title: USA300 and USA500 clonal lineages of Staphylococcus aureus do not produce a capsular polysaccharide due to conserved mutations in the cap5 locus.
abstract: The surface capsular polysaccharide (CP) is a virulence factor that has been used as an antigen in several successful vaccines against bacterial pathogens. A vaccine has not yet been licensed against Staphylococcus aureus, although two multicomponent vaccines that contain CP antigens are in clinical trials. In this study, we evaluated CP production in USA300 methicillin-resistant S. aureus (MRSA) isolates that have become the predominant community-associated MRSA clones in the United States. We found that all 167 USA300 MRSA and 50 USA300 methicillin-susceptible S. aureus (MSSA) isolates were CP negative (CP(-)). Moreover, all 16 USA500 isolates, which have been postulated to be the progenitor lineage of USA300, were also CP(-). Whole-genome sequence analysis of 146 CP(-) USA300 MRSA isolates revealed they all carry a cap5 locus with 4 conserved mutations compared with strain Newman. Genetic complementation experiments revealed that three of these mutations (in the cap5 promoter, cap5D nucleotide 994, and cap5E nucleotide 223) ablated CP production in USA300 and that Cap5E75 Asp, located in the coenzyme-binding domain, is essential for capsule production. All but three USA300 MSSA isolates had the same four cap5 mutations found in USA300 MRSA isolates. Most isolates with a USA500 pulsotype carried three of these four USA300-specific mutations, suggesting the fourth mutation occurred in the USA300 lineage. Phylogenetic analysis of the cap loci of our USA300 isolates as well as publicly available genomes from 41 other sequence types revealed that the USA300-specific cap5 mutations arose sequentially in S. aureus in a common ancestor of USA300 and USA500 isolates.
  
  
  OBJECTIVE
  The USA300 MRSA clone emerged as a community-associated pathogen in the United States nearly 20 years ago. Since then, it has rapidly disseminated and now causes health care-associated infections. This study shows that the CP-negative (CP(-)) phenotype has persisted among USA300 isolates and is a universal and characteristic trait of this highly successful MRSA lineage. It is important to note that a vaccine consisting solely of CP antigens would not likely demonstrate high efficacy in the U.S. population, where about half of MRSA isolates comprise USA300. Moreover, conversion of a USA300 strain to a CP-positive (CP(+)) phenotype is unlikely in vivo or in vitro since it would require the reversion of 3 mutations. We have also established that USA300 MSSA isolates and USA500 isolates are CP(-) and provide new insight into the evolution of the USA300 and USA500 lineages.
authors: Department of Pediatrics, Section of Infectious Diseases, The University of Chicago, Chicago, Illinois, USA sboyleva@uchicago.edu.
authors: Susan
authors: S
authors: Boyle-Vavra
authors: Xue
authors: X
authors: Li
authors: Division of Infectious Diseases, Department of Medicine, Emory University School of Medicine, Atlanta, Georgia, USA.
authors: Md Tauqeer
authors: MT
authors: Alam
authors: Timothy D
authors: TD
authors: Read
authors: Department of Pediatrics, Section of Infectious Diseases, The University of Chicago, Chicago, Illinois, USA.
authors: Julia
authors: J
authors: Sieth
authors: Division of Infectious Diseases, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA.
authors: Colette
authors: C
authors: Cywes-Bentley
authors: Minnesota Department of Health, St. Paul, Minnesota, USA.
authors: Ginette
authors: G
authors: Dobbins
authors: Department of Medicine, Section of Infectious Diseases and Global Health, The University of Chicago, Chicago, Illinois, USA.
authors: Michael Z
authors: MZ
authors: David
authors: Department of Pediatrics, Section of Infectious Diseases, The University of Chicago, Chicago, Illinois, USA.
authors: Neha
authors: N
authors: Kumar
authors: Department of Medicine, Harbor UCLA, Torrance, California, USA.
authors: Samantha J
authors: SJ
authors: Eells
authors: Department of Medicine, Harbor UCLA, Torrance, California, USA.
authors: Loren G
authors: LG
authors: Miller
authors: Minnesota Department of Health, St. Paul, Minnesota, USA.
authors: David J
authors: DJ
authors: Boxrud
authors: Division of Infectious Diseases, San Francisco General Hospital, San Francisco, California, USA.
authors: Henry F
authors: HF
authors: Chambers
authors: Minnesota Department of Health, St. Paul, Minnesota, USA.
authors: Ruth
authors: R
authors: Lynfield
authors: Division of Infectious Diseases, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA.
authors: Jean C
authors: JC
authors: Lee
authors: Department of Pediatrics, Section of Infectious Diseases, The University of Chicago, Chicago, Illinois, USA.
authors: Robert S
authors: RS
authors: Daum
mesh_terms: D016667:Bacterial Capsules / Q000235:genetics* / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D017124:Conserved Sequence; D019143:Evolution, Molecular; D005816:Genetic Complementation Test; D016680:Genome, Bacterial; D006801:Humans; D009154:Mutation; D017422:Sequence Analysis, DNA; D013203:Staphylococcal Infections / Q000382:microbiology; D013211:Staphylococcus aureus / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism*; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; C107803:Cap5 protein, Staphylococcus; D037521:Virulence Factors