staphylococcus aureus is a leading cause of human bacterial infection worldwide and is endemic in both hospitals and the community ( 1 ) . the ubiquity of s.  aureus disease is facilitated by a commensal lifestyle , and the bacterium is frequently found on the skin and anterior nares of healthy individuals . s.  aureus is a predominant cause of nosocomial infections , which arise often in individuals with predisposing risk factors such as hemodialysis or surgery . the clinical spectrum of s.  aureus infection ranges from relatively benign skin and soft tissue infections to severe and life - threatening systemic disease ( 1 ) . management of severe s.  aureus disease is confounded by the penchant of the pathogen to develop antibiotic resistance . the ability of s.  aureus to develop drug resistance is well documented and is perhaps best exemplified by the emergence of the penicillin - resistant s.  aureus lineage known as phage type 80/81 shortly after the introduction of penicillin in the 1940s ( 1 ) . resistance is conferred by a plasmid - encoded penicillinase that hydrolyzes the -lactam ring of penicillin . s.  aureus phage type 80/81 spread rapidly and became pandemic in both hospitals and the community . similarly , methicillin - resistant s.  aureus ( mrsa ) strains were first reported in the early 1960s  soon after the introduction of methicillin . methicillin resistance is mediated by a low - affinity penicillin - binding protein ( pbp2a ) encoded by meca residing on a mobile genetic element known as staphylococcal cassette chromosome mec ( sccmec ) ( 1 ) . in contrast to s.  aureus plasmid - encoded penicillin resistance , which confers resistance to penicillins only , sccmec confers a broad spectrum of resistance to the entire -lactam class of therapeutic agents and frequently non--lactam therapeutics . as a result , vancomycin , a glycopeptide antibiotic that inhibits cell wall synthesis , has become one of the last viable options for treating severe mrsa infections ( 2 ) . the high prevalence of mrsa in hospitals has led to the increased use of vancomycin for the treatment of severe mrsa infections , making vancomycin one of the most commonly used antibacterial agents in academic health centers in the united states ( 3 ) . in 1997 , the first report of an s.  aureus isolate with reduced susceptibility to vancomycin was published ( 4 ) , and resistance was found to be associated with structural changes in the cell wall ( 5 ) . according to the clinical and laboratory standards institute ( clsi ) , these strains are classified as vancomycin - intermediate s.  aureus and are defined by vancomycin mics of 4 to 8  g / ml . in comparison , vancomycin - resistant s.  aureus ( vrsa ) is defined by the clsi as having vancomycin mics of 16  g / ml . over the past decade , 12 cases of vrsa infection have been reported in the united states ( 6 , 7 - 10 ) . resistance in vrsa occurs via specific modifications of the peptidoglycan cell wall target and is mediated by the vana operon ( 11 ) . vana is carried on the mobile genetic element tn1546 in enterococcus and can be transferred to a recipient s.  aureus strain by transconjugation ( 12 , 13 ) . vrsa strains belong to the s.  aureus clonal complex 5 ( cc5 ) phylogenetic lineage . to better understand the genetic characteristics of the cc5 lineage that may permit or favor the acquisition of the vancomycin resistance plasmid from enterococci , kos et al . sequenced the genomes of all 12 known u.s . vrsa strains and compared them to the genomes of other s.  aureus lineages ( 6 ) . the authors showed that all vrsa strains are closely related in dna sequence and thus the genomes have a fairly recent ancestor in common . inasmuch as vancomycin has been used as an antimicrobial agent since the late 1950s ( 11 ) , and given that the ancestral strain is predicted by kos et al . to have existed in the early 1960s ( 6 ) , it may seem surprising that vrsa was first isolated in 2002 ( 14 ) . why did it take 40  years for vrsa to emerge , when in comparison , mrsa emerged within 2  years after methicillin was introduced to treat human infections ( 15 ) ? several factors unrelated to the composition of the vrsa genome likely contributed , including the restricted use of vancomycin until the 1980s , the need for an appropriate tn1546-containing plasmid donor ( in this case , vancomycin - resistant enterococcus [ vre ] ) , and conditions that favor polymicrobial infection with vre and s.  aureus ( 11 ) . based on the genotype of the first vrsa isolate from michigan ( vrs1 in the study by kos et al . ) , the acquisition of tn1546 by s.  aureus likely involves acquisition of the tn1546-containing plasmid from vre and subsequent transfer of tn1546 from the vre donor plasmid to a staphylococcal multiresistance plasmid ( 9 ) . these transfer events have not been reproduced in vitro using selection with vancomycin ( 9 , 13 ) , perhaps due to the time required for the induction of vancomycin resistance . in addition , vrs1 did not retain the tn1546-containing enterococcal plasmid ( 9 ) . however , several subsequent vrsa isolates , for example , the vrsa isolate from new york ( vrs3a in the paper by kos et al . ) , maintained the tn1546-containing plasmid from vre ( 10 ) , and therefore , high - level vancomycin resistance in s.  aureus does not require transfer of tn1546 into a staphylococcal plasmid or integration into the chromosome . based on these findings , weigel et al . suggested that eliminating the need for a second transfer step significantly increases the probability of the emergence of vrsa from vre - s . one possible explanation for the selective transfer of the tn1546-containing plasmid from vre to cc5 strains is that cc5 strains have a defect in one or more restriction modification systems . however , in their analyses of vrsa genomes , kos et al . found that restriction modification systems were largely intact . the authors identified several genetic features that