920. Micro-Evolution of Staphylococcus aureus During Carriage and Disease
Session: Poster Abstract Session: Bacterial Pathogenesis
Saturday, October 22, 2011
Room: Poster Hall B1
Background: Staphylococcus aureus bacteremia is a common blood stream infection with high mortality. S. aureus nasal carriage is an established risk factor for S. aureus infections, however the mechanisms by which nasally carried S. aureus develops into invasive disease are unknown. Mouse model work suggests small genomic differences can radically alter virulence of S. aureus. Regulatory protein dysfunction has recently been demonstrated to increase mortality in S. aureus bacteremia. 

Methods: We use next-generation sequencing to investigate the evolution of a single strain of S. aureus within an individual who carried the organism over 12 months before developing a fatal S. aureus bacteremia in the absence of any surgical risks or invasive devices.  Seventy-two isolates were cultured from 7 time points : 6 nasal swab cultures taken over 12 months and 2 blood cultures from the day of admission to hospital with septicemia. Extracted DNA from each isolate was sequenced using the Illumina HiSeq platform. Reads were mapped to the MRSA252 reference genome using STAMPY, with base and variant calls made using SAMTOOLS and bespoke Python scripts. De novo assembly of reads and detection of variants was performed using Cortex. 

Results: This analysis found that the isolates were highly similar, with only 29 single nucleotide variants found between the isolates, with no large insertions or deletions acquired during carriage. More than half the observed variants (16/29) occurred in the 23 isolates cultured from the last nasal swab and from blood culture, compared with only 14 variants found between 49 isolates from earlier carriage (p= 0.03). Ten variants, including all three of the observed premature stop codons, segregate disease causing isolates from the carriage strains. One premature stop codon occurs in an AraC regulatory protein, the best current candidate for a phenotype-altering mutation. 

Conclusion: This case is a unique opportunity to use whole-genome sequencing to investigate micro-evolution of a single strain of S. aureus during both colonisation and disease. We have established that case-studies can yield candidate virulence loci.

Subject Category: B. Bacterial pathogenesis, studies in animal models, molecular pathogenicity

Bernadette Young, MBBS, Microbiology, Oxford Radcliffe Hospitals, Oxford, United Kingdom, Rowena Fung, Nuffield Department of Medicine, University of Oxford, Oxford, United Kingdom, Elizabeth Batty, Statistics, University of Oxford, Oxford, United Kingdom, Rory Bowden, Statistics, Oxford University, Oxford, United Kingdom, Derrick Crook, MB BCh, University of Oxford NIHR Oxford Biomedical Research Centre, Oxford, United Kingdom and Daniel Wilson, Statistics, Univeristy of Oxford, Oxford, United Kingdom


B. Young, None

R. Fung, None

E. Batty, None

R. Bowden, None

D. Crook, Optimer Pharmaceuticals: Investigator and Scientific Advisor, Consulting fee and Institution received per-case funding to support trial patient expenses

D. Wilson, None

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.