the strain collection included 5 categories of vre and vse : 1 ) 96 animal surveillance ( bison , calves , cats , dogs , ostriches , poultry , pigs , rodents ) isolates ( 43 vre , 53 vse ) from 7 countries in africa and europe ; 2 ) 57 epidemiologically unrelated community surveillance isolates ( 20 vre , 37 vse ) from nonhospitalized persons from 7 countries in australia and europe ; 3 ) 64 epidemiologically unrelated surveillance ( fecal ) isolates ( 45 vre , 19 vse ) from hospitalized patients not linked to hospital outbreaks from 9 countries in australia , europe , and north and south america ; 4 ) 162 epidemiologically unrelated hospital isolates ( 43 vre , 118 vse , 1 not determined ) from clinical specimens ( blood , pus , and urine ) from 17 countries in africa , australia , europe , and north and south america ; and 5 ) 1 strain from each of 32 different documented hospital outbreaks ( 28 vre , 4 vse ) in 10 countries in australia , europe , and north and south america ( w. grubb and d. jonas , pers . we determined vancomycin susceptibilities for 410 isolates and ampicillin susceptibilities for 381 isolates by using standard agar dilution methods according to nccls guidelines . isolates with mic 16 g / ml for ampicillin and 8 g / ml for vancomycin were considered to be resistant . in total , 394 strains were screened for the esp gene with primer sets and amplification conditions described previously ( 24 ) . independent and combined effects of virulence and resistance markers on the abundance of complex-17 were estimated by using multiple logistic regression analysis ( stata 7.0 , statacorp lp , college station , tx , usa ) . mlst was carried out with a standard set of primers that amplify the 7 genes included in the e. faecium mlst scheme ( 14 ) . information on these loci , the latest set of primers , amplification conditions , and details of all isolates are available on the mlst web site ( http://efaecium.mlst.net ) . the eburst program was used to assess the genetic relationships of genotypes , to assign isolates to genetic complexes , and to study patterns of evolutionary descent of isolates within a complex ( 25 ) . complexes were identified by using the stringent ( 6/7 shared alleles ) group definition with 1,000 bootstrap replicates . the bland program was used to examine the relationship between pairwise allelic differences and nucleotide sequence differences ( 26 ) . if genetic diversity in e. faecium is mainly the result of accumulated point mutations , then recently diverged strains will have a high level of similarity in both their allelic profiles as well as in the nucleotide sequence of the nonidentical alleles , which results in a positive correlation between the number of nucleotide differences in nonidentical alleles and the number of allelic differences . however , such a trend will be absent when recombination plays an important role in generating the genetic diversity , since nonidentical alleles of closely related isolates can differ at multiple nucleotide sites . to assess the effect of recombination on the population structure of e. faecium in more detail , the topologies of the 7 mlst gene trees were compared by using the shimodaira - hasegawa test ( 27 ) . briefly , maximum likelihood trees for each mlst gene were obtained under a general time - reversible model , with a proportion of invariant sites and rate heterogeneity among sites assuming a discrete gamma distribution with 8 categories ( gtr+i+ model ) . paup * 4.0b10 was used to obtain the maximum likelihood trees by using a neighbor - joining starting tree followed by tree - bisection reconnection branch swapping ( 28 ) . for a given gene , the shimodaira - hasegawa test compares the difference in log likelihoods of competing tree topologies . a null distribution of differences in log likelihoods was obtained by 1,000 replicates of nonparametric bootstrapping of reestimated log likelihoods . we conducted 107 shimodaira - hasegawa tests for each mlst gene by comparing the 7 mlst gene trees and 100 random trees separately generated for each of the mlst genes . in a clonal population , the different mlst housekeeping genes have similar tree topologies , but with recombination , the different genes may have different tree topologies that may fit random trees better . associations between ampicillin resistance , presence of a novel putative e. faecium pai , and genetic clustering in complex-17 were described by linear logistic regression models : log odds = b0 + b1x1 + b2x2 + b3x3 +  + bixi . log odds denotes the natural logarithm of the proportion of samples from an epidemiologic group belonging to complex-17 , bi denotes the parameter estimated by maximum likelihood methods and xi