distinguish the cc5 phylogenetic lineage from other s.  aureus lineages , including a truncating frameshift mutation in the gene encoding dpra  a molecule known to increase dna transformation efficiency in bacteria  in all but one north american vrsa strain , the lack of a lantibiotic bacteriocin operon ( bsa ) , and the presence of a large repertoire of genes encoding lipoproteins and superantigens that may detrimentally alter host immune function ( 6 ) . whether these characteristics predispose cc5 strains to the acquisition of vancomycin resistance and enhance the spread of such strains is unknown and merits further investigation . a critical question that whole - genome sequencing and subsequent phylogenetic analyses might be able to address is whether transfer of the tn1546-containing plasmid from vre to s.  aureus has occurred multiple independent times ( i.e. , a transfer event in each patient ) or if there was a single vre - to - s . aureus transfer event followed by horizontal ( conjugal transfer ) or vertical transmission to other s.  aureus strains ( and transmission of the same vrsa strain among multiple patients ) . previous molecular genetic analyses of the vrsa isolates from three distinct geographic locations in north america , michigan , pennsylvania , and new york , suggested that there has been independent acquisition of vancomycin resistance from vre at each geographic location ( 8 , 10 , 16 ) . for example , tenover and colleagues reported that dna sequences of tn1546 obtained from the first michigan vrsa isolate ( vrs1 ) and the pennsylvania vrsa isolate ( vrs2 ) were distinct , as tn1546 in vrs2 had two insertion elements and a deletion relative to that from vrs1 ( 16 ) . in a subsequent study , zhu et al . found that four vrsa isolates from patients in michigan had unique pulsed - field gel electrophoresis patterns , albeit all were pulsed - field type usa100 or related to usa100 . the authors concluded that each michigan vrsa isolate arose by a unique genetic event ( 17 ) , that is , arose multiple times independently . although collectively these data provide support to the idea that vrsa isolates have arisen independently , a comprehensive analysis of the vrsa genome sequences was needed to better understand this process . in their whole - genome sequence analyses , kos et al . found that the tn1546 dna sequences obtained from vrsa strains segregate by region of isolation ( michigan , pennsylvania , new york , and delaware ) rather than by time of isolation from the patient , which suggests there has been independent acquisition of vancomycin resistance at each of these locations ( 6 ) . by comparison , all nine tn1546 dna sequences in the michigan vrsa isolates were either identical to each other or different by one single - nucleotide polymorphism , a finding consistent with the occurrence of a common donor strain ( i.e. , a single acquisition event ) . moreover , four michigan isolates contained identical inc18 enterococcal donor plasmids that contain tn1546 , an observation consistent with previous restriction enzyme analysis by zhu et al . ( 17 ) . on the other hand , the content of s.  aureus plasmids and , to some extent this finding is perhaps not surprising , since there is some variance in plasmid content even among s.  aureus isolates with the same pulsed - field type ( e.g. , among some usa300 isolates ) ( 18 ) . one possible explanation for the occurrence of multiple ( and seemingly unrelated ) vrsa infections in michigan is that there was a single tn1546 transfer event ( from vre to s.  aureus ) and then subsequent horizontal transfer of the enterococcal plasmid to other closely related s.  aureus clones or integration of tn1546 into an s.  aureus plasmid , followed by a horizontal transfer event . horizontal transfer of the vancomycin resistance plasmid from vrsa to mrsa has been demonstrated in vitro , providing support for the latter hypothesis ( 9 ) . such a phenomenon might also explain the phylogeny - based observations of kos et al . , in which analysis of hypervariable repeat regions of several s.  aureus molecules failed to reveal a single common line of vrsa descent ( beyond the recent common ancestor , ca . others suggest that vrsa arose independently at each geographic location , whereas the argument for independent acquisition of vancomycin resistance in each of the michigan vrsa isolates is less compelling and , in fact , provides evidence for a single acquisition event . provides new insight into the molecular basis of the emergence of vrsa and raises many important questions that require further investigation .
abstractstaphylococcus aureus 
 is a human commensal bacterium and a prominent cause of infections globally . 
 the high incidence of 
 s.  aureus 
 infections is compounded by the ability of the microbe to readily acquire resistance to antibiotics . in the united states , methicillin - resistant 
 s.  aureus ( mrsa ) is a leading cause of morbidity and mortality by a single infectious agent . 
 therapeutic options for severe mrsa infections are limited to a few antibiotics to which the organism is typically susceptible , including vancomycin . 
 acquisition of high - level vancomycin resistance by mrsa is a major concern , but to date , there have been only 12 vancomycin - resistant 
 s.  aureus ( vrsa ) isolates reported in the united states and all belong to a phylogenetic lineage known as clonal complex 5 . to gain enhanced understanding of the genetic characteristics conducive to the acquisition of vancomycin resistance by 
 s.  aureus , v. n. kos et al . performed whole - genome sequencing of all 12 vrsa isolates and compared the dna sequences to the genomes of other 
 s.  aureus 
 strains . 
 the findings provide new information about the evolutionary history of vrsa and identify genetic features that may bear on the relationship between 
 s.  aureus 
 clonal complex 5 strains and the acquisition of vancomycin resistance genes from enterococci .