) ( table ) .
* mrsa , methicillin - resistant staphylococcus aureus ; ery , erythromycin ; cli , clindamycin ; cip , ciprofloxacin ; sxt , trimethoprim - sulfamethozazole ; gen , gentamicin ; tet , tetracycline ; sst , skin and skin structure ; lft , low frequency types . 
increasing incidence of mrsa infections in this ltcf during 19972006 is attributable to 2 clonal groups ; usa100 predominated until 2003 , and usa300 predominated during 20042006 .
emergence of a new mrsa clone in healthcare facilities may be followed by a decrease in incidence of other mrsa clones ( 10 ) . unfortunately , this decrease was not observed in this ltcf , where the incidence of the previously predominant clone usa100 remained unabated . as a consequence ,
emergence of usa300 led to a 2-fold increase in mrsa incidence and an increase to 73% in the rate of methicillin resistance among s. aureus .
moreover , because usa300 has a tropism for skin and skin structure infections ( 3 ) , its emergence caused a shift in mrsa specimen sources .
although primarily encountered in urinary and respiratory specimens until 2001 in this study , as in previous studies performed in ltcfs , mrsa isolates have mostly originated from skin or skin structure since 2002 .
this finding has important implications for mrsa transmission and should be taken into account when designing infection control policies in ltcfs .
for example , nasal decolonization as a means to prevent mrsa infection implies that mrsa reservoirs reside in endogenous sources . however , skin  skin and skin 
in contrast to reports characterizing usa300 strains as typically not multidrug resistant ( 1 ) , we found that up to 30.9% of usa300 isolates were multidrug resistant .
this finding suggests that usa300 isolates were acquired under antimicrobial drug pressure in the ltcf or during a stay in another hospital rather than while in contact with the community .
results from this study and others illustrate the fitness trait of usa300 clonal lineage ( 12 ) . to prevent
further spread of multidrug - resistant usa300 in ltcfs would require enhanced infection control policies , which may include isolation or cohorting of infected patients , antimicrobial drug stewardship , and systematic use of alcohol - based handwashing products .
however , given that these policies have proven difficult to implement in tertiary care hospitals , it may be even more challenging in ltcfs , which have limited staff and infection control resources ( 5,1315 ) .
could be related to changes in sampling policies in this institution ( e.g. , more frequent sampling , surveillance cultures ) .
however , the increased incidence rate for mrsa is not likely related to these changes because the annual number of cultures positive for any pathogen gradually decreased over the study period , from 1,433 in 1997 to 862 in 2006 , as did the incidence of mssa - positive cultures , from 91 in 1997 to 38 in 2006 .
although our study was limited by its reliance on retrospective data collection , it illustrates the need for further investigations in ltcfs regarding risk factors and appropriate interventions to minimize further transmission of mrsa .
ltcfs , long thought to be reservoirs of nosocomial mrsa clones , now emerge as an important reservoir for usa300 and could play a role in the emergence of multidrug - resistant usa300 .
we performed a longitudinal analysis of 661 methicillin - resistant staphylococcus aureus ( mrsa ) isolates obtained from patients in a long - term care facility . 
 usa300 clone increased from 11.3% of all mrsa isolates in 2002 to 64.0% in 2006 ( p<0.0001 ) and was mostly recovered from skin or skin structures ( 64.3% vs. 27.0% for non - usa300 mrsa ; p<0.0001 ) .