the level of exposure , e.g. , 0 and 1 for ampicillin resistance , vancomycin resistance , and the presence of pai and 04 for the epidemiologic source of isolates : animal surveillance , community surveillance , hospital surveillance , clinical sample , and hospital outbreak , respectively . mlst of 411 e. faecium isolates resulted in 175 different sequence types ( st ) . clustering these types with the eburst algorithm ( 25 ) showed 1 large complex of genetically related types . st-22 was the primary founder ; 3 minor complexes had st-1 , -69 , and -94 as primary founders ; 6 complexes had only 2 or 3 sts ; and 57 singletons were not linked to the aforementioned complexes ( figure 1 ) . within complex-22 , st-17 represents an important secondary founder of a distinct branch designated complex-17 . this algorithm identifies the founder of a complex or genogroup of related sequence types ( st ) and subsequent patterns of evolutionary descent . the primary founder , indicated in blue , of a complex is defined as the st with the largest number of single locus variants ( slvs ) . larger complexes may contain secondary founders of additional lineages that have a number of slvs of their own . in addition , 57 sts did not group into any of the complexes and were considered singletons ( sts 13 , 15 , 30 , 31 , 36 , 37 , 38 , 39 , 42 , 51 , 52 , 58 , 59 , 62 , 65 , 67 , 68 , 70 , 72 , 73 , 74 , 83 , 84 , 86 , 89 , 95 , 96 , 98 , 99 , 100 , 102 , 105 , 107 , 108 , 110 , 112 , 114 , 116 , 118 , 126 , 129 , 131 , 142 , 144 , 145 , 152 , 153 , 155 , 162 , 163 , 164 , 167 , 168 , 169 , 170 , 171 , 172 ) . the " epidemic " genetic complex-17 derived from secondary founder st-17 is indicated . a measurement of statistical confidence in each of the assigned primary founders is made by a bootstrap resampling procedure ( 25).the predicted primary founders of the complexes 22 , 94 , 1 , and 69 have a bootstrap value of 73% , 84% , 85% , and 59% , respectively . in all , 142 of 411 isolates belonged to complex-17 , with a gradual increase in proportion among animal isolates ( 1/96 ) , human community isolates ( 3/57 ) , human hospital surveillance isolates ( 15/64 ) , and human clinical isolates ( 95/162 ) , to hospital - outbreak isolates ( 28/32 ) ( table 1 ) . ampicillin resistance , presence of the e. faecium pai ( 16 ) , and genetic clustering in complex-17 were strongly associated ( table 2 ) . * ampicillin resistant ( amr ) or susceptible ( ams ) not determined in 30 isolates , pai present ( pai+ ) or absent ( pa ) not determined in 17 isolates , vancomycin resistant ( vanr ) or susceptible ( vans ) not determined in 1 isolate . numbers in brackets refer to the percentage of isolates that belong to the complex .   the natural logarithm of the proportion of samples from an epidemiologic source belonging to complex-17 . * amp , ampicillin resistance ; pai , presence of the enterococcus faecium pathogenicity island ; gly , glycopeptide resistance ; epi , epidemiologic source ( animal surveillance , community surveillance , hospital surveillance , clinical sample , hospital outbreak ) . when controlling for individual and combined effects of ampicillin resistance , presence of pai , and vancomycin resistance , we can show that 1 ) the loglinear assumption holds for all effect parameters , and linear models describe the observed frequencies without substantial loss of goodness of fit ; 2 ) individual genetic markers exert an independent and multiplicative effect ; and 3 ) all genetic markers combined explain 48% of the category - specific abundance of complex-17 ( figure 2 ) . the effect of vancomycin resistance did not increase the explanatory value of the model , owing to the fact that determinants for vancomycin resistance could be found in equal proportions within and outside of complex-17 , likely a result of widespread horizontal transfer of vana ( table 1 ) . this finding suggests that the epidemiologic success of descendants of st-17 that results in clinical infections and hospital epidemics was at least partly related to antimicrobial resistance and the presence of putative virulence genes . the fact that 126 of 128 isolates of complex-17 were resistant to ampicillin and only 77 of 139 isolates of complex-17 contain pai ( table 1 ) suggests that e. faecium acquired ampicillin resistance first , which resulted in a selective advantage in hospitals , followed by the acquisition of pai , which further facilitated transmission . relative abundance of complex-17 in various sampled categories and proportion increase explained by combined effect of 3 parameters . 