Biofilm-associated accumulation-associated protein (Aap): A contributing factor to the predominant growth of Staphylococcus epidermidis in platelet concentrates.

BACKGROUND AND OBJECTIVES: Staphylococcus epidermidis is a predominant contaminant of platelet concentrates (PCs), outcompeting other skin flora bacteria such as Staphylococcus capitis. The accumulation-associated protein (Aap), encoded by the aap gene, is involved in formation of bacterial aggregates (biofilms) in S. epidermidis and is absent in S. capitis. In this study, the role of S. epidermidis aap in enhancing biofilm formation and conferring an advantageous growth in PCs was investigated. MATERIALS AND METHODS: Biofilm formation assays of S. epidermidis 1457, S. epidermidis 1457∆aap, S. capitis 517 and S. capitis 517 carrying S. epidermidis aap (S. capitis 517/pRBaap) were performed in glucose-supplemented trypticase soy broth (TSBg) and PCs. Additionally, competition assays with paired cultures (1:1 ratio) of S. epidermidis and S. capitis strains were seeded in PCs, followed by determination of viable counts of each organism at the end of PC storage. RESULTS: Staphylococcus epidermidis aap had no effect on biofilm formation in TSBg. By contrast in PCs, S. epidermidis 1457 showed higher biofilm formation than S. epidermidis 1457∆aap (P < 0·05). Biofilm formation was also enhanced in S. capitis 517/pRBaap compared to S. capitis 517 (P = 0·054). Competition assays showed that S. epidermidis 1457 outcompeted S. capitis 517, and importantly, S. capitis 517/pRBaap outcompeted S. capitis 517 and S. epidermidis 1457∆aap. CONCLUSION: This study demonstrated that S. epidermidis aap plays a role in biofilm formation in PCs conferring an advantageous proliferation to skin flora bacteria in this milieu. The molecular mechanisms of action of Aap merit further investigation.