2 Declaration I, Preneshni Rochelle Naicker, hereby declare that the work contained in this assignment is my original work and that I have not previously submitted it, in its entirety or in part, at any university for a degree. The ability to form biofilms contributes significantly to the virulence of Staphylococcus aureus. Virulence factors may be associated with certain S. aureus lineages. However, the contribution of the genetic background of S. aureus to biofilm formation is poorly understood. This study investigated the association between the genetic background and the biofilm forming ability of clinical invasive S. aureus isolates. Secondary objectives included investigating any correlation with biofilm formation and methicillin resistance or the source of bacteraemia. Methods:The study was conducted at a 1300-bed tertiary hospital in Cape Town, South Africa. S. aureus isolates obtained from blood cultures between January 2010 and January 2012 were included. Genotypic characterization was performed by PFGE, spa typing, SCCmec typing and MLST. Thirty genotypically unique strains were assessed for phenotypic biofilm formation with the microtitre plate assay. All isolates were tested in triplicate and an average optical density, measured at a wavelength of 490nm, was determined. The biofilm forming ability of isolates with A 490 > 0.17 was considered 'weak positive' and A 490 > 0.34 'strong positive'. Isolates with A 490 ≤ 0.17 were considered non-adherent. ANOVA with Bonferroni-adjusted post-hoc tests and t-tests were used for statistical analysis of the association between biofilm forming ability and strain characteristics.Results: Fifty seven percent of isolates were capable of forming biofilms. Weak biofilm formation occurred in 40% (n=12) and strong biofilm formation in 17% (n=5) of isolates. Thirteen (43%) of the isolates were classified as non-adherent. All 5 isolates capable of strong biofilm formation belong to one spa clonal complex (spa-CC 064). Strains from spa-CC 064 were capable of higher biofilm formation than other spa clonal complexes (p=0.00002). These 5 strains belonged to MLST CC5 and CC8. Biofilm formation did not correlate with methicillin resistance and was not related to the source of bacteraemia. Conclusion:Biofilm formation correlates with the spa clonal lineage in our population of invasive S. aureus strains. High biofilm formation was associated with spa-CC 064. MLST CC5 and CC8 strains are capable of strong biofilm formation.