0 , animal surveillance samples ; 1 , human community surveillance samples ; 2 , human hospitalized patient samples ; 3 , human clinical samples ; 4 , hospital outbreak samples ; amp , ampicillin resistance ; pai , pathogenicity island ; gly , glycopeptide resistance . to assess the effect of recombination on the population structure of e. faecium , we estimated whether single locus variants ( slvs ) from the presumed founders of complex-1 , -17 , -22 , -69 , and -94 have arisen by point mutations or by recombination ( table 3 ) . of all allelic differences between ancestor - slv pairs ( n = 30 ) , 22 ( 4 in complex-17 ) included > 1 nucleotide , were found in multiple clonal complexes , and thus were most likely a result of recombination . eight allelic differences ( 2 in complex-17 ) included only a single nucleotide change , were unique within the dataset , and thus were most likely a result of mutation . therefore , most alleles of slvs in complex-17 and in the other complexes have arisen by recombination in the initial stages of diversification rather than by de novo point mutation . an important role for recombination in genetic diversification in e. faecium was confirmed by the lack of a positive trend between the number of nucleotide differences in nonidentical alleles and the number of allelic differences ( figure 3 ) ( 26 ) . the finding of high average numbers ( 4 ) of nucleotide differences in the nonidentical alleles of slvs in the total e. faecium population as well as in complex-17 also points towards frequent recombination . * genetic complexes 1 , 17 , 22 , 69 , and 94 st , sequence type ; c , cysteine ; y , tyrosine ; h , histidine ; l , leucine ; v , valine ; n , asparagine .  single nucleotide changes that are unique in the dataset and thus are due to mutation . the average number of nucleotide ( nt ) differences in nonidentical alleles for all pairwise comparisons of the 178 enterococcus faecium sequence types ( sts ) , and the 15 sts belonging to complex-17 was calculated separately for allelic profiles that differ in 17 alleles . this computation shows no positive correlation between the number of nucleotide differences and allelic differences , which suggests that recombination has played an important role in the genetic diversification in e. faecium , including the sts that constitute complex-17 . the degree of phylogenetic congruence between the 7 mlst genes was examined in a set of 24 diverse sts . these 24 sts were separated from each other by a linkage distance of > 0.4 on a upgma ( unweighted pair - group method with arithmetic mean ) tree constructed from the pairwise comparisons of their allelic profiles ( data not shown ) and included the primary founders of cc1 ( st1 ) , cc22 ( st22 ) , cc69 ( st69 ) , and cc94 ( st94 ) ; secondary founders of important subgroups complex-5 ( st5 ) and complex-17 ( st17 ) ; 1 st ( st76 ) belonging to a small complex of 3 sts ; and 17 singletons ( sts 15 , 38 , 39 , 54 , 67 , 74 , 83 , 84 , 89 , 96 , 98 , 99 , 101 , 107 , 118 , 142 , 163 ) . the results of the congruence analysis presented in table 4 show that 25 ( 60% ) of 42 of the pairwise comparisons of the 7 mlst loci were incongruent . of the 7 genes , this analysis confirms that recombination played a substantial role in the evolution of e. faecium . * number of incongruent genes at the p<0.05 level based on a shimodaira - hasegawa ( sh ) test of tree topologies . number of random tree topologies out of 100 random trees that are better fit to the gene tree from the most incongruent multilocus sequence typing ( mlst ) gene . mlst of 411 e. faecium isolates resulted in 175 different sequence types ( st ) . clustering these types with the eburst algorithm ( 25 ) showed 1 large complex of genetically related types . st-22 was the primary founder ; 3 minor complexes had st-1 , -69 , and -94 as primary founders ; 6 complexes had only 2 or 3 sts ; and 57 singletons were not linked to the aforementioned complexes ( figure 1 ) . within complex-22 , st-17 represents an important secondary founder of a distinct branch designated complex-17 . this algorithm identifies the founder of a complex or genogroup of related sequence types ( st ) and subsequent patterns of evolutionary descent . the primary founder , indicated in blue , of a complex is defined as the st with the largest number of single locus variants ( slvs ) . larger complexes may contain secondary founders of additional lineages that have a number of slvs of their own . in addition , 57 sts did not group into any of the complexes and were considered singletons ( sts 13 , 15 , 30 , 31 , 36 , 37 , 38 , 39 , 42 , 51 , 52 , 58 , 59 , 62 , 65 , 67 , 68 , 70 , 72 , 73 , 74 , 83 , 84 , 86 , 89 , 95 , 96 , 98 , 99 , 100 , 102 , 105 , 107 , 108 , 110 , 112 , 114 , 116 , 118 , 126 , 129 , 131 , 142 , 144 , 145 , 152 , 153 , 155 , 162 , 163 , 164 , 167 , 168 , 169 , 170 , 171 , 172 ) . the " epidemic " genetic complex-17 derived from secondary founder st-17 is indicated . a measurement of statistical confidence in each of the assigned primary founders is made by a bootstrap resampling procedure ( 25).the predicted primary founders of the complexes 22 , 94 , 1 , and 69 have a bootstrap value of 73% , 84% , 85% , and 59% , respectively . in all , 142 of 411 isolates belonged to complex-17 , with a gradual increase in proportion among animal isolates ( 1/96 ) , human community isolates ( 3/57 ) , human hospital surveillance isolates ( 15/64 ) , and human clinical isolates ( 95/162 ) , to hospital - outbreak isolates ( 28/32 ) ( table 1 ) . ampicillin resistance , presence of the e. faecium pai ( 16 ) , and genetic clustering in complex-17 were strongly associated ( table 2 ) . * ampicillin resistant ( amr ) or susceptible ( ams ) not determined in 30 isolates , pai present ( pai+ ) or absent ( pa ) not determined in 17 isolates , vancomycin resistant ( vanr ) or susceptible ( vans ) not determined in 1 isolate . numbers in brackets refer to the percentage of isolates that belong to the complex . the natural logarithm of the proportion of samples from an epidemiologic source belonging to complex-17 . * amp , ampicillin resistance ; pai , presence of the enterococcus faecium pathogenicity island ; gly , glycopeptide resistance ; epi , epidemiologic source ( animal surveillance , community surveillance , hospital surveillance , clinical sample , hospital outbreak ) . when controlling for individual and combined effects of ampicillin resistance , presence of pai , and vancomycin resistance , we can show that 1 ) the loglinear assumption holds for all effect parameters , and linear models describe the observed frequencies without substantial loss of goodness of fit ; 2 ) individual genetic markers exert an independent and multiplicative effect ; and 3 ) all genetic markers combined explain 48% of the category - specific abundance of complex-17 ( figure 2 ) . the effect of vancomycin resistance did not increase the explanatory value of the model , owing to the fact that determinants for vancomycin resistance could be found in equal proportions within and outside of complex-17 , likely a result of widespread horizontal transfer of vana ( table 1 ) . this finding suggests that the epidemiologic success of descendants of st-17 that results in clinical infections and hospital epidemics was at least partly related to antimicrobial resistance and the presence of putative virulence genes . the fact that 126 of 128 isolates of complex-17 were resistant to ampicillin and only 77 of 139 isolates of complex-17 contain pai ( table 1 ) suggests that e. faecium acquired ampicillin resistance first , which resulted in a selective advantage in hospitals , followed by the acquisition of pai , which further facilitated transmission . relative abundance of complex-17 in various sampled categories and proportion increase explained by combined effect of 3 parameters . 0 , animal surveillance samples ; 1 , human community surveillance samples ; 2 , human hospitalized patient samples ; 3 , human clinical samples ; 4 , hospital outbreak samples ; amp , ampicillin resistance ; pai , pathogenicity island ; gly , glycopeptide resistance . to assess the effect of recombination on the population structure of e. faecium , we estimated whether single locus variants ( slvs ) from the presumed founders of complex-1 , -17 , -22 , -69 , and -94 have arisen by point mutations or by recombination ( table 3 ) . of all allelic differences between ancestor - slv pairs ( n = 30 ) , 22 ( 4 in complex-17 ) included > 1 nucleotide , were found in multiple clonal complexes , and thus were most likely a result of recombination . eight allelic differences ( 2 in complex-17 ) included only a single nucleotide change , were unique within the dataset , and thus were most likely a result of mutation . therefore , most alleles of slvs in complex-17 and in the other complexes have arisen by recombination in the initial stages of diversification rather than by de novo point mutation . an important role for recombination in genetic diversification in e. faecium was confirmed by the lack of a positive trend between the number of nucleotide differences in nonidentical alleles and the number of allelic differences ( figure 3 ) ( 26 ) . the finding of high average numbers ( 4 ) of nucleotide differences in the nonidentical alleles of slvs in the total e. faecium population as well as in complex-17 also points towards frequent recombination . * genetic complexes 1 , 17 , 22 , 69 , and 94 st , sequence type ; c , cysteine ; y , tyrosine ; h , histidine ; l , leucine ; v , valine ; n , asparagine .  single nucleotide changes that are unique in the dataset and thus are due to mutation . the average number of nucleotide ( nt ) differences in nonidentical alleles for all pairwise comparisons of the 178 enterococcus faecium sequence types ( sts ) , and the 15 sts belonging to complex-17 was calculated separately for allelic profiles that differ in 17 alleles . this computation shows no positive correlation between the number of nucleotide differences and allelic differences , which suggests that recombination has played an important role in the genetic diversification in e. faecium , including the sts that constitute complex-17 . the degree of phylogenetic congruence between the 7 mlst genes was examined in a set of 24 diverse sts . these 24 sts were separated from each other by a linkage distance of > 0.4 on a upgma ( unweighted pair - group method with arithmetic mean ) tree constructed from the pairwise comparisons of their allelic profiles ( data not shown ) and included the primary founders of cc1 ( st1 ) , cc22 ( st22 ) , cc69 ( st69 ) , and cc94 ( st94 ) ; secondary founders of important subgroups complex-5 ( st5 ) and complex-17 ( st17 ) ; 1 st ( st76 ) belonging to a small complex of 3 sts ; and 17 singletons ( sts 15 , 38 , 39 , 54 , 67 , 74 , 83 , 84 , 89 , 96 , 98 , 99 , 101 , 107 , 118 , 142 , 163 ) . the results of the congruence analysis presented in table 4 show that 25 ( 60% ) of 42 of the pairwise comparisons of the 7 mlst loci were incongruent . of the 7 genes , this analysis confirms that recombination played a substantial role in the evolution of e. faecium . * number of incongruent genes at the p<0.05 level based on a shimodaira - hasegawa ( sh ) test of tree topologies . number of random tree topologies out of 100 random trees that are better fit to the gene tree from the most incongruent multilocus sequence typing ( mlst ) gene . nosocomial vre , which rapidly emerged in the united states in the 1990s after their initial discovery in europe , are found in increasing rates in hospitals in europe , asia , and south america ( 57,9,23,29,30 ) . the data presented in this study show that most of these hospital - derived vre are part of a single clonal lineage . this lineage , designated complex-17 after its presumed founder st-17 , represents most hospital outbreak and clinical isolates , apparently because it successfully adapted to hospital environments . the > 400 strains analyzed in this study were selected from a large representative collection of 2,000 e. faecium isolates . a wide variety of sources were used as selection criteria : hospital - associated outbreaks ; clinical samples and stool samples from hospitalized patients , healthy persons , and animals ; and a wide geographic distribution ( 21 countries on 5 continents ) . complex-17 probably evolved from the primary e. faecium ancestor st-22 through a combination of mutation and recombination . the following observations suggest that recombination has been especially important in the genetic diversification of the e. faecium population : 1 ) within clonal complexes , most slvs ( 73% ) have arisen by recombination rather than point mutations ; 2 ) no positive correlation exists between the degree of allelic diversity and the number of nucleotide differences in nonidentical alleles ; and 3 ) most ( 60% ) of the comparisons of mlst gene tree topologies were incongruent . exploitation of a novel ecologic niche as hospital settings by e. faecium st-17 often starts with adaptive changes ( 31 ) . on the basis of our findings this amplifying selective process in which variants with a selective advantage can more easily acquire additional adaptive mechanisms has been called " genetic capitalism " ( 32 ) . after successfully exploiting the hospital environment , st-17 increased in frequency to become the dominant clone . genetic diversification over time finally resulted in a meroclone , complex-17 , of highly related genotypes , fully adapted as a nosocomial pathogen , that has spread globally ( figure 4 ) . in addition , e. faecium sts , predominantly st-78 , belonging to complex-17 , have recently also been found in the republic of south korea ( k.s . comm . ) . considering the short period in which multiresistant e. faecium emerged as a nosocomial pathogen ( 33 ) , complex-17 represents the first globally dispersed nosocomial - adapted clonal lineage . despite the frequency of recombination events , clonal complex-17 is still detectable within the e. faecium population , which suggests that the emergence of this complex is relatively recent . numbers indicate epidemiologic sources : 1 , animal isolates ; 2 , human community surveillance isolates ; 3 , surveillance ( feces ) isolates from hospitalized patients ; 4 , human clinical isolates ; 5 , isolates from documented hospital outbreaks . the existence of epidemic clones , even in recombining populations , is also seen in other bacterial species ( 34,35 ) . however , the evolution of a single epidemic and clinically relevant genetic complex , as seen with e. faecium , differs from the evolution of other gram - positive pathogens like streptococcus pneumoniae and staphylococcus aureus . in s. pneumoniae , pandemic clones such as st81 , st90 , and st156 represent major invasive and multidrug - resistant isolates that have spread globally ( 36 ) . the allelic profiles of these clones , however , are highly diverse , which suggests that they are genetically unrelated and do not constitute a single genetic lineage , as does e. faecium . furthermore , the serotype of s. pneumoniae seems a more important marker of invasiveness than the overall genotype ( 37 ) . in s. aureus isolates , major pandemic mrsa clones that are responsible for most hospital - acquired infections are found in multiple genetically unrelated lineages , though most previously identified pandemic clones are found in clonal complex 8 ( 38,39 ) . therefore , the genetic diversity of major epidemic clones as seen in s. pneumoniae or s. aureus may not have yet emerged in e. faecium epidemic populations . stress - inducing conditions in hospitals , such as antimicrobial drug use , may have favored the selection of an enterococcal subpopulation , complex-17 , with enhanced antibacterial resistance , virulence , and ability to spread . whether reducing antimicrobial selection pressure in hospitals will reestablish a susceptible and less transmissible enterococcal population is unknown and will at least partly depend on the relative fitness costs of sustaining antimicrobial resistance and virulence determinants in e. faecium . furthermore , the hospital - adapted complex-17 has rapidly spread globally during the last 2 decades . subsequent acquisition of vana- or vanb- containing transposons by horizontal gene transfer resulted in vre with pandemic potential . rapid diagnosis of complex-17 strains based on multiple locus variable number of tandem repeat analysis ( mlva ) may help control its spread ( 40 ) . whether this effort will be successful depends on the level of complex-17 endemicity in the hospital . in many european countries , a relatively large community reservoir of vre exists , a result of the massive use of the antimicrobial drug avoparcin as a growth promoter , while in general the prevalence of hospital - adapted ( complex-17 ) vre is much lower . in such a setting , hospital transmission of isolates belonging to complex-17 can be halted by using a fast genotyping scheme like mlva to discriminate between hospital - adapted ( complex-17 ) and community e. faecium strains followed by strict infection control measures . the combination of infection control measures plus genotyping controlled an outbreak of vre in a dutch hospital ( 41 ) . establishing nosocomial co - endemicity of vre and mrsa will facilitate the horizontal transfer of vana- or vanb - containing transposons , transforming mrsa into vrsa , with implications for patient care . until now , 3 sporadic cases of vana - induced vrsa have been reported in the united states in 2002 and 2004 . spread of multidrug - resistant e. faecium strains and their resistance genes will have serious implications for health care , and control efforts should focus on early detection of e. faecium isolates belonging to complex-17 .
vancomycin - resistant enterococci ( vre ) have caused hospital outbreaks worldwide , and the vancomycin - resistance gene ( vana ) has crossed genus boundaries to methicillin - resistant staphylococcus aureus . 
 spread of vre , therefore , represents an immediate threat for patient care and creates a reservoir of mobile resistance genes for other , more virulent pathogens . 
 evolutionary genetics , population structure , and geographic distribution of 411 vre and vancomycin - susceptible enterococcus faecium isolates , recovered from human and nonhuman sources and community and hospital reservoirs in 5 continents , identified a genetic lineage of e. faecium ( complex-17 ) that has spread globally . 
 this lineage is characterized by 1 ) ampicillin resistance , 2 ) a pathogenicity island , and 3 ) an association with hospital outbreaks . 
 complex-17 is an example of cumulative evolutionary processes that improved the relative fitness of bacteria in hospital environments . 
 preventing further spread of this epidemic e. faecium subpopulation is critical , and efforts should focus on the early disclosure of ampicillin - resistant complex-17 strains .